Hello

I have looked through the posts and cannot fond what I am looking for.

I have a 1981 300SD that if the car is off the battery light is on and it drains the battery in about 8 hrs.

If the car is running the altenator can over run the drain and it is fine the light goes out.

If I remove fuse number 12 the light goes out.

So that being said I pulled up the fuse panel and unscrewed the wires to fuse 12 with the fuse in and the light of course goes out. Touch either set of the wires back to the screw one at a time and the light comes back on. I have also switched out the insturment cluster to verify it and same results. Also the tach does not work on either cluster but the tach on the test cluster is good as it worked on the previous car.

One other thing I noticed is that fuse 12 does not have any wires screwed to the other end of it. It is linked internally in the fuse box to several other fuses. I also removed them 1 by 1 and still the light stays on.

I am not very good with electrical wiring so if anyone can point me in the right direction it would be greatly appreciated.

Sorry for the long post.

Thanks Ken

Does the alternator get warm when it does this?

I believe it is your regulator that is the problem. It isn't supposed to power the field with the key off, but I believe it is.

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

Thanks I will check this in the morning. I had been thinking the regulator as GM had the same issue in the '70's of bleeding back through the regulator but I did not have a Multimeter to test it with. I will check for warmth in the morning after I hook the battery back up and let it sit for an hour or so.

Thanks
Ken

tuxkwb,

It could be a bad diode in the alternator allowing the current to flow backwards thru the alternator. To test, take some resistance mesurements on your alternator and a known good one. Or maybe take your alternator and have it tested.

P E H

Thanks Guys

It turned out to be the altenator was getting warm so I went to pull it and the wire harness had been gotten hot so it was all brittle and broken causing the short. I am glad it did not cause a fire.

Anyway I have an 83 300sd for parts so it is all fixed good as new.

Thank you
Ken