Is this an alternator problem?? - PeachParts Mercedes-Benz Forum

bhatt · #1 03-22-2008, 06:18 PM

I have a 92 500SEL. Bought it with approximately 110,000 miles, and it now has 150,000 miles. Not sure what was replaced before I bought the car, but I know I haven't touched the alternator since I bought it.

Replaced the battery 2 months ago because it wasn't holding a charge. Car would run fine but not start the next morning, especially when it was cold. This fixed the problem. Not sure this is related to my problem, but thought I'd mention it in case it helps.

Now a couple of months later, I'm finding a new problem. I have what I think is low voltage, because:

1. The SRS light is starting to come on more often and stays on until I shut off and restart the car.
2. Started the car on a cold day and fired up the heated seats and rear defrost. When I hit the window button, the front 2 channels of the Bose amp cut out until I let go of the window switch.
3. Noticed last 2 weeks that the heated seat doesn't heat up as much as it used to. Putting it on high now makes it heat up less than it used to on low!

This stuff all tells me that I may have a low voltage problem with the alternator. I'm guessing I have a 150,000 mile alternator that is original to the car, and it's now starting to die.

BUT.....

I checked the battery voltage with the car at idle and it's between 14.0 and 14.1 volts!

So I guess there is no point in ordering a new voltage regulator and installing it, since I had originally thought that was the problem.

Any ideas what is causing this 'low voltage' issue? Something is definitely "dying", I just don't know what it is and don't want to wait till it happens!

Neal

Ferdman · #2 03-23-2008, 03:27 AM

Neal, check the condition of the voltage regulator brushes. If they're heavily worn it's time to replace the voltage regulator ... a real simple DIY job.

bhatt · #3 03-23-2008, 10:15 AM

I'm pretty sure the alternator has to come out of the car to do that. I remember replacing them on my 300SDL, but on this car I can't even see the back of the alternator.

I was going to pull the alternator and replace the brushes and voltage regulator (it's one unit).

But would worn brushes cause this problem? My understanding was that worn brushes would cause a low charging voltage. Perhaps I'm confusing a bad voltage regulator with worn brushes or something..

I have a voltage regulator/brush unit for this car, sitting on the shelf. Do you think replacing it will fix the problem?

Neal

Ferdman · #4 03-23-2008, 11:59 AM

Neal, check the condition of the voltage regulator brushes ... if required, replace the voltage regulator.

stevebfl · #5 03-23-2008, 01:49 PM

The issue with the brushes is that as they die the car will experience periods of NO charging. It can be intermittant and when not occuring the voltages would be OK.

Another point is that the starter alternator wiring harness on those cars is also one of the noted harness failure points.

bhatt · #6 03-23-2008, 04:20 PM

I think you may have hit the nail on the head....

I replaced a 2 year old Interstate battery with a new one, since it wouldn't properly hold a charge. The shop did do a full test on the battery and said it passed with flying colors. They only replaced it because they said that one bad cell could be the problem, and that wouldn't show up on the test.

So perhaps the alternator has been intermittent.

I did a voltage check at the battery (with the car at idle) about 2 months ago, just before I replaced the battery. It was 13.0 or 13.1 volts (can't remember).

Is it possible the alternator was charging at 13.0 volts then and 14.0 volts now? If so, I have an intermittent charging problem.

Or could the "supposedly bad" battery have caused the charging voltage to be low at 13.0 volts somehow (can't see how this could be the case, since I was testing voltage at the battery terminals...)

I'm going to pull the alternator and swap the voltage reg/brushes next weekend. Other than that and the wiring harness, is there anything else that could cause the symptoms I mentioned? The only other thing I can think of is replacing the entire alternator but let me know if there is anything else I can check before I get into that.

BoomInTheTrunk · #7 03-24-2008, 06:33 PM

sounds like the diodes inside the battery are bad and it is lossing charge over the night. with the car off see how many amps are running through your main battery cable. hook up a amp metre in series with the main power wire on you battery if you have amps then you are loosing a charge somewhere. My guess is that the diodes in the alternator are bad.