Mercedes-Benz W126 (S-class 1979–1991, sedan and coupe): common faults, maintenance guides and repair tips.
Usually the fault is in the heater control solenoid valves ("jay's eggs"). Without power the valves stay open and hot coolant flows all the time. The ...
The most common cause is the water valve: its rubber diaphragm has torn or the valve internals are broken, so warm coolant does not circulate properly...
The injector gives a cold engine extra enrichment only while cranking and only when the coolant is cool. The thermo-time switch gives it a control sig...
On KE-Jetronic injection the cause can be a leaking cold start injector that lets fuel through continuously. A break in the temperature sensor wire al...
Yes. If the regulator's spring or plate has oxidized, it does not change the pressure as the engine warms up and the injection runs at full pressure a...
Yes. During long storage fuel can turn to resin in the fuel injectors, so they clog and the engine runs poorly, especially when cold. Replacing the in...
Values vary. For the M103, 650-750 rpm is given with the gear selector in N, and about 600-700 rpm when warm. In an automatic in neutral the reference...
Yes. A dirty or stuck auxiliary air valve or idle control valve lets air pass wrongly: the revs of a warm engine stay high, the idle jerks or the engi...
Yes. A cracked cap lets moisture in, and a broken or burnt rotor weakens the spark. Symptoms are rough running, uneven idle, jerking and poor throttle...
Hold an ignition lead to a spark plug with the plug body against the chassis, and have someone else crank the engine. The spark is hard to see in dayl...
The overvoltage relay is on the bulkhead next to the ABS box. Its housing is aluminium and it has a 10 A fuse. Next to it is the fuel pump relay, whic...
The fuel pump does not suck by itself but needs fuel pushed to the supply side under pressure. From an almost empty tank not enough comes. The car usu...
The cause can be a cracked EHA (electro-hydraulic actuator), in which case the area around it is damp, or a rotted fuel vapour filter inside the left ...
A common cause is a clogged oil filter, if it was not replaced at the oil change. After a new filter the pressure rises to normal. Other suspects are ...
The chain should be replaced at about 200,000 km intervals. The stretch is seen by setting the timing marks in line and measuring how far the camshaft...
A common cause is the fan's viscous clutch, which no longer drives the fan properly. Overheating can also show up as coolant being pushed out. Do not ...
The cause can be the vacuum hose to the transmission being disconnected or leaking. The transmission then thinks the throttle is wide open and the ups...
The most common cause is too little automatic transmission fluid. Low fluid can first take away fourth gear and then third as well. Check the fluid le...
Check the level on the dipstick with the car on level ground, the transmission warm and the engine running. Cycle through the gears and leave the sele...
Common causes are a worn centre bearing (support bearing), a loose centring sleeve, worn universal joints, rubber flex discs and the support rubber. A...
A friction-type limited-slip differential needs oil made for limited-slip differentials. Ordinary rear axle oil wears out the friction plates of the l...
A shrill whine at high speeds often points to the rear wheel bearing. Check by lifting the rear wheel with a jack and testing for play. A broken handb...
The causes can be loose tie rod ends, a worn strut rod end, or worn anti-roll bar bushings and shock absorbers. The rattle and the numb feel in the st...
The most common causes are a worn steering damper, a damaged or rusted lower ball joint and a worn steering box. Symptoms are a loose steering wheel, ...
The shock absorber is removed, and the spring is compressed out with a spring compressor placed inside the spring. Alternatively the rear is slowly lo...
The most common cause is a break in the wheel sensor wire, often in the sensor's or the rear wire. The light may come on only at a certain speed or, f...
The lights can come on when the system voltage stays too low. Reported causes are failing solder joints in the overvoltage relay (YJ relay) or a fault...
The cause can be an alternator that does not charge at all, or the wire of the auxiliary fan installed in front of the radiator for the air conditioni...
Common causes are the brake light switch, fuses, oxidised connections, dried-out capacitors and cracked solder joints on the control unit's circuit bo...
Often the plastic link on the driver's side between the sunroof and the liner has broken. The sunroof then tilts and slides under the liner. The plast...