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]

You MUST replace the burned wires. Do it one at a time, using solder and shrink tubing. I've seen entire wiring looms smoke out and even fry components when left with bare exposures.

Your late model is dependant on many electrical inputs to operate correctly. Your high idle might be induced by turning on your A/C. Your ECU sees this as an extra load and compensates by increasing your idle speed. In gassers it's done by extending the impulse time to the injecters and increasing timing at idle by a couple of degrees. I'm not sure how it works on newer diesels.

I HATE to interject something unless I am 100% sure about it, but with that caveat in place…

I believe that there is an electronic idle stabilization device on these cars. What happened in your particular case is not something that I am qualified to speculate upon. But hopefully this helps lead you in the right direction.

Your situation makes me a bit nervous – I was experiencing an intermittent tachometer for a while, then I went into the wiring harness connectors at all the control boxes & simply unplugged and then reinserted the connectors to clean any corrosion that could be causing resistance or an intermittent connection. It cured the problem, but now I am curious as to the condition of the wiring harness itself.

This makes me nervous, as my elderly father’s 1994 E420 went up in flames a few months ago due to an electrical fire that apparently started under the dash…

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Current rolling stock:
2001 E55 183,000+ Newest member of the fleet.
2002 E320 83,000 - The "cream-puff"!
1992 500E 217,000+
1995 E300D 412,000+
1998 E300D 155,000+
2001 E320 227,000+
2001 E320 Wagon, 177,000+

Prior MBZ’s:
1952 220 Cab A
1966 300SE
1971 280SE
1973 350SLC (euro)
1980 450SLC
1980 450SLC (#2)
1978 450SLC 5.0
1984 300D ~243,000 & fondly remembered
1993 500E - sorely missed.
1975 VW Scirocco w/ slightly de-tuned Super-Vee engine - Sold after 30+ years.

When shorts happen these things are completely normal. Did it look more like corrosion than rust? Discoloration of the wires ?

I’d look for bad components, there’s most likely a short some ware in your system

Just examples of fried wire experience

I’ve had alternators short in 2 of my cars
First one when I was 17 and I didn’t know what a fusible link was all I knew was
That there was a wire coming off the positive terminal of my battery that looked corroded
So I replaced it with a bigger wire on, big mistake,
Had to replace the entire engine wiring harness “82 Nissan Sentra”