We have seen this problem since 1991 and it does not seem to be going away.
The insulation on wires on various Mercedes-Benz vehicles and various harnesses becomes brittle or deteriorates to the point that normal engine vibrations cause the insulation to crumble off, leaving the wires bare and exposed. The subsequent short circuits can cause anything from erroneous warning indicators to massive electrical fires.
The problem was most common on engines (injector, sensors and coil harness) of Mercedes-Benz vehicles built in 1994, 1995 and early 1996.
There are also common failures with transmission harnesses, positive cable harnesses (usually also incinerates alternator, starter and oil level and pressure sensors), and neutral safety switch / transmission range selector switch harnesses. On V12 SL600's with the 120 engine, the body-mounted engine harness often fails. The harness of this vehicle requires over 40 hours to replace. It is usually only the air mass meter portion of the harness that fails. At Autobahn Performance Inc. we have the factory air mass meter harness with enough wire to connect directly to the terminal at the engine computer. The harness can be run through a new loom that can be placed directly beside the existing wire loom. This can save about 30 hours labor if the rest of the harness is not failing.
6/22/2009 Wire harness failure in the body mounted engine harness for the auxiliary fan resistor.
We also performed the resistor update. This vehicle is a 1994 Mercedes S320 manufactured in 10/94. 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! We repaired the car for less than $500 including the fan resistors, brackets, and hardware necessary for the resistor update / modifcation.
8/20/2009 Wire harness failure in the engine harness. The wires for the knock sensors had the insulation 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, which resulted in fault codes and driveability problems with the operation of the engine.
The Mercedes parts catalog seems to have the two possible harness part numbers transposed. 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. The data card in Mercedes EPC (electronic parts catalog) online shows the vehicle does not have ASR. The first two harnesses that we received from the dealer were part number 210 540 2105; they did not match the original harness in the car. Both of the harnesses we received have boxes that were beat up and had extra delivery stickers stuck to them, indicating that they had been returned several times. The tag with the part number on the harness was missing, so the part had to be ordered by application with the V.I.N. number.
Part numbers confusion
The parts manager at the dealer decided to order the harness for the vehicle with ASR, and the part came in correctly. The part number of this harness is 210 540 3305. It indicated that it's for vehicles 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.
1993 Mercedes 400SE (140 chassis)
appears to be an original harness that has survived all these years.
We have repair harnesses in stock to repair the body-mounted engine harness on v12 600's. Usually, only the air mass sensor wires fail. The harness can be run alongside the factory harness and connected directly to the engine computers.
We are working on a very clean and original 1995 SL600 (129 chassis) that has less than 30,000 miles on it. Unfortunately, this vehicle was made by Mercedes with faulty wiring insulation that has deteriorated and caused short circuits in many components. The main engine harness (just includes the fuel injector wires and a few others) is in the worst condition. The main body-mounted engine harness has some wiring circuits that are bad, mostly the air mass meter harness. This harness costs over $5000 for the part and approximately 60 hours labor to replace. It runs from the headlights to the trunk of the car, behind the dash and under the interior. This job at the Mercedes dealer could legitimately be $14,500 just for the body-mounted harness.
Here at Autobahn Performance Inc. in Ft. Lauderdale Florida we have developed a repair kit and procedure to repair this harness without removal for less than $1800. It will work on V12 engine in the 129 (SL convertible) chassis and the 140 chassis (4 door sedan or 2 door coupe). In rare cases, more than the normal wire circuits are affected by the insulation deterioration and the harness is unrepairable. This is usually only in European-delivered vehicles and with the sedan 140 chassis. If a wire suffers from insulation deterioration, the entire length of wire must be replaced. Trying to splice into a section of the wire will not work properly and will result in short circuits and possibly a fire. The insulation will start to crumble near where you are trying to repair it and you will probably make your problems worse rather than better.
Throttle actuators (electronic controlled throttle body, TBA Throttle Body Actuators): There is one for each side of the engine. (Note: the TBA on the right side of the engine feeds the air into the left side of the engine and vice versa.) The actuators have a short wire harness attached to them: it's about 14 inches long and connects to the body-mounted engine harness. These harnesses also have insulation failure problems. The harnesses are part of the actuators and are not available as a separate piece from Mercedes. The actuators can be rebuilt and new harnesses soldered into place. Again, do not try to repair these as you will not be able to properly connect new wires to the electrical connector end of the harness. Short circuits in the wiring of the actuators commonly damage the E-GAS actuators (the control units that control the throttle actuators). These also can be rebuilt.
Jason wrote more about Mercedes wiring harness problems in Mercedes SL600 with wiring problems in his blog, Jason's Space.