Dead Battery: Alternator or Voltage Regulator? - PeachParts Mercedes-Benz Forum

burntcloth · #1 02-21-2011, 02:47 PM

Hi,
my 1982 300TD battery died.
I charged it back up, and found that the voltage is low when it is running (12.8), and under load (lights, etc.): 12.14.
So, the charge isn't keeping up.

The belts to the alternator were changed a month ago. Maybe they knocked loose something, so I'll check connections.

I know that auto stores check alternators, but will they check it on the car, or does it have to be removed?

Also, I've heard of just replacing the voltage regulator, which is a common culprit. Do I have to remove the alternator to remove the VR?

One problem I have: recently moved, not so many tools with me. I'm sure it's still cheaper for me to go out and get what I need to fix my problem, but I sure don't want to get more than I need.

Thanks!

polarisrmk · #2 02-21-2011, 02:51 PM

90 % chance its the voltage regulator brush assy. Bosch part is about $45 replace it and i am sure it will be fixed. Literally a 10 min job

lupin..the..3rd · #3 02-21-2011, 02:55 PM

I'm not familiar with your specific car, but on most 80's and 90's Bosch alternators, the voltage regulator is an assembly along with the brushes. As polaris sais, it's inexpensive and easy to swap. It's usually the brushes that are worn out, worn down to little stumps. But since it's an assembly along with the VR, you just replace the whole assembly.

If you want to confirm, hook up a volt meter. If your system voltage varies with engine speed, this part is the culprit.

burntcloth · #4 02-21-2011, 02:56 PM

Thanks Lupin,
I didn't know to check the RPM speed to volt meter reading.

Just found this pictorial on replacing the voltage regulator:

http://www.google.com/imgres?imgurl=http://www.dieselgiant.com/mercedesvoltageregulator62.JPG&imgrefurl=http://www.dieselgiant.com/mercedesvoltageregulatorinstall.htm&usg=__NI5sucxaDl2CiytqLxoEKrcNVfE=&h=480&w=640&sz=69&hl=en&start=3&zoom=1&um=1&itbs=1&tbnid=OHj--HiEqkkVTM:&tbnh=103&tbnw=137&prev=/images%3Fq%3Dmercedes%2Bvoltage%2Bregulator%26um%3D1%26hl%3Den%26client%3Dsafari%26sa%3DN%26rls%3Den %26tbs%3Disch:1&ei=ocJiTcKZLov4swOkorDXCA

bobodaclown · #5 02-21-2011, 03:30 PM

If you bring it to an Autozone or such they can test it in the car.

Daman858 · #6 02-21-2011, 03:42 PM

1. Raise the passenger side of the car and place on safe jack stand.

2. Crawl under the car and look on the back side of the alternator. That thing with the black cap and two screws is the voltage regulator.

3. Remove screws with a stubby screw driver or 1/4 inch drive ratchet with a screw driver bit. Some are regular some are phillips heads. Pull out voltage regulator. No wires to remove.

4. Examine the two brushes. If less than about 1/4 inch long, they are worn out.

5. While your down there, shine a light in the hole that the voltage regulator was in and examine the armature shaft where the brushes ride. Make sure it is smooth and shiny.

6. Put in a new voltage regulator and install screws. A trick is to wrap a zip tie around the voltage regulator to hold the brushes down (they are spring loaded). After installation, cut zip tie and side it out. This will make sure your brushes will make correct contact and not be angled in.

7. Test your charging again. This should solve your problem.

whunter · #7 02-21-2011, 03:48 PM

Does the battery light come on when you turn the ignition on?

The instrument cluster BATTERY bulb is part of the alternator field circuit = this burnt out bulb will cause zero charge.

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