Car fault diagnostics (Mercedes-Benz M-Class W163)

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

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.

Engine

If the engine does not start, the situation should be analyzed. Mandatory conditions for starting any gasoline engine are the proper supply of the air-fuel mixture to the cylinders and adequate spark formation on the spark plugs.

First of all, you should make sure that the fuel supply is working properly (see Chapter Fuel and exhaust systems).

To determine whether a spark is being produced, remove the spark plugs, insert them into the tips and apply them individually to ground. Do not hold the wire or tip with your hands, but use insulated pliers. Ask an assistant to start the engine. If there is no spark, try to determine the cause of the failure (see Chapter Engine electrical systems).

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

The engine does not turn over when trying to start it

1. The battery terminals are corroded or the wire ends are loose.

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 on the terminals.

5. The starter gear is jammed in the toothed ring of the drive disk.

6. The starter traction relay is faulty.



7. Starter is faulty.

8. The ignition switch is faulty.

The engine turns over but does not start

Petrol models

1. The startup is not performed correctly. Proceed as follows:

• 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 key in the ignition switch to the right until it stops. As soon as the engine starts, immediately release the key. If the warm engine does not start when turning it with the starter for more than 4 seconds, you should slowly press the gas pedal. Do not turn the engine more than 30 seconds in a row, wait at least 15 seconds before trying again;
• 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 - information on this issue can be obtained 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 filled with low-quality fuel.

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).



7. The battery terminal connections are oxidized or have become loose.

8. The fuel pump is faulty, or its relay is damaged - check by ear that the pump is activated properly when the ignition is turned on.

9. Ignition system components are damaged or excessively damp.

10. Spark plugs are worn or faulty, or the spark plug gap is set incorrectly.

11. The starting system wiring is broken or disconnected, or the wires are loose at the terminals.

12. The ignition coil wiring is broken or disconnected, or the wires on the coil terminals are loose.

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

14. Insufficient compression pressure.

Diesel models

1. The startup is not performed correctly. Proceed as follows:

• 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 Controls and safe operating techniques). 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. At an outside air temperature below -10°C, you should also press the gas pedal all the way. Continue turning the engine with the starter until its speed stabilizes, but not more than 30 seconds. Before restarting after an unsuccessful attempt, do not forget 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 Controls and safe operating techniques): the pressure indicator should be activated immediately after starting. If the engine is very cold, the pressure indicator will only appear 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, assess the condition of 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:

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

8. The adjustment of the moment of start of fuel supply is out of adjustment.

9. The injectors are faulty - when you loosen the union nut of the fuel line of the faulty injector, the engine speed will not drop.

10. The high-pressure fuel pump (HPFP) is faulty.

11. Compression pressure too low.

The starter operates without turning the engine

1. The starter gear is jammed.

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.

3. Paraffin has settled in the fine filter of the diesel engine.

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

Difficulty starting a hot engine

1. The air filter is clogged.

2. There is a malfunction of the power supply system components or electrical equipment.

3. Fuel does not reach the injectors of the injection system.

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

5. Insufficient compression pressure in the cylinders, the diesel engine fuel injection pump is faulty, or its adjustment is incorrect.

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 the 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.

4. The basic settings of the engine control module (ECM) are violated.

5. There is damage in the exhaust system/catalytic converter.

6. There are losses of vacuum through the gasket surfaces of the intake manifold or fuel injection unit.

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 power supply system components or electrical equipment.

3. The crankcase ventilation system valve is clogged.

4. The air filter is clogged.

5. The fuel pump does not supply sufficient fuel to the injectors of the injection system.

6. There are leaks through the cylinder head gasket - measure the compression pressure in the engine cylinders (see Chapter Engine).

7. The working projections of the camshaft cams are worn out.

Diesel models

1. The quality of the connections of the fuel hoses on the high-pressure fuel pump and fuel filter is damaged.

2. The fuel injection pump mount is mechanically damaged or loose.

3. The connections of the return line and the fuel supply line are reversed.

4. There is damage in the fuel supply line:

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

5. The fuel injection start adjustment is out of adjustment.

6. The injectors are faulty - when you loosen the union nut of the fuel line of the faulty injector, 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.

2. There is a malfunction of the power supply system components or electrical equipment.



3. The high voltage wiring is faulty.

4. Poor quality fuel has been added or 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.

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.

2. The spark plugs are faulty or dirty, or the spark plug gap is set incorrectly.

3. There is a malfunction of the power supply system components or electrical equipment.

4. There is a defect in the high-voltage electrical wiring.

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

6. Insufficient or uneven distribution of compression pressure between cylinders.

7. The ignition system is faulty.

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 clogged, or moisture or dirt has entered the fuel system.

3. There is a failure of components/information sensors of the power supply system.

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 Fuel and exhaust systems). Also check the condition of the high-voltage wiring.

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.

2. The air cleaner is clogged.

3. The spark plugs are faulty or the spark plug gap is set incorrectly.

4. The ignition coil is faulty.

5. The AT fluid level has dropped (see Chapter Automatic transmission).

6. The transmission slips.

7. The fuel filter is blocked and/or dirt/moisture has entered the fuel system.

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 (with appropriate equipment).

10. The camshaft phase shift activator is faulty.

11. Insufficient compression pressure, or its uniform distribution between the cylinders is disrupted.

Additional for diesel engine

1. The air cleaner is clogged.

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

3. The moment of start of fuel supply by the high-pressure fuel pump is set incorrectly.

4. The fuel filter or high-pressure fuel pump (injector) is clogged.

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.

2. There is a defect in the secondary circuit of the ignition system (destruction of spark plug insulators or damage to high-voltage electrical wiring).

3. The fuel injection system needs adjustment, or its components are excessively worn.

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

5. The valves are stuck.

6. The order of connecting the high-voltage electrical wiring is violated.

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.

3. The wrong type of spark plugs are installed.

4. The basic ECM settings 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. Idle speed is too high.

2. There is a malfunction of the power supply system components or electrical equipment.

3. The fuel vapor capture system (EVAP) purge valve has failed to function properly.

4. Engine operating temperature is excessive. Possible causes of this problem may be a decrease in coolant level, a thermostat failure, a blocked radiator, or a faulty water pump.

Excessive smoke from diesel engine exhaust

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

Note: Electronic control system diagnostic procedures and trouble codes are provided in Chapter Engine electrical systems.

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 systems).

2. The electrolyte level is insufficient or the battery is very discharged.

3. There is corrosion of the battery terminals or the fastening of the wire tips to them has become loose.

4. The generator does not provide the required charging current.

5. The charging circuit wiring is broken or otherwise damaged, or the wires are loose at the terminals.

6. A short circuit in the electrical wiring creates a constant leakage of current generated by the battery to ground.

7. There is an internal defect in the battery.

The charge indicator lamp does not go out when the revs increase

1. The drive belt is loose.

2. The torque converter overrunning clutch is damaged (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. The voltage regulator/generator is damaged. Disconnect the wire (D+) from the back of the generator and turn on the ignition. Next, check the condition of the generator voltage regulator.

4. The brushes are worn out.

5. The wire between the generator and the voltage regulator is damaged.

6. The instrument cluster is faulty.

The charge indicator lamp does not light when the ignition is turned on

1. The indicator lamp on the instrument panel has burned out or otherwise failed.

2. The generator is faulty.

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

The charge indicator lamp does not go out after the ignition is turned off

1. The diodes are broken.

Starter

If the starter does not turn, first check that the required voltage is present at terminal No.50 of the traction relay (minimum 10 V). If the test results are negative, the condition of the starter current circuit wiring should be assessed.

To check the starter's operation at full battery voltage, follow these steps:

a) Without engaging gears, turn on the ignition;

b) Short-circuit terminals 30 and 50 of the starter with a wire with a cross-section of at least 4 mm².

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! The electrical wiring terminals must be securely fastened and not oxidized!

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

1. The battery is low.

2. Short-circuit terminals 30 and 50 of the starter: if the starter rotates, check wire 50 to the ignition switch for a break, also assess the condition of the starter switch.

3. The ground wire is broken, or the quality of its terminal connection is poor, the battery is discharged.

4. Weakening of current strength due to poor quality or oxidation of contact connections.

5. There is no voltage at terminal 50 of the traction relay as a result of a break in the electrical wiring or damage to the starter switch.

The starter rotates slowly and does not turn the crankshaft

1. The battery is low.

2. Summer oil is poured in.

3. Insufficient current flow due to loose or oxidized connectors.

4. Carbon brushes do not fit tightly against the collector, are stuck in the guides, are worn out, broken, oily or dirty.

5. Insufficient distance between the brushes and the commutator.

6. The collector is grooved or burnt and oily.

7. No voltage at terminal 50 (minimum 8 V).

8. The bearing is broken.

9. The traction relay is faulty.

The starter catches and turns the engine jerkily

1. The gear drive is faulty.

2. The gear is dirty.

3. The flywheel ring gear is faulty.

The starter pinion does not move away from the ring gear

1. The gear drive components are dirty or damaged.

2. The traction relay is faulty.

3. The return spring is weak or broken

The starter continues to operate after the ignition key is released

1. The traction relay is jammed - immediately turn off the ignition and replace the traction relay.

2. The ignition switch does not turn off - disconnect the battery, replace the ignition switch.

Power supply system

Excessive fuel consumption

Petrol models

1. The air filter element is dirty or blocked.

2. There is a malfunction of the power supply system components, electrical equipment or electronic control.

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.

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.

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.

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 the correctness of the voltage supply to the pump (check the serviceability of the protective fuse and the reliability of the fastening of the contact terminals of the corresponding electrical wiring).

2. The fuel pump relay is faulty.

3. The injector valves are stuck. Check the injectors, replace if necessary. Check the voltage supply to the injectors - disconnect the injector connector, connect a diode test lamp to the wire and turn on the starter: the lamp should start blinking.

4. There is no signal from the Hall ignition sensor or coolant temperature sensor. Check the condition of the corresponding electrical wiring, query the memory of the on-board self-diagnostic system.

5. Fuel lines are damaged, clogged or leaky, hoses are defective.

6. The fuel filter is clogged.

7. As a result of a leak, excess air is sucked into the intake tract.

8. Vacuum lines are damaged or leaky

9. The pressure regulator is damaged - check the residual pressure.

10. The throttle position sensor is damaged.

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

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

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

14. Preheating does not work.

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

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

17. The injectors are faulty - when you loosen the union nut of the fuel line of the faulty injector, the engine speed will not drop.

18. The fuel injection pump is faulty.

Cold engine starts poorly, runs unsteadily

1. The CO content does not meet regulatory requirements - take the appropriate measurement, check the idle speed.

2. The coolant temperature (ECT) or intake air temperature (IAT) sensor is faulty.

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

4. As a result of a leak, excess air is sucked into the intake tract.

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.

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 - disconnect the injector connector, connect a diode test lamp to the wire and turn on the starter: the lamp should start blinking.

8. There is no signal from the Hall ignition sensor or coolant temperature sensor. Check the condition of the corresponding electrical wiring, conduct a survey of the on-board self-diagnostic system.

9. As a result of a leak, excess air is sucked into the intake tract.

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.

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 fuel quality, formation of vapor locks.

3. Fuel supply volume is too low.

4. The fuel filter is faulty.

5. The fuel pump is faulty.

6. The injectors are faulty.

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.

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

10. Malfunction of the fuel vapor recovery system (EVAP).

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 - disconnect the injector connector, connect a diode test lamp to the wire and turn on the starter: the lamp should start blinking.

14. No signal from the Hall Effect Ignition Sensor or Coolant Temperature (ECT) Sensor. Check wiring, interrogate ECM memory for fault codes.

15. As a result of a leak, excess air is sucked into the intake tract.

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 cause of 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 position sensor (TPS).

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.

5. The tightness of the power supply system is broken.

6. The air intake tract is not sealed.

The engine runs after being turned off

1. The injectors are not sealed.

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 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 condition of the corresponding electrical wiring.

3. The control lamp is faulty.

4. The instrument cluster is faulty.

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. The oil level has dropped.

2. There is a short circuit in the oil level sensor wiring.

3. The sensor is faulty.

Oil pressure too low at all speeds

1. The oil level has dropped.

2. The mesh filter of the oil intake in the oil pan is clogged.

3. The oil pump is worn out.

4. The crankshaft bearings are damaged

Too low oil pressure at low speeds

The pressure reducing valve is stuck open due to contamination.

Oil pressure too high at engine speed 2000 rpm

The pressure reducing valve does not open due to contamination.

Cooling system

Overheat

1. The coolant level has dropped.

2. The water pump drive belt is worn or damaged, or its tension adjustment is incorrect.

3. The internal channels of the radiator are blocked or the radiator grille is dirty (blocked).

4. Thermostat is faulty.

5. The fan impeller blades are broken or cracked.

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

7. The coolant temperature gauge is faulty.

8. The water pump is faulty.

9. The radiator cap does not hold pressure - check the cap under pressure.

Hypothermia

1. The thermostat is faulty.

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.

2. The water pump seals are damaged - coolant will leak through the inspection hole in the pump body.

3. There are leaks from the internal radiator passages or side reservoir(s).

4. There are leaks through the engine drain plug or the release plugs of the water galleries.

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 walls or in the head casting.

Coolant losses occur

1. There is too much coolant in the system.

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.

2. The cooling system is clogged. Drain the coolant, flush the system and fill it with fresh mixture. If necessary, remove the radiator and perform a reverse-flow flush.

3. The water pump drive belt is worn or damaged, or its tension is not adjusted correctly.

4. Thermostat is stuck.

Heater

The heater fan does not work

1. The fan motor fuse is faulty.

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

The pre-resistor is faulty.

The heater does not turn off the regulator

1. The switch is faulty.

2. The control mixing valve cables are damaged or do not move well.

Heating power too low

1. The coolant level is insufficient.

2. The heater control handles are difficult to operate, faulty - check the control assembly, replace the corresponding drive cable if necessary.

Noise in the area of the heater fan

1. Dirt or leaves got in - remove the fan and clean it, clean the air duct.

2. The impeller is out of balance and the bearing is damaged.

Automatic transmission (AT)

Note: Electronic transmission control system diagnostic procedures and trouble codes are provided in Chapter Engine electrical systems.

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:

• The engine can be started in transmission positions other than "P" (Park) and "N" (Neutral);
• The transmission position indicator reading differs from the actual gear selected;
• The vehicle is moving with the transmission in position "P" or "N";
• Gears shift with difficulty or randomly.

2. Adjust the gear shift rod.

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. Adjust the level in accordance with the requirements of the Specifications or change the fluid together with the filter. If the situation is not corrected, seek help from specialists.

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

Transmission fluid level is low.

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

1. The ATF level has dropped.

2. The engine management system is faulty.

3. The sensor switch or its wiring is faulty.

4. The selector drive cable is out of adjustment.

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 switch sensor is disturbed.

2. The engine control system is not functioning properly.

3. The selector cable adjustment is incorrect.

Transmission slips, jerks or makes noise when shifting gears. Car does not move when in "D" or "R" mode

1. The ATF level has dropped.

2. The sensor or its wiring is faulty.

3. The engine control system is not functioning properly.

Bridges

Extraneous noises

1. Normal road noise. Cannot be corrected.

2. Tire noise - check the condition of the treads and tire inflation pressure.

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.

Brake system

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

Increased brake pedal travel

The working circuit of the brake tract is damaged - check the system for leaks.

The brake pedal springs and sinks

1. Air has entered the brake system - bleed the system.

2. The fluid level in the master cylinder reservoir has dropped - make the appropriate adjustments, bleed 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. The hydraulic circuit is not sealed.

2. The seals in the main or working brake cylinders are damaged.

Poor braking despite high pedal force

1. Brake linings are oily.

2. Incorrect or hardened pads are installed.

3. The brake booster is faulty.

4. The brake pads are worn out.

When braking, the car pulls to the side

1. The tire pressure is not correct.

2. The treads are worn unevenly.

3. Brake linings are oily.

4. Different brake pads are installed on the same axle.

5. Brake pads are excessively worn.

6. The caliper shafts are dirty.

7. The calipers are damaged by corrosion - replace them.

8. Brake pads are worn unevenly.

Spontaneous braking

1. The compensation hole in the master brake cylinder is clogged.

2. The clearance between the drive rod and the master cylinder piston is insufficient.

Brakes get hot while driving

1. The compensation hole in the master brake cylinder is clogged.

2. The clearance between the drive rod and the master cylinder piston is insufficient.

The brakes are shaking

1. The wrong type of brake pads are installed.

2. The brake disc is corroded in places.

3. The brake disc runout is excessive.

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.

Uneven wear of brake pads

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

2. The calipers are damaged by corrosion - replace them.

3. The piston stroke is difficult.

4. The tightness of the brake system tract is broken.

Wedge-shaped wear of brake pads

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

2. The calipers are damaged by corrosion - replace them.

3. The piston is not functioning properly.

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.

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

4. The caliper shaft is dirty.

5. The brake shoe mounting springs are bent.

6. The pressure springs are stretched.

Brake pedal pulsation

1. Sign of normal ABS operation.

2. The brake disc runout is excessive.

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

ABS warning light comes on while driving

1. The supply voltage is insufficient (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. Has a faulty ABS - check the condition and reliability of the clamp connection of the ground of the return pump (in the hydromodulator).

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.

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

5. The front brakes are seized.

There is jerking, twitching or vibration

1. Wheel balance is disturbed or rims are out of ovality.

2. Wheel bearings are worn out, their tightening force has weakened, or their adjustment is out of order.

3. Shock absorbers or other suspension components are worn or damaged.

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

1. The shock absorbers are faulty.

2. Suspension components are damaged.

The steering wheel is too hard to turn

1. The fluid level in the power steering system reservoir has dropped excessively.

2. The tires are not inflated correctly.

3. The steering linkage joints are not sufficiently lubricated.

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.

2. Excessively worn suspension or steering components.

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.

2. The liquid level has dropped excessively.

3. The patency of the hoses or lines of the hydraulic system is impaired.

4. Air has entered the hydraulic system - bleed the system.

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

1. The tires are not inflated correctly.

2. Wheel balance is disturbed.

3. The wheel rims are damaged.

4. Excessive wear of suspension or steering components.

There is excessive wear on the outer edge of the treads

1. The tires are not inflated correctly.

2. Turns are made too sharply.

3. The front wheel alignment angles are out of adjustment (excessive convergence).

4. The suspension arm is bent or twisted.

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).

3. Steering components are damaged or loose.

There is localized wear of the tread

1. Wheel balance is disturbed.

2. The discs are damaged or bent.

3. There is a defect in the tires.

Windscreen wiper blade defects

Slippage

1. The rubber working elements are dirty.

2. The edges of the brushes are frayed, the rubber working elements are worn out or torn.

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

The windshield is contaminated with varnish or oil.

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

1. The rubber working element has one-sided wear.

2. The windshield wiper arm is twisted, the blade does not fit precisely to the glass.

Uncleaned surfaces

1. The reliability of the working element fixation in the brush frame is compromised.

2. The brush does not adhere evenly to the glass.

3. The lever pressing force is too low - lightly lubricate the wiper arm hinges 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 due to excessive lateral runout of the disc, too much play in the bearing joints.

Uneven wear in the middle of the tread

Static wheel imbalance, possibly due to excessive vertical runout.

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

The result of sudden braking.

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

Vehicle overload. Check the condition of the inner walls of the tires.

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

1. Wheel alignment is out of adjustment.

2. The tires are worn out.

3. Shock absorbers/torsion springs/strut assemblies are faulty.

Formation of burrs on one side of the front wheel tread

1. Wheel alignment is out of adjustment.

2. The tires are worn out.

3. Frequent movement on wavy surfaces. Fast movement 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. Has a faulty ABS - check the condition and reliability of the clamp connection of the ground of the return pump (in the hydromodulator).