PeachParts Mercedes-Benz Forum - View Single Post

Steve M · #7 10-03-2011, 01:35 PM

Deep pocketed purists and NTSB petition enthusiasts should skip this post.

The upper engine wiring harness as it comes from the factory is really two wiring harnesses that come together at the 44 pin connector that plugs into the ECU. Mine is now three harnesses that come together at the ECU.

One of the replies to my original post challenged me to imagine what the wires I couldn't see must look like. I decided (with the harness in place) to start unwrapping the wiring harness to find out. Because of the way it is wrapped, I unwrapped from the ECU end, but I only unwrapped the bundle that included the coil pack wires (the one with the black, black & white and black & yellow wires.) I basically went all the way past the engine, stopping at the thermostat. The second bundle remains unexplored.

The insulation on the wires associated with the coil packs looked terrible, and all of the other wires looked brand new. I decided to replace just the coil connectors and the wires attached to them.

As I mentioned in a previous post, the dealer supplied connectors now come with wires attached. Because they are of a much smaller gauge than the factory wires I was reluctant to use them, but the parts guy said that this is the only application for the connector, so I went with it.

I was afraid to mess with the 44 pin connector at the ECU, so I cut the three wires in the plastic trough at the top of the engine compartment and soldered the brown wires from my new connectors there.

I ran the blue wires from the new connectors along the original harness over to the switched 12 volt terminal behind the washer bottle on the left fender, and put them on a lug which goes on the same post as the existing wires, but I have it coming in from the front of the car, rather than the left, so that it fits on the post. In the same area, I cut an inch out of the large pink and red wire so that I don't have stray 12 volts floating around in the harness. I removed the large pink and red wire from the coil packs to roughly the thermostat, so I basically have abandoned some pink and red wire in the undisturbed portion of the harness. If I had it to do over again, I would not have unwrapped the harness from the thermostat back to where the coil pack wires exit the engine.

Because I could see cracks in the insulation all the way to the white goop that all of the ECU connector wires disappear into, before I soldered the three wires near the ECU, I slid heat shrink tubing down as close to the connector as possible. I did not shrink it, though. I did put heat shrink over the soldered connections themselves, which I did shrink.

When all the wiring was done I wrapped it with Tesa wiring harness wrap that I got from the dealer - the same stuff the factory used. The car seems to work fine.

A few notes: I stupidly failed to disconnect the negative terminal from the battery before I disconnected the ECU the first time. I wound up with an SRS light that stayed on for 2 minutes each time I started the car. The dealer read some undervoltage codes and cleared them out, reset the SRS, and it seems fine.

Different people, going from the VIN, quoted three different prices for a new harness. They were, IIRC, $690. $1,290 and $ 1,840. The last one seems to be the correct one. It seems that you really need the P/N from the harness. I found my P/N on a tag on two brown ground wires attached to a terminal a few inches in front of the ECU. I understand that some people find theirs on a big tag on the ECU connector itself. The date on my harness was early 1995.

In addition to the indy who worked on this car four years ago, I have found reference online to other people who have had problems after changing spark plugs, which they resolve by pushing the wires around until the engine runs ok, at which time they close it up and cross their fingers. I chose not to do it this way, even knowing that it may work for years. In looking at my wires, I'm surprised that plugs 6 and 1 weren't mis-firing also.

Two years ago while tightening down the cross tube from the MAF to the throttle body on this car while it was running, the car backfired and the crosstube exploded, sending plastic shrapnel throughout my garage. I was fortunate that I was barely injured. I now attribute this event to a mis-fire caused by the harness.