If you are going to get a Voltage Regulator get the correct one for the Alternator you have.
The below works on the Stock Alternators.
If not, set your meter to Volts AC and you should have a reading of ~100mVAC.
If the VAC is too high, then look for a rectifying diode failure. If VAC is OK but the VDC is not correct, then the problem is with the voltage regulator.

If I accept the fact that on My W123 I need to have a Bulb (2 watts) in My Charging Warning Light circuit in order to charge correctly what would happen if I installed a Volt Meter in place of the Light?

I would check and see if the SAAB 9000 has a Charging Warning Light or a Volt Meter in it and try to duplicate that circuit.

I had an issue with a non-Mercedes Bosch Alternator and took it to PepBoys and when they checked it they gave me a little receipt sized print out; in My case on what failed on it.

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84 300D, 82 Volvo 244Gl Diesel

Great info there, thanks! So you're saying the VAC test would only work for the original Alternator, but that those figures may not be accurate for the Bosch AL129X I have in the car?

And I never thought of taking it somewhere to get tested. Maybe I could swap in my stock alternator for now and see if Pep Boys can tell me the component that is failing. Was that a free service for you?

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1984 Mercedes-Benz 300D

The VAC test worked OK on My Stock Mercedes Alternator and My Chevy Van. But, that is the limit of My Experience on it.

If you get a suspect reading on the Mercedes try it on another different make of Car and see if it comes out close to the same.

Testing the Alternator was free at PeoBoys out here in CA. Just call and ask them before you remove the Alternator.

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84 300D, 82 Volvo 244Gl Diesel

Worn brushes could be hopping around on the sliprings at shakey idle.
I agree check brushes and brush springs.

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Retired Marine Engineer
2005 E320 CDI
1987 300 Turbo Diesel @ 300,000 kM going
1974 240 D 300,K Miles when given away gone
1970 200 D gone
1969 190 D gone

I think this might be it. I've observed that since yesterday, there is no more shaking volt meter at idle. I did quite a bit of driving yesterday after work and I observed the volt meter now just sits at 14v even at idle and running. I remember when everything was working, it was closer to 12.5v-13v at idle, and would hit 14v at certain running speeds. I'm picking up a new V-Reg on the way home, so I'm confident that should resolve it, but we'll see.

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1984 Mercedes-Benz 300D

Is your AL129X new or used? Does it use the plug in connector or hard wired?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The AL129x is hard wired.

I think it is the 80 amp one like Eagleautoelectric sells on dBay that can use the stock Connector.

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84 300D, 82 Volvo 244Gl Diesel