Car fault diagnostics (Mercedes-Benz S-Class W140)

Description of diagnostics of electronic control systems and diagnostic codes are given in Chapter Engine electrical equipment.

This Section offers the simplest scheme for identifying the causes of malfunctions and failures occurring in vehicle units and systems. Failures and their possible causes are divided into groups according to their relationship to specific components or systems of the vehicle, such as the engine, cooling system, etc., and the text also provides references to Chapters and Sections related to these issues.

Remember that successfully completing a troubleshooting is not some black magic reserved for experienced professional mechanics, but a combination of knowledge and a patient, systematic approach to investigating the problem. Always move from simple to complex, take each check to its logical conclusion, and try not to miss the obvious - anyone can forget to fill the fuel tank or leave the lights on overnight.

Finally, you should always try to get a clear picture of the progression of the fault and take appropriate steps to prevent a recurrence. If the electrical equipment failure occurred due to a poor contact, check the condition of all other contacts and electrical connectors in the system at the same time. If the same fuse continues to blow several times in a row, there is no point in replacing it further - you should try to find out the cause of the failure. Remember that the failure of a minor component may be a sign of a malfunction of a more important unit or the entire system.

The engine does not turn over when trying to start it

1. There is corrosion of the battery terminals or the fastening of the wire tips on them is loose. Check the condition of the terminals and tips. Tighten the terminal clamps, remove traces of corrosion if necessary.

2. The battery is discharged or defective. If the cable ends are found to be clean and securely connected to the battery terminals, turn the ignition switch to the ON position, then turn on the headlights and/or windshield wipers. If these electrical equipment do not function, the battery is discharged.

3. The transmission is not correctly set to the "P" position.

4. The wiring in the starting system circuit is broken or the wires are loose at the terminals. Check the condition of the wires, terminals and electrical connectors of the battery, starter solenoid relay and ignition switch.

5. The starter pinion is jammed in the toothed rim of the drive disk. Try turning the engine by the crankshaft pulley. Remove the starter as soon as possible and check the condition of the pinion and toothed rim.

6. The starter traction relay is faulty (see Chapter Engine electrical equipment).

7. The starter is faulty (see Chapter Engine electrical equipment).

8. The ignition switch is faulty.

The engine turns over but does not start

Petrol models

1. The start is not performed correctly (see the corresponding Section of the Chapter User manual).

Attention! The order of starting a gasoline engine:

• Set the parking brake.
• Engage neutral gear (manual transmission)/move the selector lever to position "P" or "N" (automatic).
• Without pressing the gas pedal, turn the ignition key to the right until it stops. As soon as the engine starts, immediately release the key. If the warm engine does not start when cranking it with the starter for more than 4 seconds, slowly press the gas pedal. Do not crank the engine for more than 30 seconds at a time. Wait at least 15 seconds before trying again.
• Monitor the engine oil pressure gauge (see the relevant Section in Chapter User manual): The pressure indicator should be activated immediately after starting. If the engine has cooled down significantly, the pressure indicator will only appear some time after starting. Do not increase the engine speed until the oil pressure indicator is activated.
• At unstable speeds (for example, when misfiring) to avoid failure of the catalytic converter, try to run the engine only for a short time and do not allow heavy loads - immediately drive the car to a Mercedes-Benz service station.
• In areas where the outside temperature often drops below -20°C, it is recommended to install a coolant heater. You can get information on this issue at any Mercedes-Benz service station.

2. The engine immobilizer is faulty or not disabled.



3. The fuse for the electric fuel pump or the electronic injection system is faulty (see Chapter On-board electrical equipment).

4. The fuel tank is empty or poor quality fuel is added.

5. The air filter is heavily contaminated. There are vacuum leaks in the air supply elements, malfunctions in the fuel injection and ignition control systems.

6. The battery is discharged (the engine turns over too slowly). Check the functioning of electrical equipment.

7. The battery terminal connections are corroded or loose.

8. The fuel pump is faulty, its fuse is blown or the pump relay is damaged. Check by ear the noise of the pump operation when the ignition is turned on (see Chapter Power and exhaust systems).

9. Damaged or excessively damp ignition system components (see Chapter Engine electrical equipment).

10. Spark plugs are worn or faulty, or the spark plug gap is set incorrectly (see Chapter Engine electrical equipment).

11. The wiring of the electrical circuit of the starting system is broken or disconnected, or the fastening of the wires at the terminals is loose.

12. The ignition coil circuit wiring is broken or disconnected, or the wires are loose at the coil terminals (see Chapter Engine electrical equipment).

13. Fuse 15 of the engine control unit is damaged, one of the sensors is faulty: Hall, pulse sensor, coolant temperature (ECT) or intake air temperature (IAT) sensor (see Chapter On-board electrical equipment).

14. Compression pressure too low. Replace cylinder head gasket. Repair engine if necessary.

Diesel models

1. The launch was performed incorrectly.

Attention! The order of starting a diesel engine:

• Set the parking brake.
• Engage neutral gear (manual transmission)/move the selector lever to position "P" or "N" (automatic).
• Turn the key in the ignition switch to position 2 - the pre-glow indicator lamp on the instrument panel should light up (see the corresponding Section of the Chapter User manual). The engine should be started no earlier than the indicator lamp goes out - without turning on the on-board power consumers and without pressing the gas pedal, turn the key in the lock to the right until it stops.
• Caution! At outside temperatures below -10°C, you should also press the gas pedal all the way down. Continue turning the engine with the starter until its speed stabilizes, but not for more than 30 seconds. Before restarting after an unsuccessful attempt, remember to first return the key to position 0 and wait at least 15 seconds.
• Monitor the engine oil pressure gauge (see the relevant Section in Chapter User manual) the pressure indicator should be activated immediately after starting. If the engine is very cold, the pressure indicator will appear only after some time after starting. Do not increase the engine speed until the oil pressure indicator is activated.
• If the glow plug indicator lamp does not light, there is a fault in the glow plug system that must be repaired immediately at a Mercedes-Benz service station.
• In areas where the outside temperature often drops below -20°C, it is recommended to install a coolant heater. You can get information on this issue at any Mercedes-Benz service station.

2. The engine immobilizer is faulty.



3. The fuse for the electric fuel pump or pre-heating system is faulty.

4. The engine is not warmed up: check the proper functioning of the pre-heating device.

5. There is no voltage on the electromagnetic shut-off valve. Connect a voltmeter or LED tester to the switch. Turn on the ignition. The LED should light up. Otherwise, you need to find the break in the wiring and fix it.

6. The electromagnetic shut-off valve is faulty. Check the reliability of the electromagnetic switch fastening, check the contacts. Turn the ignition on and off. A click should be heard in the switch.

7. There is damage in the fuel supply line or air in the lines:

• a) The pipes or hoses are mechanically damaged or clogged - clean the fuel pipes and remove air from the lines and filter;
• b) The fuel filter is clogged - replace the filter element;
• c) In winter, check the filter and pipes for ice or wax - drive the car into a heated garage, add gasoline to the system;
• d) The tank ventilation or fuel intake mesh filter is clogged - clean it.

8. The adjustment of the moment of start of fuel supply is knocked down. Check, make adjustments.

9. The injectors are faulty. Check. Unscrew the union nuts one by one - when you unscrew the nut of the faulty cylinder, the engine speed will not drop.

10. The high-pressure fuel pump (HPFP) is faulty. Install a new or known good HPFP for testing.

11. Compression pressure too low. Replace cylinder head gasket. Repair engine if necessary.

The starter operates without turning the engine

1. The starter gear is jammed. Remove the starter (see Chapter Engine electrical equipment) and check its condition.



2. The starter gear teeth or flywheel ring gear are worn or damaged.

Difficulty starting a cold engine

1. The battery is discharged or not sufficiently charged.

2. There is a malfunction of the components of the power supply system or electrical equipment (see Chapters Power and exhaust systems and Engine electrical equipment).

3. Paraffin in the fine filter of the diesel engine. Replace the filter element. Fill with winter fuel or dilute with gasoline (refer to the subsection Operating a car in winter conditions in the Chapter User manual).

4. The glow plugs of the diesel engine are faulty.

Difficulty starting a hot engine

Please also refer to Section Checking and adjusting the control position of the camshafts.

1. The air filter is blocked (see Chapter Power and exhaust systems).

2. There is a malfunction of the components of the power supply system or electrical equipment (see Chapters Power and exhaust systems and Engine electrical equipment).

3. Fuel does not reach the injectors of the injection system (see Chapter Power and exhaust systems).

4. The fuel injection advance angle of the diesel engine is out of order.

5. Low compression in the cylinders, faulty or out of adjustment fuel injection pump of the diesel engine.

The starter engages too noisily or with difficulty

1. The starter gear teeth or flywheel ring gear are worn or damaged.

2. The starter mounting bolts are missing or their tightening force has weakened.

The engine starts but immediately stalls

1. The engine immobilizer is faulty.



2. The electrical wiring is faulty or the wires on the coil or generator terminals are loose.

3. There is a malfunction of the components of the power supply system or electrical equipment (see Chapters Power and exhaust systems and Engine electrical equipment).

4. The basic settings of the electronic control unit (ECM) are violated.

5. Damage to the exhaust system/catalytic converter.

6. There are vacuum losses through the gasket surfaces of the intake manifold or fuel injection unit. Make sure that the fastening bolts/nuts are tightened securely, check the fact and quality of fastening on their fittings on the intake manifold of all vacuum hoses. Check the condition of the hoses and their correct connection (specifically to your fittings).

The stability of the engine at idle speed is impaired

Petrol models

1. There is a loss of vacuum. Make sure that the mounting bolts/nuts are tight, check the fact and quality of fastening on their fittings on the intake manifold of all vacuum hoses. Listen to the running engine with a stethoscope or a section of fuel hose. The presence of a hissing sound will help to identify the source of the vacuum "leak". A solution of soapy water can be used for checking no less effectively.

2. There is a malfunction of the components of the power supply system or electrical equipment (see Chapters Power and exhaust systems and Engine electrical equipment).

3. The crankcase ventilation system valve is blocked (see Chapter Engine).

4. The air filter is blocked (see Chapter Power and exhaust systems).

5. The fuel pump does not supply sufficient fuel to the injectors of the injection system (see Chapter Power and exhaust systems).

6. There are leaks through the cylinder head gasket. Measure the compression pressure in the engine cylinders.

7. The working projections of the camshaft cams are worn out (see Chapter Engine).

Diesel models

1. The quality of the fuel hose connections on the high-pressure fuel pump and fuel filter is damaged. Replace the fuel hoses, secure them with clamps, tighten the screws of the corner connections.

2. The fuel injection pump mount is mechanically damaged or loosened. Check the mount.

3. The connections of the return line and the fuel supply line are reversed. Put the connections back in place. The screw for connecting the return line is marked "OUT".

4. There is damage in the fuel supply line:

• a) Pipes or hoses are mechanically damaged or clogged - clean fuel lines;
• b) The fuel filter is clogged. Replace the filter element.
• c) In winter, check the filter and pipes for ice or wax - drive the car into a heated garage, add gasoline to the system;
• d) The tank ventilation or fuel intake mesh filter is clogged - clean it.

5. The fuel injection start adjustment is out of adjustment. Check and make adjustments.

6. The injectors are faulty. Check. Unscrew the union nuts one by one - when you unscrew the nut of the faulty cylinder, the engine speed will not drop.

7. The high-pressure fuel pump (HPFP) is faulty. Install a new or known good HPFP for testing.

There are misfires at idle speed

1. Spark plugs are worn or dirty, or the spark plug gap is set incorrectly (see Chapter Engine electrical equipment).

2. There is a malfunction of the components of the power supply system or electrical equipment (see Chapters Power and exhaust systems and Engine electrical equipment).

3. The spark plug wires are faulty (see Chapter Engine electrical equipment).

4. Low-quality fuel was added. The fuel filter is clogged.

5. There are losses of vacuum in the intake manifold or through hose connections.

6. Insufficient or unevenly distributed compression pressure in the cylinders. Measure the compression (see Chapter Engine).

7. There are malfunctions in the engine control system.

There are misfires while driving in gear

1. The fuel filter is blocked or the fuel system is contaminated (see Chapter Power and exhaust systems).

2. The spark plugs are faulty or dirty, or the spark plug gap is set incorrectly (see Chapter Engine electrical equipment).

3. There is a malfunction of the components of the power supply system or electrical equipment (see Chapters Power and exhaust systems and Engine electrical equipment).

4. There is a defect in the spark plug wires (see Chapter Engine electrical equipment).

5. Components of exhaust gas toxicity reduction systems are faulty.

6. Insufficient or uneven distribution of compression pressure between the cylinders. Remove the spark plugs and measure the compression pressure (see Chapter Engine).

7. The ignition system is faulty (see Chapter Engine electrical equipment).

8. There are vacuum losses at the throttle body, intake manifold or through vacuum hoses.

The engine stalls spontaneously

1. Idle speed adjustment is incorrect.

2. The fuel filter is blocked and/or moisture or dirt has entered the fuel system (see Chapter Power and exhaust systems).

3. There is a failure of the power supply system components or sensors (see Chapter Power and exhaust systems).

4. Components of exhaust gas toxicity reduction systems are faulty.

5. The spark plugs are faulty or dirty, or the spark plug gap is set incorrectly (see Chapter Routine maintenance). Also check the condition of the spark plug wires (see Chapter Engine electrical equipment).

6. There are losses of vacuum at the throttle body or through the vacuum hoses.

The engine does not develop full power

1. There is a malfunction of the components of the power supply system or electrical equipment (see Chapters Power and exhaust systems and Engine electrical equipment).

2. The air cleaner is clogged. Clean or replace.

3. The spark plugs are faulty or the spark plug gap is set incorrectly (see Chapter Routine maintenance

4. The ignition coil is faulty (see Chapter Engine electrical equipment).

5. The AT fluid level does not meet the requirements (see Chapters Routine maintenance and Automatic transmission).

6. The transmission slips (see Chapter Clutch).

7. The fuel filter is blocked and/or dirt/moisture has entered the fuel system (see Chapter Power and exhaust systems).

8. The wrong type of fuel has been added. Fill the tank with fuel with the required octane rating.

9. The turbocharger control valve is damaged

10. The camshaft activator of the timing phase shift is faulty

11. Insufficient or uneven compression pressure across the cylinders. Perform a compression test, the analysis of the results of which will reveal the presence of valve leaks and/or a leak in the cylinder head gasket (see Chapter Engine).

Additional for diesel engine

1. The air cleaner is clogged. Clean or replace.

2. Uneven fuel supply from the high-pressure fuel pump to the cylinders. Adjust on a warm engine.

3. The fuel injection pump start time is set incorrectly. Set it.

4. The fuel filter or high-pressure fuel pump (injector) is clogged. Wash the pre-filter, change the fine filter element, clean the sprayer or replace the pump (injector).

5. The maximum crankshaft speed adjustment is not correct.

There are pops in the intake system or shots in the exhaust system

1. There is a malfunction of the components of the power supply system or electrical equipment (see Chapters Power and exhaust systems and Engine electrical equipment).

2. There is a defect in the secondary circuit of the ignition system (destruction of spark plug insulators or defects in spark plug wires) (See Chapter Engine electrical equipment).

3. The fuel injection system needs adjustment or its components are excessively worn (see Chapter Power and exhaust systems).

4. There are vacuum losses at the throttle body, intake manifold or through vacuum hoses.

5. The valves are stuck (see Chapter Engine).

6. The order of connecting the spark plug wires is violated (see Chapter Engine electrical equipment).

When accelerating or driving uphill, detonation sounds occur

1. Low quality fuel was used.

2. There is a malfunction of the components of the power supply system or electrical equipment (see Chapters Power and exhaust systems and Engine electrical equipment).

3. The wrong type of spark plugs are installed. Check the spark plugs and coils for damage (see Chapter Engine electrical equipment).

4. The basic settings of the electronic control unit are violated.

5. The knock sensor is faulty.

6. There are losses of vacuum.

Pre-ignition takes place (the engine continues to run after the ignition is turned off)

1. The idle speed is too high (see Chapter Engine).

2. There is a malfunction of the components of the power supply system or electrical equipment (see Chapter Engine electrical equipment).

3.The fuel Vapor Capture system (EVAP) purge valve is damaged.

4. The engine operating temperature is excessive. Possible causes of this malfunction may be a decrease in the coolant level (see Chapter Cooling and heating systems), thermostat failure, radiator blockage or water pump malfunction (see Chapter Cooling and heating systems).

Diesel engine smokes

1. Black smoke:

• a) The air cleaner is dirty - wash and fill with fresh oil or replace the element;
• b) Incorrect fuel has been used - flush the tank and change the fuel;
• c) The moment of start of injection of the high-pressure fuel pump is set incorrectly.

2. Blue smoke:

• a) Oil burning in the combustion chamber due to worn piston rings, high oil level in the air cleaner, worn seals in the turbocharger covers, leaks in the gasket between the crankcase and the turbocharger - repair the engine, set the normal oil level, replace the seals, tighten the turbocharger mounting bolts or replace the gasket;
• b) Fuel is not supplied to one of the injectors - check by heating the outlet pipes of the manifold;
• c) Poor fuel atomization by the injector due to a faulty valve or a broken atomizer - grind the valve or replace the injector.

3. White or brown smoke:

• a) Low temperature in the cooling system - check the thermostat;
• b) Poor operation of the injectors due to wear or breakage of the nozzle - replace the nozzle.

Engine electrical equipment

Caution: Electronic control system diagnostic procedures and fault codes are provided in Chapter Engine electrical equipment.

The battery does not hold a charge

1. The alternator drive belt is worn or damaged, or its tension adjustment is incorrect (see Chapter Engine electrical equipment).

2. The electrolyte level is insufficient or the battery is very discharged (see Chapter Engine electrical equipment).

3. There is corrosion of the battery terminals or the fastening of the wire tips to them has become loose (see Chapter Engine electrical equipment).

4. The generator does not provide the required charging current (see Chapter Engine electrical equipment).

5. The charging circuit wiring is broken or otherwise damaged, or the wires are loose at the terminals (see Chapter Engine electrical equipment).

6. A short circuit in the wiring creates a constant leakage of current generated by the battery to the ground (refer to Chapter Engine electrical equipment and Schematic diagrams of electrical equipment).

7. There is an internal defect in the battery.

The charging indicator light does not go out when the revs increase

1. The drive belt is loose. Check the V-belt tensioner.

2. Damage to the overrunning clutch (turbodiesel only). Remove the generator and turn the generator drive shaft to the left. If the working surface of the pulley does not rotate, then the damage is usually in the overrunning clutch.

3. Voltage regulator/generator damaged. Disconnect the (D+) wire from the back of the generator and turn on the ignition. If the test lamp does not light now, the fault is usually in the voltage regulator or generator.

4. Brush wear. Visually inspect the brushes, replace if necessary.

5. Damage to the regulator. Check and replace if necessary.

6. Generator damage. Check and replace if necessary.

7. Damage to the wire between the generator and the voltage regulator.

8. Damage to the instrument panel unit.

The charging indicator lamp does not light when the ignition key is turned to the ON position (when the ignition is turned on)

1. The control lamp on the instrument panel has burned out or otherwise failed (see Chapter On-board electrical equipment).

2. The generator is faulty (see Chapter Engine electrical equipment).

3. There is a defect in the printed circuit board, the wiring inside the instrument panel unit or the lamp socket (see Chapter On-board electrical equipment).

The charge indicator lamp lights up after the ignition is turned off

Diodes are broken. Check diodes, replace diode board.

Starter

1. If the starter does not turn, first check that the required voltage of at least 10 V is present at terminal 50 of the traction relay. If the voltage is lower than this value, check the wires included in the starter current circuit according to the diagram.

2. Whether the starter operates at full battery voltage can be checked as follows:

• a) Without engaging gears, turn on the ignition;
• b) Short-circuit terminals 30 and 50 of the starter with a wire of at least 4 mm² cross-section, refer also to the Section "Checking the traction relay" of Chapter Engine electrical equipment).

3. If the starter works perfectly, the cause of the malfunction should be sought in the wires to the starter. If the starter does not work, it should be checked with it removed.

Attention! Test condition: The wires at the connection points must be well secured and must not be oxidized.

The starter does not turn when the ignition switch is turned on

1. The battery is discharged. Charge the battery.

2. Jumper terminals 30 and 50 of the starter: The starter rotates. Wire 50 to the ignition switch is broken, the starter switch is faulty. Fix the break, replace the defective parts.

3. Broken ground cable or connector, battery is discharged. Check battery cables and connectors, measure battery voltage, charge if necessary.

4. Insufficient current flow due to loose or oxidized connectors. Clean the battery terminals and poles, check the connections between the battery, starter and ground.

5. No voltage at terminal 50 of the traction relay. Wire break, starter switch faulty.

The starter rotates slowly and does not turn the crankshaft

1. Battery is low. Charge the battery.

2. Summer oil is filled. Fill with all-season oil.

3. Insufficient current flow due to loose or oxidized connectors. Clean the battery terminals and poles, check the connections between the battery, starter and ground.

4. Carbon brushes do not fit the commutator, are stuck in the guides, are worn, broken, oily or dirty. Check the brushes, clean or replace. Check the guides.

5. Insufficient distance between brushes and commutator. Replace brushes and clean brush guides.

6. The collector is grooved or burnt and oily. Treat the collector or replace the anchor.

7. No voltage at terminal 50 (min 8 V). Check the starter switch or traction relay.

8. The bearing is broken. Check the bearing, replace if necessary.

9. The traction relay is faulty. Replace the traction relay.

The starter catches and turns the engine jerkily

1. The pinion drive is faulty. Replace the pinion drive.

2. The gear is dirty. Clean the gear.

3. The flywheel ring gear is faulty. Treat the ring gear, replace the flywheel if necessary.

The starter pinion does not move away from the ring gear

1. The pinion drive or threads are dirty or damaged. Clean or replace the pinion drive.

2. The traction relay is faulty. Replace the traction relay.

3. The return spring is weak or broken. Replace the return spring.

The starter continues to operate after the ignition key is released

1. The traction relay is stuck and does not turn off. Immediately turn off the ignition, replace the traction relay.

2. The ignition switch does not turn off. Disconnect the battery immediately and replace the ignition switch.

Power supply system

Excessive fuel consumption

Petrol models

1. The air filter element is dirty or blocked (see Chapter Power and exhaust systems).

2. There is a malfunction of the components of the power supply system, electrical equipment or electronic control (see Chapters Power and exhaust systems and Engine electrical equipment).

3. Leaks in the air intake system.

4. Damage to the exhaust system/catalytic converter.

5. Insufficient tire pressure or incorrect size tires installed.

Diesel engine

1. Clogged return line. Blow air through the return line from the high-pressure fuel pump to the fuel tank.

2. Idle speed and maximum speed are too high. Adjust.

3. The engine has mechanical damage. Check the compression, repair the engine if necessary.

There are fuel leaks and/or a smell of gasoline

1. There are leaks in the fuel supply lines or vent lines (see Chapter Power and exhaust systems).

2. The fuel tank is full. Refuel only until the gun switches off automatically.

3. There are leaks or evaporations from the fuel system lines and exhaust gas toxicity reduction (see Chapter Power and exhaust systems).

The engine does not start

1. When the starter is turned on, the electric fuel pump does not work (no noise from work can be heard). Lightly tap the pump body to release the jammed part. Check if voltage is applied to the pump. Check the fuse for proper operation. Check the electrical contacts for good conductivity.

2. The fuel pump relay is faulty. Check the relay.

3. The emergency fuel supply switch under the driver's seat has been activated. Press the switch button.

4. The injector valves are stuck. Check the injectors, replace if necessary. Check the voltage supply to the injectors. To do this, disconnect the injector connector. Connect the diode test lamp to the wire and turn on the starter. The test lamp should blink.

5. No signal from the Hall ignition sensor or coolant temperature sensor. Check the electrical wires. Perform a fault memory query.

6. Fuel lines are damaged, clogged or leaky, there are defects in the hoses.

7. The fuel filter is clogged.

8. The intake system is not tight. Additional air is sucked in.

9. Vacuum lines are damaged or leaky.

10. The pressure regulator is damaged. Check the residual pressure.

11. The throttle position sensor is damaged.

12. There is no power supply to the engine control unit.

13. Presence of ice or wax in the filter or lines of the diesel engine.

14. The fuel tank ventilation is clogged, the filter in the tank is clogged.

15. Preheating does not work.

16. The diesel engine fuel shut-off valve is damaged.

17. The moment of start of fuel supply is disrupted.

18. The diesel engine injectors are damaged. Loosen the union nuts one by one and check whether the corresponding cylinder is working.

19. The fuel injection pump is faulty.

Cold engine starts poorly, runs unsteadily

1. CO content does not meet requirements. Check CO content and idle speed.

2. The coolant/intake air temperature sensor is faulty. Check the sensor.

3. The fuel pressure does not meet the required value. Check the fuel pressure.

4. The intake system is not tight. Additional air is sucked in.

A warm engine starts poorly and runs unsteadily

1. The air intake system is leaky. Check the intake system. To do this, leave the engine idling and cover the seals and connections on the intake tract with gasoline. If the RPM increases briefly, eliminate the leaky areas.

Warning: Gasoline vapors are poisonous, do not inhale them.

2. The fuel pump check valve is damaged.

3. Leaks in the fuel system.

4. Increased fuel pressure in the fuel system.

5. Malfunction of the fuel vapor recovery system.

6. The return fuel line to the tank is clogged or crushed.

7. The injector valves are stuck. Check the injectors, replace if necessary. Check the voltage supply to the injectors. To do this, disconnect the injector connector. Connect the diode test lamp to the wire and turn on the starter. The test lamp should blink.

8. No signal from the Hall ignition sensor or coolant temperature sensor. Check the electrical wires. Perform a fault memory query.

9. The intake system is not tight. Additional air is sucked in.

10. Vacuum lines are damaged or leaky.

11. The pressure regulator is damaged. Check the residual pressure.

12. The throttle position sensor is damaged.

13. There is no power supply to the engine control unit.

The engine runs intermittently

1. Electrical connections to the fuel pump are intermittently interrupted. Check the condition of the plug connections and connectors of the electrical wires of the fuel pump, air flow meter and fuel pump relay. Check the fuse and contact locations on the fuel pump relay. Clean the contacts, replace if necessary.

2. Low quality fuel, formation of vapor locks. Fill with quality fuel.

3. Fuel supply volume is too low. Check the pump performance.

4. The fuel filter is faulty. Replace the fuel filter.

5. The fuel pump is faulty. Check the fuel pump.

6. The injectors are faulty. Check the injectors.

7. The lambda probe is faulty, there are problems in the mixture quality control circuit, or the lambda probe is not heated.

8. The throttle position sensor is faulty. Check the throttle position sensor.

9. The exhaust manifold or downpipe of the exhaust system is damaged (there are exhaust leaks).

10. Malfunction of the fuel vapor recovery system.

11. The fuel hoses have become loose on the high-pressure fuel pump and fuel filter of the diesel engine.

12. When connecting, the supply and return pipelines of the high-pressure fuel pump were mixed up.

13. The injector valves are stuck. Check the injectors, replace if necessary. Check the voltage supply to the injectors. To do this, disconnect the injector connector. Connect the diode test lamp to the wire and turn on the starter. The test lamp should blink.

14. No signal from the Hall ignition sensor or coolant temperature sensor. Check the electrical wires. Perform a fault memory query.

15. The intake system is leaky. Additional air is sucked in.

16. Vacuum lines are damaged or leaky.

17. The pressure regulator is damaged. Check the residual pressure.

18. The throttle position sensor is damaged.

19. There is no power supply to the engine control unit.

The engine runs intermittently in transient modes and in idle mode

1. The air intake system is leaky. Check the connections of the intake system. Start the engine and lubricate the contact points and connections in the intake tract with gasoline. If the engine speed increases briefly, eliminate the leak.

2. Incorrect idle speed adjustment. Check the idle speed control valve, throttle position sensor, lambda adjustment.

3. The full load sensor is faulty or incorrectly adjusted. Check the throttle sensor.

Hot engine won't start

1. Incorrect adjustment of CO content. Check CO content and idle speed.

2. Too high pressure in the fuel system. Check the fuel pressure, replace the pressure regulator if necessary.

3. The return line between the pressure regulator and the fuel tank is clogged or bent. Clean or replace the line.

4. The engine temperature sensor is faulty. Check the temperature sensor.

5. The fuel system is leaking. Visually inspect all connections in the engine and electric fuel pump area. Tighten all connections.

6. The air intake system is leaking. Check all connections of the intake system.

The engine runs after being turned off

1. Injectors are leaky. Check injectors.

2. The fuel shut-off valve on the diesel engine does not work.

Lubrication system

The indicator lamp does not light after the ignition is turned on

1. The oil pressure sensor is faulty. Turn on the ignition, disconnect the wire from the oil pressure sensor and short it to ground. If the lamp lights up, replace the sensor.

2. There is no voltage supplied to the sensor, the contacts are corroded. Check the electrical wires and contacts.

3. The indicator lamp is faulty. Replace the indicator lamp.

4. The control device is faulty. Replace the control device.

The indicator light does not go out after starting the engine

The oil is overheated. If the indicator lamp goes out after gas is supplied, nothing needs to be done.

The indicator light does not go out after gas is applied or lights up while driving

1. Oil pressure is too low. Check the oil level, top up if necessary. Check the oil pressure.

2. The wire to the oil pressure sensor has a short circuit to ground. Disconnect the cable from the sensor and move it to the side (without shorting it to ground), turn on the ignition. If the lamp lights up, check the wire.

3. The oil pressure sensor is faulty. Replace the sensor.

Oil pressure too low at all speeds

1. There is too little oil in the engine. Add oil.

2. The mesh filter is clogged. Remove the oil pan, clean the mesh filter.

3. The oil pump is worn out. Remove the oil pump and check it, replace it if necessary.

4. Bearing damage. Remove the engine.

Too low oil pressure at low speeds

The pressure reducing valve is stuck open due to contamination. Remove and check the valve.

Oil pressure too high at engine speed 2000 rpm

The pressure reducing valve does not open due to contamination. Remove and check the valve.

Cooling system

Overheat

1. Not enough coolant in the system (see Chapter Cooling and heating systems).

2. The water pump drive belt is worn or damaged, or its tension is not adjusted correctly (see Chapters Routine maintenance and Engine).

3. The internal channels of the radiator are blocked or the radiator grille is dirty (blocked) (see Chapter Cooling and heating systems).

4. The thermostat is faulty (see Chapter Cooling and heating systems).

5. Broken or cracked fan blades (see Chapter Cooling and heating systems).

6. The fan switch sensor is faulty.

Attention! When the red-yellow and red-white wires of the sensor are short-circuited, the fan should run at half speed.

7. The coolant temperature gauge is faulty. Check the sensor.

8. The water pump is faulty. Remove and check the pump.

9. The radiator cap does not hold pressure. Test the cap under pressure.

Hypothermia

1. The thermostat is faulty (see Chapter Cooling and heating systems).

2. Temperature meter readings are inaccurate.

External coolant leaks

1. The hoses are damaged or destroyed as a result of aging of the material or their fastening on the fittings has become loose. Replace the hoses and/or tighten the clamps of their fastening (see Chapter Cooling and heating systems).

2. The water pump seals are damaged. In this case, the coolant will leak through the inspection hole in the pump body (see Chapter Cooling and heating systems).

3. There are leaks from the internal radiator passages or side reservoir(s). The radiator needs to be repaired.

4. There are leaks through the engine drain plug (see Chapter Engine) or squeeze plugs of water galleries (see Chapter Cooling and heating systems).

Internal coolant leaks

Caution! Internal coolant leaks are usually detected by checking the engine oil. Inspect the dipstick blade and the inside surfaces of the cylinder head(s) for traces of water and oil foaming.

1. There are leaks through the cylinder head gasket. Perform a pressure test of the cooling system.

2. There are cracks in the cylinder walls or in the head casting. To check, disassemble the engine (see Chapter Engine).

Coolant losses occur

1. Too much coolant has been added to the system (see Chapter Cooling and heating systems).

2. The coolant boils away as a result of engine overheating.

3. There are internal or external coolant leaks.

4. The radiator cap is faulty. Check the cap with pressure.

Coolant circulation is disrupted

1. The water pump is not functioning properly. The easiest way to check that the pump is functioning is to pinch the upper radiator hose while the engine is idling. If you feel a jolt of fluid inside the hose when you release it, the pump is functioning properly (see Chapter Cooling and heating systems).

2. The cooling system is clogged. Drain the coolant, flush the system and fill it with a fresh mixture (see Chapter Cooling and heating systems). If such a need arises, remove the radiator (see Chapter Cooling and heating systems) and perform a reverse flow flush.

3. The water pump drive belt is worn or damaged, or its tension is not adjusted correctly (see Chapters Routine maintenance and Engine).

4. The thermostat is jammed (see Chapter Cooling and heating systems).

Heater

The heater fan does not work

1. The fan motor fuse is faulty. Check the fan fuse and replace if necessary.

2. The fan switch is faulty. Check if voltage is applied to the pre-resistors, remove and check the fan switch.

3. The electric motor is faulty. Check if voltage is applied to the fan motor contacts with the ignition on and the fan switch closed. If voltage is applied, replace the motor.

The heater fan does not work at one of the stages

Pre-resistor is faulty. Replace the connection board.

The heater does not turn off with the regulator

1. The switch is faulty. Check the switch.

2. The control mixing flap cables are damaged or do not move well. Replace the cables.

Heating power too low

1. Coolant level is too low. Check coolant level, top up if necessary.

2. The heater control unit has a heavy stroke, is faulty. Check the heater control unit, replace the cable if necessary.

Noise in the area of the heater fan

1. Dirt, leaves got in. Remove the fan and clean it, clean the air duct.

2. Impeller imbalance, bearing failure. Remove the fan motor, check for ease of movement.

Automatic transmission (AT)

Caution: Electronic transmission control system diagnostic procedures and trouble codes are provided in Chapter Engine electrical equipment)

Due to the complexity of the AT design, it is advisable to carry out diagnostics of its faults and repair of components in a car service workshop or a representative office of Mercedes-Benz.

Common problems associated with the operation of the switching mechanism

1. The following can be attributed to failures associated with a malfunction of the shift rod adjustment:

• a) The engine can be started in transmission positions other than "P" (Park) and "N" (Neutral);
• b) The transmission position indicator readings differ from the actual gear selected;
• c) The vehicle is moving with the transmission in position "P" or "N";
• d) Gears shift with difficulty or erratically.

2. Adjust the gear shift rod (see Chapter Gearshift box).

The transmission slips, shifts with difficulty, makes extraneous noises, or does not allow the vehicle to move when shifted into one of the forward or reverse gears

1. There are many possible causes for the problems listed in the title, but only one of them falls within the competence of the amateur mechanic - the incorrect transmission fluid level.

2. Before driving the car to the auto service workshop, check the level and condition of the transmission fluid (see Chapter Routine maintenance). Adjust the level according to the requirements of the Specifications) or change the fluid together with the filter. If the situation is not corrected, contact specialists for help.

There are transmission fluid leaks

1. AT fluid is dark red. Its leaks should not be confused with engine oil, which can be carried to the transmission housing by the oncoming air flow.

2. To identify and locate the source of the leak, first remove all traces of dirt and grease from the transmission housing. Use a suitable degreaser and/or steam clean. Then drive the vehicle for a short time at low speed (so that traces of a leak are not carried away by the oncoming flow far from its source). Stop, jack up the car and visually inspect to determine the source of the leak. Most often, these are:

• a) Transmission oil pan - tighten mounting bolts and/or replace oil pan gasket;
• b) Fluid filler tube - replace the rubber seal where the tube enters the transmission housing;
• c) Transmission fluid lines - tighten fittings or replace lines;
• d) Vent tube - the transmission is overfilled and/or water has entered it.

The oil is brown in color or smells burnt

Low fluid level in the box (see Chapter Routine maintenance).

Kickdown mode does not turn on when the pedal is fully pressed (the downshift does not engage)

1. Low fluid level in the box (see section Chapter Routine maintenance).

2. Malfunction in the engine management system (see Fuel injection and fuel supply system of a gasoline engine - general information and Diesel Engine Fuel Injection System - General Information Chapters Power and exhaust systems).

3. Sensor or wiring faulty (see Automatic transmission 722.6 Chapters Automatic transmission).

4. The selector cable adjustment is incorrect (see Automatic transmission 722.6 Chapters Automatic transmission).

The engine does not start in any position of the selector or starts in any position except "P" and "R"

1. The adjustment of the start permission sensor is disturbed (see Automatic transmission 722.6 Chapters Automatic transmission).

2. Malfunction in the engine management system (see Fuel injection and fuel supply system of a gasoline engine - general information and Diesel Engine Fuel Injection System - General Information Chapters Power and exhaust systems).

3. The selector cable adjustment is incorrect (see Automatic transmission 722.6 Chapters Automatic transmission).

Transmission slips, jerks or makes noise when shifting gears. Vehicle does not move when shifting into D or R mode

1. Low transmission fluid level (see Chapter Routine maintenance).

2. Sensor or its wiring is faulty (see Automatic transmission 722.6 Chapters Automatic transmission).

3. Malfunction in the engine management system (see Fuel injection and fuel supply system of a gasoline engine - general information and Diesel Engine Fuel Injection System - General Information Chapters Power and exhaust systems).

Bridges

Extraneous noises

1. Normal road noise. Cannot be corrected.

2. Tire noise. Check the condition of the treads and the inflation pressure of the tires.

3. Wheel bearings are worn or damaged, or their tightening force has weakened.

Vibrations

Check the condition of the wheel bearings by jacking up the appropriate corner of the car and rotating the wheel manually. Listen to the sounds coming from the bearing. Remove the bearings and check their condition.

Oil leaks

The drive shaft seals are damaged (see Chapter Clutch).

Brake system

Caution: Before coming to a final conclusion about the presence of problems with the braking system, make sure that the tires are in satisfactory condition and inflated to the required pressure, the front wheel alignment is not disturbed, and the vehicle is not loaded unevenly.

Increased brake pedal travel

The brake circuit is out of order. Check if brake fluid is leaking.

The brake pedal springs and sinks

1. Air in the brake system. Remove air from the system.

2. Not enough fluid in the reservoir. Add new brake fluid, remove air from the system.

3. Formation of steam bubbles. Occurs mainly under heavy load on the brakes. Replace the brake fluid, remove air from the system.

Reduced braking effect, pedal falls through

1. Leaky pipeline. Tighten the pipeline connections or replace the tubes.

2. Damage to the seals in the master or working brake cylinders. Replace the seal. Replace the internal elements in the master brake cylinder, replace the master cylinder if necessary.

3. The rubber gasket is damaged. Repair the caliper.

Poor braking despite high pedal force

1. The brake linings are oily. Replace the brake pads.

2. Incorrect or hardened brake pads installed. Replace the pads. Install genuine Mercedes-Benz brake pads.

3. The brake booster is faulty. Check the brake booster.

4. The brake pads are worn out. Replace the brake pads.

When braking, the car pulls to the side

1. The tire pressure is not correct. Check and adjust the tire pressure.

2. Uneven tire wear. Replace worn tires.

3. The brake pads are oily. Replace the pads.

4. Different brake pads are installed on one axle. Replace the pads. Install genuine Mercedes-Benz pads.

5. Poor condition of brake pads. Replace the pads.

6. The caliper wells are dirty. Clean the guides and seating surfaces of the pads in the caliper.

7. Corrosion of brake caliper cylinders. Replace the caliper.

8. Uneven wear of the brake pad. Replace the brake pads (on both wheels).

Spontaneous braking

1. The compensation hole in the master cylinder is clogged. Clean the master cylinder and replace the internal components.

2. The clearance between the drive rod and the master cylinder piston is too small. Check the clearance.

Brakes get hot while driving

1. The compensation hole in the master cylinder is clogged. Clean the master cylinder and replace the internal components.

2. The clearance between the drive rod and the master cylinder piston is too small. Check the clearance.

3. The throttling hole in the special valve on the bottom is clogged. Clean the main brake cylinder, replace the internal elements and brake fluid.

The brakes are shaking

1. The wrong type of brake pads are installed. Replace the pads, install genuine Mercedes-Benz pads.

2. The brake disc is corroded in places. Carefully polish the brake disc.

3. The brake disc has lateral runout. Treat or replace the disc.

The brake pads do not move away from the brake disc, the wheel is difficult to turn manually

Corrosion in brake caliper cylinders. Repair the caliper, possibly replace it.

Material taken from a web resource [MercedesMan.ru]

Uneven wear of brake pads

1. The wrong type of brake pads are installed. Replace the pads, install genuine Mercedes-Benz pads.

2. The caliper is dirty. Clean the caliper well.

3. Heavy piston stroke. Fix.

4. The brake system is not tight. Check the tightness of the brake system.

Wedge-shaped wear of brake pads

1. The brake disc does not rotate parallel to the caliper. Check the caliper mating surfaces.

2. Corrosion in the calipers. Remove contamination.

3. The piston is not working properly. Check the piston position.

Squeaking brakes

1. Often the reason is atmospheric influence (air humidity). If the creaking appears after a long period of standing in high humidity and then disappears, there is no need to do anything.

2. The wrong type of brake pads are installed. Replace the pads, install genuine Mercedes-Benz pads, coat the base plates with anti-squeak (anti-squeal) paste.

3. The brake disc does not rotate parallel to the caliper. Check the mating surfaces of the brake caliper.

4. The caliper well is dirty. Clean the caliper well.

5. The brake shoe mounting springs are bent. Replace the mounting springs.

6. The pressure springs are stretched. Replace the pressure springs.

Brake pedal pulsation

1. ABS in action. Normal, no assistance required.

2. Too much lateral runout or thickness tolerance of the brake disc. Check runout and thickness. Rework or replace the disc.

3. The brake disc does not rotate parallel to the caliper. Check the caliper mating surface.

ABS warning light comes on while driving

1. The supply voltage is too low (below 10 V). Check the voltage. Check whether the generator indicator lamp goes out after the engine is started. If everything is OK, check the generator drive V-belt.

2. ABS damage. Check for reliable contact and corrosion of the ground connection at the return pump (in the hydraulic block).

Suspension and steering

The car pulls to one side when moving

1. Tires are unevenly inflated.

2. There is a defect in the tires.

3. Excessive wear of suspension or steering components (see Chapter Suspension and steering).

4. It is necessary to adjust the front wheel alignment angles.

5. The front brakes are seized (see Chapter Brake system).

There is jerking, twitching or vibration

1. Wheel balance is broken or rims are out of ovality. Have them balanced at a tire shop.

2. Wheel bearings are worn out, their tightening force has weakened or their adjustment is disturbed (see Chapter Suspension and steering).

3. The shock absorbers or other suspension components are worn or damaged (see Chapter Suspension and steering).

There is excessive rocking of the car/its nose "digging" in when turning or braking

1. The shock absorbers are faulty. Replace the entire set (see Chapter Suspension and steering).

2. Suspension components are damaged. Check the condition of the components (see Chapter Suspension and steering).

The steering wheel is too hard to turn

1. The fluid level in the power steering system reservoir has dropped excessively (see Chapter Suspension and steering).

2. The tires are not inflated correctly.

3. The steering gear joints are not sufficiently lubricated (see Chapter Suspension and steering).

4. The front wheel alignment angles are out of adjustment.

5. The power steering does not develop the required power.

There is excessive play in the steering

1. The tightening force of the front wheel bearings has weakened (see Chapter Suspension and steering).

2. Excessive wear of suspension or steering components (see Chapter Suspension and steering).

The power steering system does not develop the required force

1. The power steering pump drive belt is worn or damaged, or its tension adjustment is incorrect (see Chapter Suspension and steering).

2. The fluid level has dropped excessively (see Chapter Suspension and steering).

3. The hydraulic system hoses or lines are clogged. Check and replace if necessary.

4. Air has entered the hydraulic system. Bleed the system (see Chapter Suspension and steering).

There is excessive wear on the treads (not in one particular area)

1. The tires are not inflated correctly.

2. Wheel balance is disturbed. Contact a tire shop.

3. Wheel disks are damaged. Check and replace if necessary.

4. Excessive wear of suspension or steering components (see Chapter Suspension and steering).

There is excessive wear on the outer edge of the treads

1. The tires are not inflated correctly.

2. Turns are made too sharply. Reduce speed.

3. The front wheel alignment angles are out of adjustment (excessive convergence). Contact a workshop for professional adjustment.

4. The suspension arm is bent or twisted (see Chapter Suspension and steering).

There is excessive wear on the inner edge of the treads

1. The tires are not inflated correctly.

2. The front wheel alignment angles are out of adjustment (divergence). Make adjustments at a car service workshop.

3. Steering components are damaged or loose (see Chapter Suspension and steering).

There is localized wear of the tread

1. Wheel balance is disturbed.

2. The disks are damaged or bent. Check and replace if necessary.

3. There is a defect in the tires.

Windscreen wiper blade defects

Slippage

1. The rubber bands are dirty. Clean the rubber bands with a stiff nylon brush and a detergent solution or alcohol.

2. The edges of the brushes are frayed, the rubber bands are worn or torn. Replace the rubber bands.

3. Old rubber bands, cracked surfaces. Replace the rubber bands.

In the cleaning field, the remaining water immediately collects into drops

The windshield is contaminated with varnish polish or oil. Clean the windshield with a clean cloth and remover of grease, oil, silicones.

The brush cleans one-sidedly - one way is good, the other is not

1. The rubber band is deformed to one side "does not tip over anymore". Install a new rubber band.

2. The wiper arm is twisted, the blade does not fit precisely on the glass. Carefully turn the arm until the vertical position is reached.

Uncleaned surfaces

1. The elastic has come loose from the frame. Carefully insert the elastic into the frame.

2. The brush does not fit evenly to the glass because the spring bars or plates are bent. Replace the brush. This malfunction primarily occurs when the spare brush is installed carelessly.

3. The force of the lever pressing is too low. Lightly lubricate the wiper arm pivots and springs or install a new arm.

Tyre defects

Severe tire wear on both sides of the working surface

Tire pressure too low.

Severe wear of the tire in the middle of the working surface along the entire circumference of the tread

Tire pressure is too high.

Uneven tread wear

Static and dynamic imbalance of the wheel. Possibly too much lateral runout of the disc, too much play in the bearing joints.

Uneven wear in the middle of the tread

Static wheel imbalance. Possible consequence of strong vertical runout.

Severe wear of individual areas in the middle of the tread working surface

Traces of blocking under full braking.

Saw-tooth tread wear, often accompanied by an invisible tear in the tire's fabric base

Overloading the vehicle. Check the inside of the tire for base tears.

Rubber tongues on the side edges of the tread (scaly wear)

Incorrect wheel alignment. Tire is worn. Also check the condition of the shock absorbers.

Formation of burrs on one side of the front wheel tread

Incorrect toe adjustment. Tire is worn. Frequent driving on wavy surfaces. Fast driving on turns.

Cord breaks. At first only visible from the inside

Driving over sharp rocks, tire impacts, etc. at high speed.

One-sided wear of the working surface of the tread

1. Check the camber adjustment.

2. ABS damage. Check the reliability of the connection to the housing at the return pump (in the hydraulic block).