Battery light has been on since I have been driving her for about 9 months now. Replaced the voltage regulator about 8 months ago, old one was shot, but light is still on. With winter just around the corner I would like get this taken care of.

Tested at the battery.....

Engine off = 12.54
Engine @ idle 13.52
Engine @ 2k RPM = 13.92

From what I have read these numbers seem to be in-line, please correct me if I am mistaken. Would you just replace the alternator or does the problem lie somewhere else?

Thanks!

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'87 300D, w124, 171k
'82 300CD, w123, 200k or so. (sold)
'82 300TD, w123, 168k (sold)
'81 300D, 118k, w123 (sold)

You have a problem with the main leads from the alt to the battery. I have had this problem and ended up installing a new main wire from the back of the alt to the battery. Its the thick wires that are the problem or it may be the contacts in the connectors.

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

run a booster cable from the alternator to the battery, both pos and neg. If the light goes out, you have a cable problem.

Is the light on bright or dim? it might just be a bad diode.

I had a dim light on my 300TD for a while, it ended up being the alternator. I replaced it, and the dim light went away.

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Also check the ground strap between the engine and the chassis. Remove it, make sure the terminals, the bolts, and the places they screw into are clean and free of corrosion.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

If you have a bad diode the voltage can still be ok but the light will come on. Check for a/c voltage at the junction box (the battery smooths it out), if you have more than .1vac then it's time for new diodes.

Hi, thanks! I was able to check last night. Put the meter on 200VAC setting and was getting .24. Does that sound like it is not concerting all of the energy to DC? Does new diodes = new alternator?

Thanks again.

__________________
'87 300D, w124, 171k
'82 300CD, w123, 200k or so. (sold)
'82 300TD, w123, 168k (sold)
'81 300D, 118k, w123 (sold)

An AC reading of 0.24 volt is low so the diodes are probably OK. They are built into the alternator so yes, replacing them means replacing the alternator. Your numbers in the first post look OK. I think the alternator and regulator are good.

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

How about 24.7. VAC?

__________________
'87 300D, w124, 171k
'82 300CD, w123, 200k or so. (sold)
'82 300TD, w123, 168k (sold)
'81 300D, 118k, w123 (sold)

24.7 means your diodes are blown. If you can find replacements, diodes are replaceable, but they are internal to the alternator and if you don't have some heavy duty soldering equipment you won't be getting them out and in. I've replaced them on a few alternators, but on domestic vehicles so I'm really not sure on parts availability for the Mercedes.

Thanks! I pretty much knew the answer, but just wanted to make sure. Thanks for the education on the matter! Guess I will be investing in a rebuilt Bosch.

__________________
'87 300D, w124, 171k
'82 300CD, w123, 200k or so. (sold)
'82 300TD, w123, 168k (sold)
'81 300D, 118k, w123 (sold)