87 300d - battery light stays on while key is out of ignition

[ou2mame]

I was driving last night and the radio started making this weird humming that went up and down with the RPM. I turned the radio off and kept going. I noticed that my interior lights were more dim than usual, and my headlights were pretty dim too. I tried the wiper and it was very slow. I was on the highway but was able to check at the voltage at the cigarette lighter without stopping and it was only showing 8 volts. Around this time the ABS light came on. Luckily it's diesel so it'll run I just won't have electrical power. I was able to make it home and the battery was pretty much dead.

I charged the battery today and drop tested it and it checks out. The battery light now stays on as long as the battery is connected, even if the key isn't even in the ignition. I ordered a new voltage regulator because that's what my instinct tells me, but I'm kind of stumped. The car can start with the battery charged and now shows about 12v while idling.

Am I on the right track? Oh, and the ABS light is now off.

[Sugar Bear]

Yes, light staying on is a bad diode. If the regulator doesn't fix it which it should then it is time to dig into or replace the alternator. I use a local rebuilder for that vs. big box rebuild suppliers.

Good luck!!!

[ou2mame]

Originally Posted by Sugar Bear

Yes, light staying on is a bad diode. If the regulator doesn't fix it which it should then it is time to dig into or replace the alternator. I use a local rebuilder for that vs. big box rebuild suppliers.

Good luck!!!

Thanks! That makes me feel a little better. After checking the battery my first thought was the regulator and I figured it was a cheap gamble. I saw FCP has a reasonably priced alternator.

[Mxfrank]

It's not the regulator. A bad diode is likely, but even so, I'm having difficulty figuring how this could happen without a short in the wiring or a bad ignition switch.

[ou2mame]

Originally Posted by Mxfrank

It's not the regulator. A bad diode is likely, but even so, I'm having difficulty figuring how this could happen without a short in the wiring or a bad ignition switch.

Everything seems to function, just functioning as if there's not enough voltage. Which wires do you think would be shorted in this case? The ignition switch seems to work fine. If the battery's charged, the car starts every time, it runs, all the accessories work, etc. It just seems like the battery isn't charging because eventually it dies. I guess I'll replace the voltage regulator and go from there.

[pimpernell]

Originally Posted by ou2mame

Everything seems to function, just functioning as if there's not enough voltage. Which wires do you think would be shorted in this case? The ignition switch seems to work fine. If the battery's charged, the car starts every time, it runs, all the accessories work, etc. It just seems like the battery isn't charging because eventually it dies. I guess I'll replace the voltage regulator and go from there.

As mentioned by other posters, a bad diode may be the problem and has nothing to do with any shorted wires. If a diode in the alternator goes bad, is allows voltage to drain via the alternator body at a slow rate. After a couple of hours, the battery will start to run down and eventually fail. Good luck!!!

[ou2mame]

Ok, so I put the new voltage regulator on and still the same problems persist. Maybe someone can help me out.

Here's a list of what I know.
1) The battery light is on with the key out of the ignition as long as *a* battery is connected. The car battery, jump pack, doesn't matter
2) It wouldn't start without the jump pack, but the battery checks out with my drop tester when fully charged. It cranks slow
3) The battery light turns off once the car is running
4) The alternator charged at roughly 13v when I started it
5) I checked the pulleys and accessories and everything spins freely

[Sugar Bear]

Unplug the alternator, if the light goes out it is a bad diode in the alternator. Pull it and have it rebuilt and tested locally if you can.

Good luck!!!

[ou2mame]

Originally Posted by Sugar Bear

Unplug the alternator, if the light goes out it is a bad diode in the alternator. Pull it and have it rebuilt and tested locally if you can.

Good luck!!!

AAAAnd the light turns off when I unplug the alternator. Why didn't I think of that lol

[ou2mame]

So I got a rebuilt bosch with 2 year warranty for under 200 bucks at a local euro mechanic. Put it on and it runs! Its charging at 14.5 volts now, but I think I need a new battery. It makes sense that its ruined, its been low charged for who knows how long, and drained at least 5x by me. I really didn't think it was the alternator cause it was generating power, and that battery light threw me off. Oh well lesson learned! Thanks everyone!