I have been over on the Benz world forum and there is a guy with a W123 that said he keeps frying alternators even when they are replaced.

I can't remember if a bad ground would cause his issue or not. If you have a reason post here and I will post the answer on benzworld or go to his thread and comment.

https://www.benzworld.org/threads/alternator-woes.3041438/

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84 300D, 82 Volvo 244Gl Diesel

The alternator itself can’t lose ground, since the case is bolted to the engine at more than one point. What can happen is an interruption of the charging circuit, either due to a bad battery ground or a loose B+ connection. When the charging circuit is opened suddenly, a huge inductive surge can form in the stator, causing a little lightning storm. That’s why you should never use the stupid dog trick of undoing a battery cable while the engine is running to test the alternator.

What Frank said ! .

If you don't have the proper equipments to test alternators you can disconnect the ground cable with the engine running above idle but you MUST NOT RECONNECT IT UNTIL YOU STOP THE ENGINE as the resulting spike of charging current will likely do it in .

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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

The OP in the thread over on BW says it's the same alternator that keeps cooking. The question is what is cooking on it? Diodes? Stator? Rotor? Regulator? Since it's the same alternator failing over and over again, it could be a combination of bad slip rings and a cheap/generic regulator. Burn the regulator out, it runs away, burns up trying to overcharge the battery.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Typically this will blow the diodes to open circuit allowing AC to enter the system......

On older generator system the surge would cook the cutout contacts on the voltage regulator leading to over charging and "motoring" of the generator when the engine was shut off .

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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

" I've had it repaired by a reputable vendor at least 3 times now." I suppose I am assuming that the reputable vendor tested it before giving the alternator back.

If it was repaired properly would that not be the same as a rebuilt alternator.

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84 300D, 82 Volvo 244Gl Diesel

Yes it can lose the ground. If where the Alternator Bracket mounts to the Engine becomes rusted enough you won't have a good ground.

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84 300D, 82 Volvo 244Gl Diesel

I forgot there is another place you can have a bad ground. I read about this when someone powder coated their alternator housing. One end of the Voltage Regulator is grounded where one of the screws goes through the alternator. I assume corrosion would do the same. See attached picture.

Then there is that springy contact finger inside of the Alternator that has to make good contact with the ground strap. see the pic of the attached corroded one.

Attached Thumbnails

   

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84 300D, 82 Volvo 244Gl Diesel

I don't know about the Motorola-style alternator in my 300D and GM's, but in my old Chryslers w/ external voltage regulator (Vreg), if the Vreg doesn't have a good ground, it will sense the output voltage low and cause the alternator to put out excessive voltage. That is common in the factory wiring since they just used a sheet-metal screw (now rusty) into the body for the ground. I don't know that kills the alternator itself, but makes the headlight burn bright and short, and other electrical issues. Use a cigarette-lighter voltmeter to monitor your running voltage.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Most of them have Bosch Alternators.

We know from that from the mod in this thread that increasing the resistance to ground increases the output voltage on the type of Bosch alternator that is on my 300D.

Bosch Alternator Regulator Modification with diode to increase resistance

http://www.detomaso.nu/~thomast/alternator/

More on Alternator Mod with diode

http://www.peachparts.com/shopforum/showthread.php?p=2427184#post2427184

So it is not a stretch to imply that some other reason to increase the resistance to ground would cause the same increase in output voltage. However, I am guessing there has to be some current flow.

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84 300D, 82 Volvo 244Gl Diesel

He is not overlooking a bad battery?

I've experienced a bad ground at the regulator causing it to not charge. Basically, it was slight corrosion of the housing. It kept occurring until I dabbed it with a bit of conductive grease. Since the alternator is mounted low, it may be the cause of many "failures".

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Sam

84 300SD 350K+ miles ( Blue Belle )