Dear Kiwi,

I have replaced the engine wiring harness on my 03/1996 C36 (M104). Only last week I helped a friend replace the engine wiring harness on his 12/1994 W124 220E. So I say the following based on some personal experience.

Mercedes wiring harnesses fitted on all Mercedes cars built between about 1992 and about 1997 were made using insulation that hardens with time but also rapidly degrades with high temperatures. So, at some time the engine wiring harnesses (due to engine bay temperatures) on all these affected cars must be replaced. You will not fix the problem by buying any number of new sensors, new MAFs, new computers etc, etc (I have been there and got the T-shirt).

You can easily check the condition of your engine wiring harness by looking for signs of disintegration, cracking and dust power (that was once insulation material). When changing the headgasket on the M104, the necessary physical manipulation of removing and replacing the engine wiring harness is almost guaranteed to cause engine problems due to shorting as the old temperature-affected insulation breaks down under stresses that it suffers. If your engine harness is showing the critical signs indicated above, then you must replace the harness with a new one (or have the old harness re-built with new wiring). When you buy a new harness, look to see the date of manufacture is not before 1998. The date is shown on a label along with the MB part number and attached to the earth lead.

Good luck..