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Old 08-30-2004, 11:11 PM
pberku's Avatar
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Join Date: Oct 2003
Location: Montreal, Canada
Posts: 737
Mystery Solved - But Problem persists

Last week on another forum I posted a question about my '95 E300 Diesel. The question basically concerned an occasional rise in idle speed to about 900RPM and simultaneously the AC compressor would rapidly cycle On-and Off. The problem would usually last for a few minutes and occur maybe once or twice a month.

The consensus was that the problem is probably related to the Over-Voltage relay. I subsequently removed the relay, cleaned the contacts, changed the fuse, and put it back. I the waited to see if the problem would return.

Well last night it returned with a vengeance. I was driving through extremely heavy rain, when all of a sudden the following symptoms occurred almost simultaneously:

a) Idle speed went up to around 900RPM
b) Air Condition stopped working
c) Tachometer stopped working
d) ABS light came on

These symptoms were now permanent. I drove the car home, opened the hood and went straight to the Over-Voltage relay. To my relief the fuse was burned. I figured this is an easy fix, replaced the fuse, and everything was back to normal, but only for about 10 seconds. The problem recurred. I changed the fuse again, and 10 seconds later it popped again. I committed to change the fuse just one more time and if it pops again then I will look further for the cause. I changed it for the 3rd time, waited the now standard 10 seconds and everything was still working, when all of a sudden I see smoke coming from behind the Battery. Not good. Shut-off the engine and proceeded to remove the Battery so that I can locate for the source of smoke.

All I saw was a wire harness covered with a bit of burned wire wrapping fabric. By now, the battery was disconnected and on the floor, yet I needed to start the car so that I can pin-point source of the smoke without the Battery being in the way. Using booster cables, I connected the Battery to the car's empty Battery terminals, started the car and confirmed that the smoke was indeed emanating from the wire harness.

Shut everything off again, Disconnected the booster cable, and unwrapped the fabric wire harness sheathing. I was shocked at what I saw next. There were about 15 individual wires within that particular wire harness, and ALL of their insulation had completely disintegrated. Many of the now bare metal wires were touching each other, as well as shorting to the chassis.

The bare strands were rusty, and many of them had broken off. I was staring at the dreaded W124 Chassis wire harness problem straight in the eye, or should I say that it was staring me in the eyes.

About 4-6 inches of wire harness was totally damaged, the rest of the harness appeared to be OK. So after 3 Hrs. of splicing, soldering, cursing, and electrical taping the damaged wire harness section, I put everything back started the car and everything functioned as before with one exception. My idle speed, which previously was around the 600-650 RPM mark, has now increased slightly. I can't tell exactly what it is since the graduations on the tachometer are not fine enough, but I am certain that its now around the 700RPM mark. The spec for this (606) engine calls for an idle speed of 580 - 680 RPM. 630 being the norm.

I now don't know what the next step should be. Should I look at the wiring harness again, (I hate to do that)? Should I have the idle speed adjusted by a qualified mechanic? (I don't know how to do that myself. Or should I just leave things alone, since its not that bad.
In the interim, I just hope that my rather amateurish attempts to repair the damaged section of wires will survive for at least several years. To maximize my chances, I placed a plastic bag behind the battery to divert some of the water flowing down from the windshield to a location that is not near the wiring harness.

One day when I have more patients, a better soldering gun, and am more motivated, I'll consider redoing my fix more professionally. Until then, I am just praying that my fix lasts.

By the way, does anyone know if a Diesel engine idles at a higher speed when cold, and subsequently at a lower speed when hot? (as gasoline engines do), or does it always idle at the same speed, regardless of engine temperature?


Phil
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'95 E300 Diesel, 264,000 Miles. [Sold it]

Last edited by pberku; 08-31-2004 at 12:35 AM.
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