Mercedes Benz – Wire Harness Insulation Failure

We have seen this problem for over 12 years now and it appears to be a common occurrence. The wire insulation on various Mercedes vehicles and various harnesses turns brittle or deteriorates to the point that normal engine vibrations cause the insulation to crumble off leaving the wires exposed. The subsequent short circuits can cause anything from the annoyance of erroneous warning indicators to massive electrical fires.

The problem was most common on engines (injector, sensors and coil harness) built in 1994, 1995 and early 1996. There are also common failures with transmission wire harnesses, positive cable harness (usually also incarcerates alternator, starter and oil level and pressure sensors), neutral safety switch / transmission range selector switch harness. On V12 SL600’s with the 120 engine the body mounted engine harness often fails. The harness on this vehicle takes over 40 hours to replace. It is usually only the air mass meter portion of the harness that fails. At Autobahn Performance, we have the factory air mass meter harness with enough wire to connect directly to the terminal at the engine computers. The harness can be rerouted through a new loom that can be placed directly beside the existing wire loom. This can save about 30 hours labor (assuming the rest of the harness is OK).

We fixed the body mounted engine harness for the auxiliary fan resistor. We also preformed the resistor update. We replaced the defective wires only, from the fuse / relay box to the Aux fan resistor that is mounted between the auxiliary fans. Wire harness replacement would be over $4000 parts and labor! Repair was done for less than $500 including the fan resistor’s, brackets and hardware necessary for the resistor update / modification.

The wire insulation for the knock sensors had the severely deteriorated. The engine was replaced by another shop. When the harness was switched from the original engine to the replacement. The unavoidable movement and flexing of the harness caused the wires to short circuit between the knock sensor wires. That resulted in codes and drive ability problems with the engine.

The Mercedes parts catalog must have the two possible harness part numbers reversed or switched. Our vehicle does not have ASR (automatic slip regulation) or traction control. There is no ASR computer and the part number for the throttle actuator corresponds to a non ASR vehicle. Also the data card in Mercedes EPC online shows the vehicle does not have ASR.

The first two harnesses we got from the dealer were part number 210 540 2105 they did not match the original harness in the car. Both of the harnesses we got had boxes that were beat up and had extra delivers stickers stuck to them, indicating they had been shipped and returned several times. The part number tag on the harness was missing so the part had to be ordered by application with the VIN number. The parts manager at the dealer finally decided to order the harness for the vehicle with ASR and the part we received was now the correct part. The part number of this harness is 210 540 3305. It indicated it is for vehicle with ASR. It seems Mercedes has accidentally switched or incorrectly matched the non ASR harness with the ASR equipped harness in their electronic parts catalog.

We had almost the same situation with incorrectly updated part numbers for the front wheel speed sensors on 1990 through 1993, 129 chassis SL vehicles. If you ordered both front sensors one would come as a plastic housing and the other a metal housing. If both were installed on the same vehicle there would be incorrect wheel speed indicated, causing several problems and of course, those loveable warning lights.


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