Hey guys,

So I noticed something strange in my '84 300D. Before I get into it, I should mention I have a Bosh AL129X upgraded alt in there. I put it in about a year ago and it's worked fine since then with no issue. Anyways, I noticed at idle, the volt meter needle was flicking back and forth lightly, nothing major, but it usually just sat still around 13v. Then at random it would go crazy and jump between what looks like about 11-15 volts, and it would jump back and forth and the lights would dim and brighten until I applied a bit of throttle, then it would smooth out.

It would operate fine at any speed besides idle, and sat steady between 13.5-14v. But as soon as I got to a stop and was idling, it would sit there flickering lightly, and then with no prompting, would start doing the major flickering. I was reading up on causes of this and was already suspecting a failing voltage regulator.

Then today I got in to go to work, and I noticed that as soon as I started the car, the volt meter was pretty much pegged at 14v, from idle to operating speed, it was at 14v. I can only imagine it's not good to run like that.


Do these sound like the symptoms of a failed Voltage Regulator? I've already checked all of my cables, cleaned connections, tightened all connections, etc. There is no cable that was overlooked. All connections are tight and cables are secure and appear to be functioning. All grounds have been checked and cleaned to verify proper grounding. I checked to make sure the volt meter isn't faulty, which it's not.

Also, I have one of the smaller stock Bosch alternators that I had bought just a couple months before I bought the AL129X unit. It's still sitting in my garage and has hardly any miles on it. After looking at these 2 units, it looks like they might use the same Voltage Regulator. Would anyone know anything about that?

Thanks!

-Tony

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1984 Mercedes-Benz 300D

It might be a loose connection that is noticed because the engine shakes at idle.

14v is considered normal.
Are you saying that your meter does not show voltage beyond 14v?

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85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.

Go ahead and pull the regulator, inspect the brushes.

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83 SD

84 CD

Check for loose belts. Also, check your grounds, battery-chassis and especially the chassis-engine.

I'm thinking that a failing regulator would be more evident immediately after starting. That is when the greatest load is placed on it for recharging the battery. Your symptoms sound more like something "loose".

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Sam

84 300SD 350K+ miles ( Blue Belle )

Was all over that thought. Even found my positive battery terminal loose and tightened it and thought I had fixed it. Once it started happening again, I inspected all cables and tightened all connections.

Ah, that makes sense. This will actually be my first dealing with a failed regulator so I didn't know you could visually inspect wear on them. If the brushes look okay, is it possible for another component in the regulator to fault out?

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1984 Mercedes-Benz 300D

Your diodes might be going belly up as well.
Time to throw an regulator in there. It's a 15 minute job and they cost around $40. If that doesn't fix it at least you'll know it's not the regulator!
Fluctuating voltage usually means diodes, Then loose connection, then gremlins, then...............

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'70 F100 shortbed
'82 Diesel Westy
'83 Euro 300TD
Curtlo Viper
Yeti ARC
Surly long haul trucker

I like your thinking! Excuse my lack of knowledge, but are diodes internal to the alternator? As in, if it was the diodes, it would require repair or replacement of the alternator?

Also, any word on if that regulator out of the stock Bosch alternator will fit? If so, like I said it's almost new and I can pull it off tonight. I don't think the regulator on the current alternator is actually Bosch branded, so I can't compare part #'s. However, if they are the same shape and size, would the AL129X use a different regulator since it's higher amperage, or would that not matter?

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1984 Mercedes-Benz 300D

Amperage does not matter to the regulator. I think it's a different part number than the 55/65 amp alternators though.
The diodes are internal to the alternator and require you to split the alternator and re solder them on. At that point it's time to just swap it out for a new unit anyway.

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'70 F100 shortbed
'82 Diesel Westy
'83 Euro 300TD
Curtlo Viper
Yeti ARC
Surly long haul trucker

Ah, I gotcha. Would the diodes going out have anything to do with the pegged reading of 14v? When everything was working properly, it would only show 14v at certain running RPM's. Now it was showing 14v at idle and looked like it might've gone a bit above 14v at one point. Does that tilt it more towards regulator or diodes at all?

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1984 Mercedes-Benz 300D

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 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked