Hey everyone, I've been reading on here for a while but I've finally joined so I can get help with an issue that is driving me nuts.

I have a 1987 300D (W124 with the OM603) and recently I've been having alternator/voltage regulator issues. Last week I replaced the voltage regulator (which looked original) with an AutoZone regulator, and it worked for about 200 miles before my lights dimmed and I had low voltage again. I pulled the negative wire off the battery and tested the voltage from the alternator; 6ish volts. Not good.

This afternoon I took the regulator back to AutoZone and exchanged it for a new one. I put it on, started the car, and once again I'm getting 5-6 volts out of it.

So did I screw something up? Or are AutoZone regulators incredibly crappy, and I should try a Hella?

Thankfully it's a diesel and I can still drive with no voltage, but electronics issues make me wish for the sweet release of death.

Yes, you should try Hella but also your alternator can be dying.
I'm driving with an original one from 1987 and replaced the regulator because of the same issue (dim cluster lights, multiple lights on).
The issue came back twice after the replacement for a short while, went away immediately after revving the engine. I haven't checked the alternator with any tool, but I got an opinion that it is reaching its end of life. So I'm driving with a reman spare in the trunk.

Could be the alternator, they don't last forever. Try to sand the commutator with 600 grade sand paper, blow the dust off with compress air or solvent. Reinstall the voltage regulator and check.

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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.

You did not mention the Battery being recharged.

It is possible to jump start a vehicle and the vehicles batter is in poor condition.
Fully charge the battery and get it load tested.

If your battery (or the connections are poor) is not good the alternator is not going to work properly.

Also what was the reason for removing the ground cable?

Note, I had a battery that had 12 volts but not much amps. It started and ran my volvo diesel but at my next stop and re-start nothing. As said that volt meter showed 12 volts but when I used a jumper wire there was only a tiny soft spark indicating not much amps.
I walked across the street to an Autozone and bought another Battery and that fixed the issue.

It does not happen often but batteries can develop issues inside with the plates or simply reach the end of life.

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84 300D, 82 Volvo 244Gl Diesel

Thanks for the comments.

Is there anything besides the alternator or regulator that could cause me to get only 5 to 6 volts? The reason I unhooked the battery ground was to check alternator output with my volt meter.

Diesel911, are you saying if I recharged my battery the new regulator might be fine? It's low now and I did have to jump start the car.

Also the battery is new; replaced in January 2020. OReilly tested it and said it was good. And the alternator seems to be in great shape when I look inside it with a light while the regulator is removed.

The only other thing that I've seen cause a low voltage charge to the battery is having a massive discharge from the glow plug relay not timing out and NOT shutting off, but if you're measuring that right off the alternator I would be looking to just replace it.

Removing the battery ground very likely screwed something; it causes a large voltage spike. Even if nothing was damaged, the voltage reading of an open circuit will be inaccurate; the reading must be taken with a complete circuit, under load.


A fully charged battery is necessary to obtain a correct reading of the alternator charging voltage.

If you have a volt/ohm meter you don't have to do that; you just touch the appropriate probes to the batter terminal.. If you are using some sort of volt meter you mount on the dash board you might have to do as you did.

You might as well remove the alternator and take it to be bench tested. If the alternator tests good and you charged the battery and have it load tested and that is good your issue is in the circuitry/wiring to include the battery to chassis ground and the chassis to engine ground and cables.

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84 300D, 82 Volvo 244Gl Diesel

Make sure the belt is tight.

I think you have a diode or 2 blown.

FWIW, I just replaced my voltage regulator with Meyle and it works great. It's a good inexpensive option, at $15ish.

You did not say you recharged the Battery before installing the newest voltage regulator.
You don't need to remove any Battery Cables to test the charging or battery voltage.

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84 300D, 82 Volvo 244Gl Diesel

Checking for a bad diode.
AC voltage across battery
VAC across battery should be under 100 mV; excess AC voltage on the DC line means one or more of the rectifier diodes in the bridge are leaking or shorted. Such a problem is relatively rare but unfortunately means replacing the entire alternator. (unless you can replace the diode yourself)


Another description:
How to test an Alternator Diode
If the car is running do an ac voltage drop test from the positive on the alternator to the positive battery terminal. there should be less than 0.1v ac. if there is greater than 0.1v that would indicate a failed diode.

Another way is to remove the alternator from the vehicle and disassemble it, exposing the diode trio. using a multimeter on the diode setting, test for continuity on each side of each diode. continuity should be detected only one way on each diode. if this is not the case on any of the 3 diodes, then the diodes have failed and are in need of replacement.

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84 300D, 82 Volvo 244Gl Diesel

You may easily have just a bad alternator. Convention is to remove it and have it tested. Usually free.

I would just get a used alternator from a salvage yard before buying a cheap chain store alternator. If your altenator tests bad. Cars do not get scrapped because the altenator is bad. Chain stores can provide properly rebuilt altenators reconditioned by the original manufactuer but they will cost you.

The chain stores tend to buy so called rebuilt alternators from Mexico. They are hit and miss for too many being bad. Or having a very short lifespan.

Remember starting a car and pulling a battery cable is not a good ideal. Nor is boosting one. You may get away with it especially on these old cars. When you do not on some cars. It can really cost you dollars.

Mikey ;

? Did you look at the commutator slip rings ? .

If they're black I'd polish them, never, EVER use coarse grit abrasives ! .

On slip rings I use metal polish, in a pinch you can use Crocus Cloth or 1,500 or finer grit abrasives .

The volt meter testing for AC leakage is a good idea, thrift and pawn shops often have good quality meters for under $50, Fluke or similar, I'm using a Fluke copy sold by Caterpillar, 20 plus years and it still works like new .

FWIW, disconnecting the negative cable isn't going to help you charging tests and re connecting it is what causes voltage spikes and blown diodes so don't do that .

You've been told repeatedly to not buy off brand regulators and now you're seeing why .

I'd not buy a non BOSCH BRANDED rebuilt because the other brands don't re epoxy the rotor windings, this means they're pretty much guaranteed to fail in a year at most .

I'm sure you can find another good used alternator locally, if not contact me off list as I have a few, I think with shipping to TEXAS will be about $70 sans regulator .

Do remove the battery cables and clean the ends with baking soda and polish where they attach to the starter and body .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better