Charging System Trouble

hELLO: i HAVE A 1993 300SD. The battery light in the instrument
cluster stays on till revved up to approx 1700 RPMs when
starting the car. Once the light goes out the light stays out at any
RPM. I checked the output of the alt at idle with the light on and
there is no output. After the light is out at idle the output is
between 45 & 47 amps. At 2500 RPMs output is only 50 amps
with all accessories on.[should be near 100 amps] I pulled the
instrument cluster and checked continuity of the exciter wire
going to the alt. It was okay, and then I checked the continuity
of the wire going to the alt for continuity to ground and there was
continuity to ground with the wire was disconnected at both the alt and cluster. Is this a problem with the wiring?? Has anybody
else experienced simular problems? I have been running like this
since 1999. At that time I took it to a dealer and they worked on
it a full day, on the phone with Mercedes reps and were unable
to diagnose the problem. I never went back. Thank you anybody
and everybody.

[P.E.Haiges]

rghtara,

Just some thoughts:

Why do you think the output should be more than 50 amps even when the accessories are on? The 50 amps you measure may be all you are using to run the accessories. There may be an alternator reserve to charge the battery when you have all the accessories on or when running the engine at a slower speed.

On way to findout would be to measure the current in or out of the battery when the alternator is putting out 50 amps with the accessories on. If no current is coming out of the battery, the alternator is supplying all the necessary current. There should also be some current going into the battery depending on its state of charge.

One other thing, did you check the brushes in the alternator. Maybe thay are not worn but its possible that one of the springs is weak and not pushing the brush against the alternator slip rings hard enough.

Was the voltage regulator been replaced when the shop did the attempted repairs? I would think it was because shops tend to just replace parts until the problem is fixed and of course charging you for all the replaced parts telling you they were necessary.

P E H