As requested by several people, here's a short DIY to repair your engine wiring harness so you don't have to spend $800-$1500 to replace it.
Take the cover off where you would replace the spark plugs to see the three wires (on a C280) that lead to the 3 coils. These wires are 12V DC, and are the most common failure points.
Picture 1 is a view of the part of the harness on my car that failed. Carefully cut off the outside insulation of the harness between the plug and where the harness enters a rubber section.
Picture 2 is a view of the harness after I removed the outside insulation. The picture is kind of fuzzy, but you can see the green corrosion on the wires and how there is no insulation around them...this is bad!
Picture 3 shows how I separated the wires and wrapped electrical tape around them, and then wapped more tape around both wires to hold them together. Be very careful to separate the wires entirely, especially in the plug and in the rubber housing.
Connect everything back up, making sure to have the air tube that crosses over the engine back in place, and then start her up. She should fire on all cylinders now, but if she doesn't then just jiggle the two wires around a bit near the plug and rubber until it works.
Last step, to reset the Check Engine Light, turn the engine off and pull fuse number 32 in the fuse box on the driver's side of the car (LHD cars), and leave it out for about 30 seconds. Replace the fuse, turn the car on, and the Check Engine Light should be off.
The engine wiring harness suffers from a cracked insulation in several places, including connections to the ECU. You're better off replacing the darn thing.
Note: This guide is intended for those who don't want to or who don't have the cash for replacing the harness at a cost of $800-$1500 for the part alone. Although replacing the harness is best, it's not always a feasible option.
Repairing the harness in the way I described won't put your ECU at risk. In fact, you're more likely to protect your ECU by performing this repair, rather than just leaving it broken (with the wires shorted out and the cylinders misfiring).
I don't mean that doing this repair could damage the ECU; what I mean is that leaving the bad engine harness in place could damage the ECU so even if you perform this repair, there are other areas of the harness which still have problems so the best bet is to replace the entire harness.
I just got a quote to replace my harness for 1508.00 from a MB dealer here in the Toronto area... I think I am having the same problem...Sometimes the engine runs real smooth and at other times seems like it is missing on a couple of cylinders and has absolutely no power... Does this sound like the same problem to you and if so can an average guy (pretty mechanical) take this apart and fix it..
It is possible, as demonstrated by my photos, to repair the harness without replacing it. Unfortunately most dealerships are no longer offering "goodwill repairs" on the C-Class engine harness, so a temporary repair as per my instructions will work.
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