Car fault diagnostics (Mercedes-Benz C-Class W203)

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. In addition, the text provides references to Chapters and Sections related to these problems.

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 quality, 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.

Engine

If the engine does not start, the situation should be analyzed. For a gasoline engine to start at all, two basic conditions must be met. The fuel-air mixture must enter the cylinders, and a spark must be generated at the spark plug. Therefore, the first thing to check is whether fuel is being supplied. Refer to Chapter Power and exhaust systems.

To determine whether a spark is produced, remove the spark plugs, insert them into the tips and apply them individually to ground. Do not hold the tip or ignition cable with your hands, but use well-insulated pliers. Ask an assistant to start the engine. If there is no spark, refer to Chapter Engine electrical equipment.

Diagnostic procedures for the electronic control system and fault codes are provided in Chapter Engine electrical equipment.

Starting the engine

Please also refer to Section Vehicle control and auxiliary systems.

All engines

Manual transmission: Apply the parking brake, fully depress the clutch pedal and hold it. Set the transmission to neutral. The disengaged clutch makes it easier to start the engine, especially at low outside temperatures, since there is no friction from the transmission.

Automatic transmission: Place the selector lever in position "P" or "N". Press and hold the brake pedal.

Caution! Do not turn on the starter for more than 30 seconds continuously, otherwise the starter and cables will overheat.

Gasoline engine

Turn the ignition key and engage the starter without applying the gas. As soon as the engine starts, release the key. If the engine does not start within 5 seconds, press the gas pedal slightly and try to start it again. If the engine does not start or stops immediately, try to start it again with the gas pedal half depressed, and if necessary, start it again with the gas pedal fully depressed.



When the engine is hot, press the gas pedal slowly during starting.

Be sure to start moving immediately, only in severe frost let the engine warm up for 30 seconds.

Warning! Successive unsuccessful attempts to start the engine can damage the catalytic converter, as unburned gasoline gets into it and explodes when heated.

Diesel engine

On a cold engine: Turn on the ignition and wait until the pre-glow indicator lamp goes out. Immediately after the indicator lamp goes out, start the engine without giving gas. If irregular flashes occur during starting, keep the starter on until (but not more than 20 sec), until the engine starts running on its own. If the engine still does not start, turn the ignition key to position "0" and wait about 30 seconds. After that, preheat again and repeat the start as described above.

Attention! While preheating is in progress, do not turn on powerful electrical consumers, otherwise the battery will be unnecessarily overloaded.

Note: A direct injection diesel engine generally requires preheating only at temperatures below 0°C.

Preheating is not required on a warm engine. Start the engine immediately without giving it gas.

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 at the first opportunity and check the condition of the pinion and toothed rim.

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

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

8. The ignition switch is faulty.

The engine turns over but does not start

1. The startup is not performed correctly (Refer to the section Starting the engine using an auxiliary battery).

2. The engine immobilizer is faulty or not disabled.

3. The fuse is faulty:

• Electric fuel pump;
• Electronic injection system.

For checking fuses, refer to Chapter Body.

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 system (Section Replacing the engine air filter element).

6. The battery is discharged (the engine turns over too slowly). Check the operation of the electrical equipment (refer to Section Automotive chemicals, oils and lubricants).

7. The battery terminal connections are corroded or loose (refer to Section Automotive chemicals, oils and lubricants).



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

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

10. Worn or faulty spark plugs or incorrectly gapped spark plugs (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 (refer to Section Automotive chemicals, oils and lubricants).

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

13. The engine control unit fuse is damaged, one of the sensors is faulty: Hall, pulse sensor, coolant temperature sensor or intake air temperature sensor (Chapter Engine electrical equipment).

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

Diesel engine:

1. The launch was performed incorrectly.

2. The engine immobilizer is faulty.

3. The fuse is faulty:

• Electric fuel pump;
• Preheating systems.

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 lines or hoses are mechanically damaged or clogged. Clean the fuel lines 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 lines for ice or wax. Park the car in a heated garage, add gasoline to the system.
• d) The tank ventilation or fuel intake mesh filter is clogged. Clean it.

8. Fuel injection start adjustment is out of adjustment. Check and 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 (Chapter Engine electrical equipment) and check its condition.

2. The teeth of the starter gear 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 power supply system components or electrical equipment (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 low-octane gasoline.

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

Difficulty starting a hot engine

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

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

3. Fuel does not reach the injectors of the injection system (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 teeth of the starter gear 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 power supply system components or electrical equipment (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

1. There are leaks in the intake air tract. Make sure that the mounting bolts/nuts are tight, check the fact and quality of fastening on their nipples 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 power supply system components or electrical equipment (Chapters Power and exhaust systems and Engine electrical equipment).

• The injector activation (Y62) and injection duration control are faulty.
• There are leaks through the injectors/the injection pattern is damaged.
• Pre-catalytic lambda probe 1 is faulty (G3/3 or G3/4).
• The ignition voltage level is insufficient.

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

4. The EGR valve is stuck or leaking.

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

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

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

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

Diesel engine:

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) The pipes or hoses are mechanically damaged or clogged. Clean the fuel pipes.
• b) The fuel filter is clogged. Replace the filter element.
• c) In winter, check the filter and lines for ice or wax. Park the car in 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 when the engine is idling

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

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

Insufficient ignition voltage level; the MAF air mass meter (B2/5) is faulty.

3. The spark plug wires are faulty (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. Perform a compression measurement (Chapter Engine).

7. There are malfunctions in the engine control system.

There are misfires when the car is moving in gear

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

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

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

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

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

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

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

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

The engine stalls

1. Idle speed adjustment is incorrect.

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

3. There is a failure of the power supply system components or sensors (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 (Chapter Routine maintenance). Also check the condition of the spark plug wires (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 power supply system components or electrical equipment (Chapters Power and exhaust systems and Engine electrical equipment).

• Pre-catalytic lambda probe 1 is faulty (G3/3 or G3/4).
• The coolant temperature (ECT) sensor is faulty (B11/4).
• The mass air flow (MAF) sensor is faulty (B2/5).
• The functioning of the resonant control system of pressure pulsations in the intake manifold is disrupted; the functioning of the detonation control system is impaired.

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

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

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

5. The AT fluid level does not meet the requirements (Chapter Routine maintenance).

6. Clutch slips (Chapter Clutch and drive shafts).

7. The fuel filter is blocked and/or dirt/moisture has entered the fuel system (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 phase shift activator is faulty.

11. The three-way catalytic converter (TWC) is not working properly.

12. Insufficient or uneven compression pressure across the cylinders. Perform a compression test, the results of which can reveal the presence of valve leaks and/or a leaky cylinder head gasket (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 power supply system components or electrical equipment (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) (Chapter Engine electrical equipment).

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

4. There are vacuum losses at the throttle body, intake manifold or through vacuum hoses (refer to Section 8).

5. Valves are stuck (Chapter Engine).

6. The order of connecting the spark plug wires is violated (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 power supply system components or electrical equipment (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 (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. Excessively high idle speed (Chapter Engine).

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

3. The EVAP canister purge valve is damaged.

4. Excessive engine operating temperature. Possible causes of this malfunction may be a decrease in the coolant level (Chapter Cooling and heating systems), thermostat failure, radiator blockage, or water pump failure (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 used. Flush tank and change 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) No fuel is 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 atomizer. Replace the atomizer.

Engine electrical equipment

Diagnostic procedures for the electronic control system and fault codes are provided in Chapter Engine electrical equipment.

The battery does not hold a charge

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

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

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

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

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

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

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. 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 and 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 is turned on

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

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

3. There is a defect in the printed circuit board, wiring inside the instrument panel unit, or lamp socket (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

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.

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

• Without shifting into gear, turn on the ignition.
• Short-circuit terminals 30 and 50 of the starter with a wire of at least 4 mm² cross-section, also refer to the materials of Chapter Engine electrical equipment.

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 in the removed state.

Test condition: At the connection points, the wires must be well secured and must not be oxidized.

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

1. The battery is discharged. Charge the battery.

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

3. Ground cable or connector is broken, 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 collector, 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 collector. 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 in jerks

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.

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The starter gear 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.

Electronic control system

Cold engine does not enter warm-up mode

1. The coolant temperature sensor ECT (B11/4) is faulty.

2. The MAF air mass meter (B2/5) is faulty.

3. There are leaks in the air intake tract.

A warmed-up engine does not exit warm-up mode, or fuel consumption is increased

1. Pre-catalytic lambda probe 1 is faulty (G3/3 or G3/4).

2. The purge control valve of the adsorber (Y58/1) is stuck in the open position.

The engine jerks

1. The MAF air mass meter (B2/5) is faulty.

2. The ignition voltage level is insufficient; pre-catalytic lambda probe 1 is faulty (G3/3 or G3/4).

The electronic acceleration control system (EA) has entered emergency mode

1. The nominal value potentiometer in the pedal position sensor (B37) is faulty.

2. The potentiometer of the actuator of the electronic acceleration/cruise control/idle speed stabilization system (EA/CC/ISC) is faulty.

Power supply system

Excessive fuel consumption

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

2. There is a malfunction of the components of the power supply system, electrical equipment or electronic control (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 (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 and exhaust gas toxicity reduction system lines (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. Power supply from ECM is interrupted (N3/10); the driver authorization system (DAS) is faulty.

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. Insufficient ignition voltage. No signal from the Hall ignition sensor or coolant temperature sensor. Check the electrical wiring. Perform a fault memory scan.

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. The diesel engine preheater does not work or is not fully preheated.

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.

20. Compression is impaired due to excessive increase in oil pressure.

Cold engine starts poorly, runs unsteadily

1. There are leaks in the air intake tract.

2. The ignition voltage level is insufficient.

3. The coolant temperature sensor ECT (B11/4) is faulty. Check the sensor.

4. The MAF air mass meter (B2/5) is faulty.

5. The injector activation (Y62) and injection duration control are faulty.

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

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

A warm engine starts poorly and runs unsteadily

1. There are leaks in the air intake tract. 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 speed increases briefly, eliminate the leaks. Note: Gasoline vapors are poisonous, do not inhale them.

2. The injector activation (Y62) and injection duration control are faulty.

3. The fuel pump check valve is damaged.

4. Leaks in the fuel system.

5. Increased fuel pressure in the fuel system.

6. Malfunction of the fuel vapor recovery system.

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

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. Throttle position sensor is faulty. Check the throttle position sensor.

9. The exhaust manifold or exhaust pipe is damaged (there are exhaust leaks).

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

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

The engine runs intermittently in transient and idle modes

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 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 speeds above 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 (Chapter Cooling and heating systems).

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

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

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

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

6. The fan switch sensor is faulty. Check: when the red-yellow and red-white sensor wires are shorted, the fan should run at half speed.

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

8. The coolant 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 (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 (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 (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 (Chapter Engine) or squeeze plugs of water galleries (Chapter Cooling and heating systems).

Internal coolant leaks

Note: Internal coolant leaks are usually detected by checking the engine oil condition. 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 mirror or in the head casting. To check, disassemble the engine (Chapter Engine).

Coolant losses occur

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

2. The cooling system is clogged. Drain the coolant, flush the system and fill it with fresh mixture (Chapter Cooling and heating systems). If such a need arises, remove the radiator (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 (Chapters Routine maintenance and Engine).

4. Thermostat is stuck (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 when the ignition is on and the fan switch is 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)

Note: 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 reading differs 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.

Adjust the gear shift rod (Chapter Automatic transmission).

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 (Chapter Routine maintenance). Adjust the level according to the requirements of the Specifications or change the fluid together with the filter. If the situation does not improve, 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 the 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 the mounting bolts and/or replace the oil pan gasket.
• b) Fluid filler tube: Replace the rubber seal where the tube enters the transmission case.
• c) Transmission fluid lines: Tighten fittings or replace lines.
• d) Vent Tube: Transmission is overfilled and/or water has entered the transmission.

The oil is brown in color or has a burning smell

Low fluid level in the box (refer to 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 (refer to Section Checking the Automatic Transmission Oil Level).

2. Malfunction in the engine management system (refer to Chapter Engine electrical equipment).

3. Sensor or its wiring is faulty (refer to Chapter Automatic transmission).

4. The selector cable adjustment is incorrect (refer to Chapter Automatic transmission).

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

1. The start enable sensor adjustment is incorrect (refer to Chapter Automatic transmission).

2. Malfunction in the engine management system (refer to Chapter Engine electrical equipment).

3. The selector cable adjustment is incorrect (refer to Chapter 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 (refer to Section Checking the Automatic Transmission Oil Level).

2. Sensor or its wiring is faulty (refer to Chapter Automatic transmission).

3. Malfunction in the engine management system (refer to Chapter Engine electrical equipment).

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 one by one 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 (Chapter Clutch and drive shafts).

Brake system

Note: Before you come to a final conclusion about the presence of problems with the brake system, make sure that the tires are in satisfactory condition and inflated to the required pressure, the front wheel alignment is not out of alignment and the vehicle is not unevenly loaded.

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. The brake pads are incorrect or hardened. Replace the pads. Refer to the section Purchase of spare parts.

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 the same axle. Replace the pads. Refer to the section Purchase of spare parts.

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. Refer to the section Purchase of spare parts.

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 by hand

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

Uneven wear of brake pads

1. The wrong type of brake pads are installed. Replace the pads. Refer to the section Purchase of spare parts.

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

3. Heavy piston stroke. Fix it.

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, coat the base plates with anti-squeak paste. Refer to the section Purchase of spare parts.

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 the reliability of the contact and the presence of 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. Excessively worn suspension or steering components (Chapter Suspension and steering).

4. The front wheel alignment angles need to be adjusted.

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

There is jerking, twitching or vibration

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

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

3. Worn or damaged shock absorbers or other suspension components (Chapter Suspension and steering).

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

1. The shock absorbers of the suspension struts are faulty. Replace the struts entirely (Chapter Suspension and steering).

2. Suspension components are damaged. Check the condition of the components (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 (Chapter Routine maintenance).

2. The tires are not inflated correctly.

3. The steering linkage joints are not sufficiently lubricated (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 front wheel bearings have become loose (Chapter Suspension and steering).

2. Excessively worn suspension or steering components (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 (Chapter Routine maintenance).

2. The liquid level has dropped excessively (Chapter Routine maintenance).

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

4. Air has entered the hydraulic system. Bleed the system (Chapter Routine maintenance).

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

1. The tires are not inflated correctly.

2. The wheels are out of balance. Contact a tire shop.

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

4. Excessively worn suspension or steering components (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 deformed (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 (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 exactly 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, as 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, around the entire circumference

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 in certain areas in the middle of the working surface

Traces of blocking during sudden braking.

Saw-tooth tread wear. Often with a tear in the fabric base that is not visible from the outside

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

Rubber tongues on the side edges of the tread

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 worn working surface

1. Check the camber adjustment.

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