1985 300sd batttery light staying on - PeachParts Mercedes-Benz Forum

JgBenz · #1 08-08-2010, 07:50 PM

Got to work this evening and noticed when I turned car off battery light still on. Cranked the car and light goes off. What is going on. I took ground off of battery so I would not drain. Did notice it had a spark on the terminal. Again this is with the key out of ignition.

whunter · #2 08-08-2010, 09:55 PM

The most common answer is a shorted voltage regulator.

NOTE:
The 1985 300SD requires the following Long Brush regulator...

Voltage Regulator, Internal Type (Long Brush)
MB# 002 154 72 06

Fastlane: Voltage Regulator
http://catalog.peachparts.com/item.wws?sku=W0133-1813438

Have a great day.

layback40 · #3 08-08-2010, 11:46 PM

X2 its probably the reg. You could try unplugging the reg & see if the light goes out. This will remove the slight chance of it being a wiring problem.

JgBenz · #4 08-09-2010, 03:47 PM

I removed the voltage regulator from alternator and still have a battery light with the key out of ignition... I removed the ground from battery to prevent drainage of battery to get home this morning. One other thing, when I crank the engine I do NOT have a battery light.

layback40 · #5 08-09-2010, 10:50 PM

If you had the reg unplugged as well as removed, then it looks like you have a wiring problem. Probably the wire from the idiot light to the reg (the thin one) is some how getting power to it, probably a short in the loom or the plug.

If you didnt have the reg unplugged, you have proved nothing.

Meta6981 · #6 12-26-2010, 07:30 PM

Similar problem for me on my 81 300sd. I have been having to operate the past few weeks with disconnecting the batt at every stop more than an hour or so. If i leave the batt connected it will eventually drain the battery. I have already replaced the voltage regulator. When the Alternator is unplugged the car operates normally. ALso with the alternator unplugged the gauges zero out and GP light operates normally. This is a 911 so help asap would be appriciated. Thank you

layback40 · #7 12-26-2010, 08:48 PM

First thing check all earths, it sounds like that is the problem. If they are all good, then start looking for a short down to earth. There are threads describing how to do that starting by removing fuses.

Meta6981 · #8 12-29-2010, 04:53 AM

My brother went through most of the fuses with a test light. We think we have narrowed the short to something in the alternator or something from the GP relay. As soon as i get some excess cash I am going to put a new alternator in.

TX76513 · #9 12-29-2010, 07:00 AM

Resistor Pack is bad allowing voltage to feed back through system when ingintion is off

thayer · #10 12-29-2010, 08:41 AM

have you tried jiggling the key right before you pull it out of the switch....the light being on while the key is out would point me to the ignition switch.

Just another thought if you can't find a short anywhere. I dislike electrical problems.

paddo · #11 12-29-2010, 02:24 PM

Quote:

Originally Posted by thayer

have you tried jiggling the key right before you pull it out of the switch....the light being on while the key is out would point me to the ignition switch.

Just another thought if you can't find a short anywhere. I dislike electrical problems.

Good point, it sounds like the ignition switch maybe is not turning off the car as it should, it's just leaving it in maybe accessory position. But if that was the case, the radio should still be able to be switched on when key is removed.

But the light is still on with regulator removed - that suggests something else. I think battery is applied to the alternator light, through a resistor then to the alternator field. So it depends if it goes to the field via the regulator, or just straight to the brushes, by-passing the regulator. A way to test that would be to remove the regulator on a normally running car, and see if the battery light still goes on when key is switched on.

Meta6981 · #12 12-30-2010, 08:46 PM

At paddo, The light turns off when the alternator is removed, have replaced the voltage regulator already, I thought it was the Ignition switch as well, but i don't think that is it.

Does anyone have a wiring diagram for these beasts?

Meta6981 · #13 01-05-2011, 10:32 PM

Update for my car: the alternator was Toast, got it rebuilt and its been working fine since, these cars have some wacky electrical systems.

JgBenz · #14 01-06-2011, 12:16 AM

It was the alternator. Got a rebuild on it and fixed all problems.