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Old 12-22-2003, 10:34 PM
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Zeus Zeus is offline
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Join Date: Feb 2000
Location: Canada
Posts: 1,774
Question Charging help - 300E - alternator OK?

I've had a few weird symptoms lately that have caused me to worry about my charging system in my 1989 300E. My windshield washer jets have lacked power intermittently and the horn seemed weak a few times. I've been doing lots of city driving lately and it's snowing here so lots of defrosting/rear defrost, etc.

I took the car to an independent garage today for an alternator check and they said it needs to be replaced - they said it was not putting out enough current. I was skeptical and I pulled the multimeter out when I got home from work and here's what I found...

Battery reads 12.96V at rest, engine off.

With engine running at idle, NO LOAD, RPM 600, battery reads 13.74V.

With a pretty FULL LOAD (lights, defrost on high, rear defrost, stereo), RPM 800-900, battery reading goes down to 12.46V.

Kicking the throttle up to about 2000 RPM, with the same FULL LOAD, the battery reading went up a notch to 12.57V.

I pulled the voltage regulator and the brushes are ok. They're worn but both surfaces are clean and are definitely making contact. The voltage regulator itself was very dirty though and I cleaned it before re-installing. I'm done for tonight, so I haven't driven the car yet. I checked the readings again and with the engine running at idle, 600 RPM and NO LOAD it was slightly higher - 13.96V.

With these readings can anyone tell me if my alternator is OK? Could these intermittent symptoms be related to the fact that it's a stock 70A alternator and at low RPMs it is just not putting out enough current (amps) to handle the load and hence the drain?

I guess the voltage regulator could be ok, but something inside the alternator could be worn?? Diodes?

Any help would be much, much appreciated before I drop big $$$ for a new alternator!
2007 E550 4Matic - 61,000 Km - Iridium Silver, black leather, Sport package, Premium 2 package
2007 GL450 4Matic - 62,000 Km - Obsidian Black Metallic, black leather, all options
1998 E430 - sold
1989 300E - 333,000 Km - sold
1977 280E - sold
1971 250 - retired

"And a frign hat. They gave me a hat at the annual benefits meeting. I said. how does this benefit me. I dont have anything from the company.. So they gave me a hat." - TheDon

Last edited by Zeus; 12-27-2003 at 11:09 PM.
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