I purchased my 1994 E320 two years ago in 2001 and recently began having problems with the idle control and check engine light. I did my research on this site and a variety of other sources and found that the wiring harness and throttle actuator are common problems due to premature breakdown of the insulation covering the individual wires. Eventually, the wires touch and cause many strange symptoms. It is states in the forums that technicians who are knowledgeable about this model will not put serious effort into troubleshooting it until the original harness has been replaced.
There are all kinds of examples of folks who had a water pump replaced, or head work done that required moving the harness aside. The problems begin as the car is re-assembled, with resulting shorts inside the harness. When you cut into one of these, it is no wonder.
Without going into my specific troubleshooting details, I determined that I needed to replace my main engine harness. Here are the telltale signs that my harness was original and in poor shape.
You can see the insulation cracking and flaking off, leaving bare wire exposed.
I cut into the harness near the ignition coil. The insulation is crispy here too, and its amazing that the car ran as well as it did! The bare wires are a bit green from corrosion.
Results may vary, but I replaced my harness in 1 hour and 20 minutes. There are 24 plugs on the harness and they only snap on one way. How hard could it be Really, this is as easy as changing your spark plugs. There are no special tools required. Begin by removing the battery. You need to remove it to access the harness, clear the trouble codes, and finally, you sure dont want to short anything out or apply voltage where it doesnt belong during this job. Then yank the crossover tube and pull the top engine cover off, just like you were going to change your plugs. Heck, while youre in there treat your car to a set of plugs I did!
Here is what you start with - fresh new wires!
Here is the battery removed and the old harness on the way out. This is about 15 minutes into the job. Note, the old harness has been unplugged and the new harness is lying across the battery tray. Just start from one end and replace as you go. I counted the number I unplugged and then make sure I plugged in just as many. This is easy stuff. Note the fresh, new OVP on the right side of the picture.
This next picture shows the new harness plugged in, routed cleanly and ready to go.
This next shot is looking messy. I have the new harness sitting on top of the old one. Being the first time Ive done this, I didnt want to just go along and remove the entire old harness.
Here is the old harness on its way out. Coil connections, fuel injectors and a few others.
Here is the same area. Out with the old, in with the new
New harness in place.
Working from back to front.
The front of the engine. Just use care here to route things cleanly. You dont need to understand what all these things do to replace the harness. Unplug the old harness and plug in the new one. There are no extra wires, plugs or anything strange to sort out. If you find anything strange, you probably have the wrong harness. There are a couple of plugs on the drivers side fenderwell that I didnt take a picture of.
Done, picture 1.
Done, Picture 2
Done, Picture 3.
Hey, Ive seen big spiders in my garage, but this is ridiculous!
While the harness install was no biggie, my problem also required replacement of the infamous throttle actuator. Again, a simple part to replace. Maybe 20 minutes to get it out, and 20 to put the new one in. The part is expensive, but dont let that trick you into a big installation expense. Here is the engine bay with the old one out. It lifts up through the intake manifold. Nothing difficult about it.
This is my old throttle actuator. Note, wiring is in sad shape. My car is now 100% healthy. Cruise control works again too!!
Good luck. If you are worried about this job, pay a professional. I consider myself an advanced DIYer, although I think this can be done by a beginner. However, you are working around a lot of expensive parts, so do what makes the most sense for you. Thats my disclaimer!!