Just started reading How To Diagnose and Repair Automtive Electrical Systems by Tracy Martin, who say, "Poor electrical connections are the most common cause of many electrical problems. While this staement may seem obvious, in reality, few mechanics think to look first . . ."

Funny, just read that before lunch and was digesting it. Thinking that changing the part and giving it a solid connection probably fixed most of my problems. Years ago I'd change starter, alt, bat and cables all at the same time.

Before changing out the alternator, I'd suggest cleaning the connector by scraping the blades on the alternator with a screwdriver and squeezing the female terminals to increase contact tension. Also snug up the large nut (17 or 19 mm wrench?- can't remember which) on the starter motor (disconnect battery terminal first). Most alternator charge wire goes to the starter before going to the battery.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Look, this is relatively easy. The D+ connection provides voltage to the VR during engine startup. It does this by providing the ground for the instrument dash light (the light and VR are in series). Once the alternator is running, it self-energizes D+, and so the light will have the same voltage on both leads, and will be dark. No magic.

If the bulb is lit, it's because the internal voltage produced by the alternator to power the regulator is less than the voltage at the B+ terminal. There aren't too many things that can cause this:

1) Slipping belt
2) Carbon buildup on the slip rings
3) Corrosion on the B+ or D+ leads
4) Bad ignition switch (really rare)
5) bad regulator
6) A shorted battery cell
7) Bad diode in the alternator
8) Open or shorted winding in the alternator.

And that's it. Don't bother taking apart the dash to clean grounds or resoldering traces. Ignore silly stuff like "blackout diodes". Don't look for a zener (unless this is a British motorcycle). A lit alternator bulb is ALWAYS indicative of a problem, period.

Since you've eliminated the regulator (and the brushes), try cleaning the slip rings (the copper rings that the brushes ride on). Try cleaning the alternator terminals. Charge the battery and make sure it holds (take it somewhere to test if you can). If none of those things work, you only have two choices: slaughter a chicken and consult the entrails, or replace the alternator.

Did you clean/check the commutator inside the alternator when you replaced the Voltage Regulator? I normally send it down with 600+ grit sand paper and blow it with compress air. It is normally the VR problem when the battery light is dimly lit. Your old VR shows the brushes are not of equal length and badly worn.

I would at least try cleaning the commutator, replace the alternator before checking ground, in that order. I believe grounding is a red-herring all the time.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

So I thought I found the problem, but I guess not. Interestingly on a very mild winter day yesterday the battery light wasnt on or visible. I had played with the white cloth covered wire that goes from the alternator to what looks like the terminals fastened to the wheel well. What is this wires function can anyone say?

Theres was a poor connection done as an accessory is tapped into that wire for power. So I took the connection apart and soldered it. Taking it apart I could see a thin layer of greenish corrosion on the connection but also seemingly around the wire. Its a good connection now.

At idle with a multimeter connected to the battery terminals I get about 14.1V As soon as I turn on a the heater fan, it goes down to 12.7 or so. Is that normal?

After pulling out of the Garage I noticed the Battery light was dimply on again, Its much colder today, but I dont know if that has anything to do with it.

I have a 1981 TD and I have the Same problem, with the break light very dimly lit as well...

I took out the old Voltage Regulator, and the little wire with the electrical component (?) fell out. I don't know what this piece is, so Ijust replaced it with a new VR...

I have a brand new battery, and new ALT belts as well. I just had the ALT off to put in the new VR (cause someone used bolts to put the old one in, and screwed it all up). Whilst I had the ALT out, I lightly sanded the 3 male connectors on the back with 600 grit and put a light amount of dielectric grease on them. Put the whole assembly back together and still have a dim light!!

I did check the voltage with a not so accurate test light and at first the voltage improved.. but looks about the same now as it was... hmmm!?!? I gotta test it with a true volt meter to be sure tho...

I think the 3 screw terminals on the wheel well are suspect.. they are a bit corroded.

I have the same model and year you have, I measured my battery voltage with engine running, it was 13.6 volts I changed the regulator and now a perfect 14.2 and it thing the lights are a little brighter. I have only had the car a few weeks so it may be my imagination. The module is cheap and easy to change you might want to have a 90 degree phillips on hand, I didn't but a needle nose vice grip pliers got one screw that was being interfered by a hose.

I am not sure that the brake warning light would subject to the same problem. Those clusters can be subject to grounding issues and to breaks in the circuit traces from being handled (re/re).

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Hmm ya that could be it..

I just had the instrument cluster out last week and switched the gauges into a cluster that had the plexi glass.. mine was missing...

While I was in there I soldered the dimmer. It was nice to see the instrument lights start to work!!

But the problem existed before and after the swap.. so could be a trace.

Hmmm