when is an alternator considered bad?

I just changed my W123's battery. The voltage of the batt is okay at 12.4-12.5.

but with the engine at idle, the voltage drops to about 12.1! And on revving, it goes up to about 13.1.

Is the alternator is on its way out? has it anything to do with the voltage regulator? thanks.

This does sound as if it's weak. However, a fully charged battery will not show a voltage as high as one that is down. If the battery stays charged, I'd say you're okay. Keep an eye on it. Is this a high mileage alternator? If so, it may voltage regulator/brush assembly time.

If you resort to a new alternator, you will be money ahead in the long run to have a Bosch certified rebuilt one. I've tried all kinds, and these are the only ones that will last a long time.

Good luck,

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Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

[420SEL]

I have read that MB alternators do not usually have to be replaced. Replacing the brushes inside was enough. Ask your MB mechanic if this is true or not.



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Jason Priest
1986 420SEL

[Marc]

I have purchased the alternator rebuild for a GM truck before and done the work myself. Really inexpensive and not to difficult to do. I called the local MB dealer to see if there was a kit available for my car, they acted as if I was speaking a foreign language.

Obviously not German

thanks for your responses. will monitor how the batt holds up and decide later if i should change the alternator brushes or the whole thing.

btw, does anyone here have the part number for the brush "kit"? Its for a W123 200. thanks.

[Benzmac]

A drop in voltage is, sort of, normal. The reason for this is the low idle. You can only check an alternator with an amperage check. If the amperage is low, replace the alternator.

You can try a set of brushes (regulator), but when they fail, the alternator doesn't work at all. If it is working about 1/2 way, the alternator is faulty.

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Benzmac:
Donnie Drummonds
1991 GMC Syclone
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

what kind of amperage is considered low for the alternator? thanks again, for all the invaluable responses.

roland

[stevebfl]

Your alternator should have a rating label on it. US cars had a couple different alternators on different 123 body cars. They usually were rated 55 or 65 amps. These actually were real POS alternators. They commonly only produced 50% of rated power at idle. If the A/C is on the blowers alone take 30amps or more. Many MB models through the eighties had the same problem.

They are now using alternators with outputs over 100amps and they get 90% of rating at idle.

The brush number from Bosch is either short # 11 011 or long number 1 127 014 011. We never replace just the brushes as they come attached to the regulator and with extra labor you might as well replace the regulator bruch holder.