Vehicle: 98 ML320, 110k miles.

I rebuilt the alternator last year, new voltage regulator, new bearings, it was all good till this week. the charging indicator is on while engine is running. Test voltage, and shows only 12.3V with engine running. So I thought the regulator was bad, replaced with a new one, and still no go.

Does this mean the rectifier diodes are gone, and I need to replace the whole alternator? I can find remanufactured Bosch alternator that fits the ML. Is this pretty much the route to go?

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98 ML320 (gone but not forgotten)

If you remove the "small" field wire from the alternator & check for it voltage with the key ON & engine OFF,
Do you have voltage there??

IF yes, then yes you need a complete alternator.

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MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Doc,
Let me get this straight,

check voltage of the small field wire after it is removed from the D+ terminal?

or

check voltage of the D+ terminal itself?

or

the voltage across D+ and B+ terminals?

base on what I read, voltage across D+ and B+ terminals should be zero. Is that what you meant by check the voltage there?

thanks.

steve

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98 ML320 (gone but not forgotten)

If you remove the "small" field wire from the alternator & check for it (the wire) FOR voltage, you should have 12V there.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

with key on and engine off, there should be voltage going to that wire, how does that determine the alternator is bad? I am not following the logic.

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98 ML320 (gone but not forgotten)

I replace the alternator with a remanufactured bosch (actually remanufactured by REMY). it is generating 13.6-14.2V now. Life is good.

Thinking back, after I replaced the battery 2 weeks ago, I checked the charging voltage and it only showed 13.1-13.2 V. That's probably a tell tale sign that the alternator is dying a slow death.

Lesson learn: If the alternator is already 11 years old with 100k miles on it, don't bother rebuilding it on your own, it is more cost effective to go get a remanufactured unit. The rectifiers will die too and the rectifier assembly is not very easy to source.

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98 ML320 (gone but not forgotten)