My old alternator was failing and replacing the voltage regulator last year bought me about one more year. But it started acting up again so I took it out. Old alternator bench tested bad. I bought a 65amp Bosch rebuilt alternator and bench tested good. A couple days ago I installed it and started it up. I was getting about 11 volts with engine idling.

I figured I could go replace my 6 month old battery for free just to be safe and did so. Today I installed new battery.

Here is what I have now:
12.5 volts from battery with car off
11.9 volts with car idling
about 12.3 volts with car at about 2000rpm
belts are correct tightness and there is no slipping

I don't know much when it comes to electrical problems, so any help would be appreciated.

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82 300D 225k

check the grounds. Engine to body, and battery to body.

Does the battery light on the dash come on when the key is in the glowplug position?

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1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg

I recently had a similar problem and it turned out to be corroded connections on the back of the alternator (inside the plastic connector). I took it apart and redid the connectors. That solved the problem. I discovered the problem when I noticed that the voltage changed is I wiggled the cable to the alternator.

A Voltage Drop test will lead you straight to the problem. Use a DC Digital Voltmeter across each link in the chain. When you come to a point where more than 1 Volt or so shows up, THAT is your bad connection. Fix it.

Turned out to be the junction box above the right wheel well. There was a loose wire in there.

Thanks for all the help!

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82 300D 225k

Glad you got it sorted out.

Turns out, it was my glow plug relay. It was failing to shut off power to the plugs, so it was a tremendous drain on the alternator/battery. everything seems to be working normally now with a new relay...

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82 300D 225k

Mine went out too this past spring/summer. I replaced it with the afterglow one from Phil. Other than the fact that I burnt out my old glow plugs when I switched from it glowing for 2 minutes afterwards, I like it. I think the old relay took out my battery and alternator. Both are new since replacing the relay and all is well for me.

What color is your 300D? I saw a classic beige one on 94 East right by the 280 north exit last week in the evening.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD