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.

voltage regulator brushes worn would be my guess since the light goes out after you rev up the car, so it's not making enough juice up to that point. Could be a contact problem. Was this item replaced? I heard that these can sometimes be faulty too.

xp

Were you loading it when you got those output readings? A stable system will put out a couple amps more than what is necessary and no more. If you turned on 50 amps worth of users and only got 55amps, it would be what I would expect.

To get full output from the alternator it must be loaded externally (as the cars loads will usually be less than rated output - except for the cars made in the 80's)

Hi stevebfl: I did have all accessories on but perhaps you are
correct that the load wasn't enough to cause the alt to put out
more amps. But, what about the continuity to ground on the wire
which goes to the alt from the cluster. Should that be???? I know
the battery light should not stay on till 1700 RPMs have been
reached, so something is wrong. If you can shed more light on this problem I would be estatic. The voltage regulator has been
changed. THANK YOU

In the full day at the dealer with the higher-up MB service reps., did anyone suggest swapping in a new alternator as a test? I'm not a fan of throwing parts at a problem, but after one FULL day of diagnosis at a dealer service facility with NO conclusion, I'd be trying a new/different alternator. One failed diode in the triad?...sextad?...whatever little "loop" circuit is internal to the alt., could be your problem. Bosch rebuilt for your car.. less than $200 at Fastlane. Reliable and non-annoying charging system..priceless ;) FWIW

Hello: the dealer took the alt out and sent it to a specialty shop
where they said they replaced a diode. Symptoms still the same.
Thanks

the voltage reg was replaced