Troubleshooting Alternator
 

I’ve have ‘83 300 Turbo Diesel. My battery recently went dead while driving the car. The strange thing is that my alternator light never went on to indicate I was having a problem with the battery charge. I recharged the battery and the car starts but I am not getting a charge back to the battery. I’m monitoring the battery with a volt meter and over a weeks period I see the battery slowing loosing its charge. I have noticed that the battery check light and the yellow brake light that momentarily turn on during the ignition test are not lighting up. Only once in the last week I have seen the lights go on during ignition start up so I know they are not burned out. Which leads me to believe that my alternator my not be the source of the problem but some other bad connection. I had my alternator replaced about 5 years ago, the belt is fine and the alternator is turning. Any suggestions on how to trouble shoot this problem myself before I take it to my mechanic where he will suggest a new alternator?

Thanks

The first thing to do is to check the voltage at the battery, then start the engine and check it again while the engine is running. If the readings are the same - it's a charging problem. You should get a reading somewhere in the mid 14v range while your engine is running. If you do suspect your alternator, double check all the connections before replacing/rebuilding it (including the terminals of the battery itself).

I would check voltage regulator (brushes). Should be about a $20 part -- $45-$50 for Bosch/MB. 2 screws and 8 minutes later and you are good -- and that's if you have to crawl under the car as I did on my W124.

My dash lights did not light with the key on position 2. They do again now.

This was on a car with 115k. I replaced the ones on my older car with 166k proactively. They were almost totally gone!

I don't think the dash lamp will light if there is no circuit to the alternator rotor winding, as will be the case if the brushes are worn out.

That means it won't indicate lack of output, either.

However, if you have some bad diodes, you have the correct voltage, but no current, so you run the battery down starting and using the headlights, no dash indicator until the last diode gets weak. This happened on the 220D -- I would have to jump it once in a while, and then recharge the battery. Ate alternator belts, too.

I took the alternator out (after I got a used on at the u-pull-it that was also bad), figured the bearings were bad since it was noisy.

Took it to the rebuild shop and got essentially a new alternator in return for $110. New rotor, new stator, new diodes, the guts were knocking together! Bearings were fine. Note that these were actually used parts -- new was more like $200.

Peter