Have a 1984 300SD...today adjusted the valves...all were tight. Runs quieter now with somewhat more response. Cleaned the motor with engine cleaner and hot water...no problems.
Then drained coolant and added a radiator flush and started the engine...now have the alternator light on. Checked the usual connections...found the junction box next to the alternator on the fender well was wet and somewhat corroded. Took it all apart and used some electrical grease on the connections. Still have the light on. Checked the voltage to the battery with the engine running...only getting battery voltage at around 12.5 volts and dropping.
What could I have done. I did get some coolant on the alternator when I pulled the heater hose off next to the alternator. Could I have shorted the alternator? Any suggestions?

I had the same thing happen after working on my car. Didn't use water or anything like that and wasn't even near the alt. Actually I did some welding and left the battery hooked up so I figured I fried something in the alt. In my case it was just coincidence as my brushes were shot. I puchased replacement set locally and installed without removing the alternator.

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Currently driving a very clean 1985 300SD from the West Coast.

Most common problem is the voltage regulator.

Just two screws hold it in.

Cheap from Bosch even.
With engine running...tap on the regulator with screwdriver handle to see if it resets the brushes..

Other possibility is loose and wet alternator belt.



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I think I really did it this time. I was cleaning all the connections to the alternator, and the brushes then ran the alternator by mistake with out it being connected to the main wiring. Did I fry it?

An alternator needs to be connected to the battery to create a "generation field" (to get itself going)....so I think disconnected it just spins/does nothing. Hook it up and find out.....my bet is that its fine.

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Nope...still doesnt work. May try a new voltage regulator. Is there any way of testing the old regulator? If this doesnt work will have to eat it and buy a reman alt.

I know this sounds strange, but you test an alternator's bearings (the only other common thing to fail) by placing a wood dowel or broom handle on the atl. 9watch out for moving parts) while engine is running. You will hear the bearings if bad. Check out a few other spots on the engine for expereince. If the bearings are good, change the voltage regulator. New/rebuilt atls cost a good night out. Votlage regulators cost two cases of good beer. Leaving the key on without the engine runnig for a long time can fry the regulator. This is a 5-10 minute job. Watch as you remove the old reg. You have to trun it to remove. Parctice with the old one so the new one won't get damaged with trial & error. A short screwdriver is the only tool needed. Unplug battery!!!!!