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Old 01-03-2000, 10:59 AM
Registered User
Join Date: Jul 1999
Location: Tampa, Florida, USA
Posts: 214
Last night while driving home, the headlights would occasionally dim a little bit, or flicker back and forth between being bright and dim. The dash lights, console lights, and dome lights did the same, simultaneously.

Does this sound like a failing (or maybe just dirty) voltage regulator? I checked the battery when I got home; it was cool and dry and not bulging at all, so I'm guessing that the periods of bright lights were normal voltage and the dim times were less-than-normal voltage (as opposed to the dim times being normal and the bright times being higher-than-normal voltage). It was 3 AM so I didn't spend any time trying to diagnose it with a voltmeter.

Can I visually inspect the voltage regulator for any clues, or will a visual inspection not give me any information? From reading other posts, I gather that I don't need to pull off the whole aternator to get the voltage regulator off... not that it's terribly difficult to get the alternator off and on. If I remember correctly, this alternator is one that I got from a junker about 25,000 miles ago, and it seemed OK when I had it tested.

- Nathan
'83 240D, 250k miles
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Old 01-03-2000, 11:16 AM
Aaron's Avatar
Join Date: Aug 1999
Location: Cincinnati, Ohio
Posts: 1,935


What you described is definitely a failing voltage regulator. You can see how they wear if you compare the contacts of the new one to the old one. They're not hard to get at, it's screwed onto the back of the alternator. It might be tricky getting the top screw out, so be patient and work it gently so you don't strip anything. If you can't get it out with a screwdriver, than a set of ignition channel locks will do the trick. Hope this helps!

Aaron Greenberg
MB technician
Precision Motorcars, Cincinnati, Ohio
'67 250SE Cabriolet
'77 450SL
'79 6.9
'80 300SD
'85 380SE
'89 420SEL
'93 300E 2.8

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Old 01-03-2000, 09:41 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
The screws in the back of the regulator might be very tight. They appear to be phillips, but if you look really close, you will see little dash marks on them indicating that they are posi-drive. This is a weird little phillips bit that is spaced a little different. (This is just for your info)

Do replace the regulator first before trying anything else. Be sure the belt and the alt. pulley is tight.

1981 280GE SWB
1987 16V

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Old 01-04-2000, 11:25 AM
non member
Posts: n/a
Be SURE when going in with new voltage regulator to CAREFULLY position the commutator brushes so they don't bind or
break off upon reinserting in the back of the alternator case...I had to tie down the brushes with a sring in order insert the regulator.
I changed out a VR on sunday in my 240D.
You'll find it will likely be a spring on the brush set that failed to hold tension on
the carbon brush hence not allowing good electrical continuity..I've had two VR's
fail because of faulty springs.
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Old 01-04-2000, 02:58 PM
Posts: n/a
I seem to be having the same problem with my 1976 230.4 It just started about a month and a half after I had the alternator replaced with a used one in better working condition. Let me know how it goes!


1976 230.4
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Old 01-04-2000, 04:52 PM
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Join Date: Jul 1999
Location: Tampa, Florida, USA
Posts: 214
Argh, the dealer wants $81 for a new voltage regulator. I think my dad has a dead alternator with a working VR on it. If not, I guess I try the PartsShop, or drive around with a half-working voltage regulator. Maybe I'll pull it off and see if I can clean it up or recondition it a little bit.. that worked wonders for the power window switches on the console inside.

I pulled the VR out and immediately saw the problem - the little 120ohm resistor on the back of the VR had become unsoldered on one side and broken on the other side. I couldn't solder it back in, since the wire was broken right next to the resistor itself, so I got my hands on a replacement resistor, soldered it into place, and voila`! it works. There seemed to be plenty left of the commutator brushes.

I'm getting about 12.8V across the battery terminals at idle, and about 13.7V with the engine at normal operating speed. So, I guess I'm good to go.

I love this forum.

- Nathan
'83 240D, 250k miles

P.S. Finding a replacement resistor was a PITA. I went to Radio Shack, and despite their marketing slogan of "You've got questions, we've got answers" the only help they could give me is "We don't carry it, and we don't know where else you could go to get one." I guess a 49-cent resistor isn't worth their salesman's time.. not that he was busy doing anything else. I eventually went to the Electrical Engineering department of the university I'm attending/employed by, and they were quite helpful - the EE shop guy found me a working resistor and just gave it to me free of charge. Happy, happy day.

[This message has been edited by ncarter (edited 01-13-2000).]
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