My 93 300TE has displayed rapid changes in the coolant temp, from about "78" to about "88", like a light switch. I thought I had a skinned wire grounding out, or maybe a sticking thermostat, or a slipping belt.

So I tightened the belt, did not back off the tensioner lock bolt enough, stripped the adjustment. Replaced tensioner, found fan bearing bracket bearings failing, and also water pump hard to turn. Replaced both (water pump is an all day activity). And replaced original thermostat twice.

Finally would stay on about 78 most of the time. Then on the way to work, ABS light on, then SRS, then AC shuts off, then starts running rough, then dies within 1/2 mile. Voltmeter on battery reads 6.5 volts.

Turned out to be alternator brushes, which of course never caused idiot light to stay on. But from an electrical standpoint it makes sense. As the brushes touched or did not touch, the voltage changed quickly, and the guage responded, being a simple series circuit.

So if your coolant temp on a W124 is twitching, reach for the back of the alternator. One of the few places under the hood with plenty of space to work... And the regulator / brushes are simple to change - two screws. By the way, the voltage across the battery won't be the 14.1 - 14.2 you might be used to on Detroit iron. MB at that time liked 13.6. And now my temp is 88 (splits the top of the 8 in 80).

Do you think this would apply to an 87 like mine? I'm not having problems, but its good to know the warning signs.

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Streetside: 1987 Mercedes 300E Petrol Blue/Green
2014 Ford Mustang Grabber Blue
1999 Ford Ranger Green

Dockside: 1968 Eltro 17
1985 Silverton 34C

Welcome aboard Savannah Buzz.

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Jim

My charge light flicked on and off from time to time. I took out my old voltage regulator and found the brushes worn down about 50% after 113k. A new regulator seemed to fix the issue. Noticed the voltage was a bit under 14 volts too but was good enough! Simple repair!

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1989 300E

2005 Acura TL

2011 Hyundai Elantra Limited

Swing hard! Take chances!

If your (electric) temperature gauge fluctuates during varying alternator load, it means that the engine-to-body ground is faulty.

The sensor is grounded to the engine, and the gauge to the body. A faulty ground causes the engine to be at a different potential than the body.

So you fixed one problem, but another remains.

Which grounds would you check?

Thanks.

glenmore
1991 300CE
1999, 2000 C280
2000 C230K

I've had this happen multiple times when my stock alternator was eating brushes. Car ran almost 40 miles on battery power each time (go Optima!). Started with one idiot light and then the rest tumbled. Was able to drive for about 15 minutes once all the lights came on.

The OP's problem with the coolant gauge was resolved with a new alternator. I recall having similar twitches with the coolant gauge when my brushes were going out.

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http://i193.photobucket.com/albums/z...-RESIZED-1.jpg
1991 300E - 212K and rising fast...

Hmm, my '93 300TD was giving 14.2V with an original looking Bosch regulator even with worn out bushes, the replacement regulator gave 14.0V....

Not having a thermostat in it when I got the car used to cause the temp gauge to flutter wildly though.....

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cheers!!

My early warning was cruise control shutoff at 70 mph. Got my attention.

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Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles