Mercedes-Benz W140 (S-class 1991–1998, sedan and coupe): common faults, maintenance guides and repair tips.
In an early-1990s Mercedes, the wire insulation is biodegradable and cannot withstand heat. Near the engine the insulation turns brittle and crumbles,...
The most common cause is the throttle or accelerator pedal position sensor not giving the control unit the correct data. The brake light switch, fuses...
The most common cause is a wheel speed sensor or its wiring. ABS and ETS/ASD use the same sensor data, so one fault can turn on both lights and also d...
The base module is the control unit of the W140's electrical system. It is in the engine bay on the passenger side at the base of the windshield, unde...
The typical cause is the regulator on top of the blower motor burning out. The blower then works only at low speed, not steplessly, or not at all. The...
A common cause is the blower motor wire connector, which has overheated and loosened. Check the connector first and make sure it sits tightly. Other c...
The locking, trunk lid and suction doors work on vacuum, which is produced by an electric pump under the rear seat. If all of these stop working at th...
Usually the door lock mechanism is stuck. A small spring-loaded link or the handle lever can seize because of dried-out grease, rust or ice. A piece o...
The distributor cap and rotor can oxidize or crack, leaving a burnt trace between the terminals. The result is misfiring, an uneven idle or cylinders ...
The engine runs unevenly or the revs fluctuate. The car can also run rich or go into emergency mode, in which the engine control unit uses fixed value...
Common causes are a faulty idle control valve, small vacuum hoses that have come off near the throttle body, and the lambda sensor. The symptom often ...
A typical place for an air leak is the O-rings of the transparent tubes in the fuel lines. When the rings were replaced, the air bubbles and the idle ...
The turbo wastegate works on vacuum. If the vacuum hose has come off or the turbo otherwise does not get vacuum, boost pressure does not rise, the rev...
Yes, because the rubber wears fastest and instead of a new blade you can replace just the rubber. The old frame can be kept. The rubber may have to be...
The cause can be an old relay, whose contacts should be cleaned, or the wiper motor's park switch. The relay is in the engine bay on top of the fuse b...
The cause is usually an electrical fault: a ground fault, or a break in the cluster circuit board or connection that interrupts the power supply. The ...
The lights are T5 wedge-base bulbs with an original rating of 1.2 W. The cluster also has a T10 bulb that only becomes visible once the cluster is tak...
The most common causes are too much fluid, vacuum hose leaks and a worn transmission. Start with the fluid level and the vacuum lines, because they ar...
The shift points are affected by the adjustment of the throttle cable and the kickdown cable. In the engine bay next to the throttle body are two poin...
Yes. Driveshaft vibration typically appears at about 80–100 km/h. A worn center bearing can cause it, and replacing the bearing has removed the vibrat...
The cause is usually wheel runout, a bent rim, worn suspension joints or a wheel bearing. Even a vertical runout of a few millimetres is felt in the s...
The whine is usually caused by the rear axle bearings. Bearing play affects the backlash between the gears, and later the bearing starts to growl. If ...
A typical cause is a dried-out lower ball joint. The noise has stopped after grease or oil was applied under the joint's rubber boot. If the joint is ...
A momentary rise is normal if the temperature drops in under 5 minutes. If the gauge stays constantly above 90 °C, for example in city driving, there ...
A warm engine that has been driven over 10 km runs at about 80 degrees or more. If the temperature stays clearly below 70 degrees even in winter, the ...
About 1 bar at hot idle is a normal reading. As revs rise, the pressure must rise. If the reading drops to zero when you accelerate, the sensor is a c...
If the system pressure is zero, the refrigerant has leaked out. The leak is looked for with UV dye, which can be seen in the engine bay and under the ...
There is no official jump-start terminal in the engine bay. According to the owner's manual the car also must not be tow-started. Opinions differ on w...
Check the brake light switch first. If that does not help, the fault is often in the cruise control unit, whose solder joints or dried-out capacitors ...
The filter is in front of the windscreen under the bonnet, in the air intake housing on the front passenger side. Opening the large Phillips screws is...