Early Warning for W124 Alternator Failure

[Savannah Buzz]

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).