The engine warning light on the dashboard of my 2002 Mercedes C180 Kompressor recently came on. At a Mercedes dealer, it was determined that oil was leaking into the wiring harness. I was advised to replace the harness as soon as possible. The same thing happened to my previous Mercedes, the 1995 box-shaped model. A friend who has a Mercedes that is about 30 years old has never had this kind of problem. Why is it happening to mine? And is it a common occurrence on newer models?
Frank, certain Mercedes engines, including the M271 engine, use solenoids for adjusting the camshafts to achieve variable valve timing.
The solenoids, called cam magnets in this application, are under the control of the engine control unit (ECU) and, for this purpose, there are wiring plugs at the front of the cylinder head. In time, these plugs develop internal leaks, allowing hot engine oil to leak into the connectors and, from there, into the sleeve of the wiring loom.
The oil migrates downward inside the loom and the oxygen sensor is usually the first to be affected. This is the probable cause of the warning light switching on.
The oil alters the resistance of the sensor connections, sending mixed signals to the ECU. If unattended, the oil will eventually reach the Mass Air Flow sensor and the ECU, at which stage repair costs begin to skyrocket.
Fortunately, Mercedes has developed a cure: when replacing the cam magnets — they must be replaced if their plugs have started leaking — you have to install a short extension cable, called a pigtail in the trade, at the wiring plug on each magnet. The pigtail allows the sleeve of the loom to be moved away from the plug so that if the latter starts leaking again, the oil will drip off the pigtail and run down the front of the engine instead of entering the loom.
The oil-contaminated loom can be cleaned by disconnecting its ends, injecting an oil solvent into its sleeve and blowing out the oil with compressed air. The process is repeated until you have got rid of all the oil. Cars sometimes have to be brought back after a while for the last of the oil residues to be flushed out, but the dealership I consulted reported that their customers find this far more acceptable than the cost of a new loom.
For further information about the problem and its cure, see this helpful thread on the Mercedes-Benz World forum.