How about him checking the diodes ... is this one of the easy alternators for doing that?

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

Im not sure how to change the diodes on these, they might even be in the regulator part, usually a diode problem will give other symptoms. OReillys should have found that.

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1982 300CD Turbo (Otis, "ups & downs") parts for sale
2003 TJ with Hemi (to go anywhere, quickly) sold
2001 Excursion Powerstroke (to go dependably)
1970 Mustang 428SCJ (to go fast)
1962 Corvette LS1 (to go in style)
2001 Schwinn Grape Krate 10spd (if all else fails)

Could the glowplugs be sticking on? I know the '80 diesels have relays that sometimes stick on and can cause what you are seeing, but I can remember if the '78s have relays. Either way, when the glow plugs are on, even when everything else is working perfectly, the alternator tries and tries its little heart out, but it just can't quite keep up and the voltage sags. 12V sounds about right from the threads I've read (hasn't happened to me personally.. yet).

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'83 300DTurbo http://badges.fuelly.com/images/smallsig-us/318559.png

Broadband: more lies faster.

Does the alternator light come on when you turn on the key, before you start the car? If not, the bulb is burned out or you have a loose wire in the bulb circuit. The bulb provides the initial field current for the alternator; without it you will see either no charging or poor charging.

yep, forgot that.should always be the first question.....

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1982 300CD Turbo (Otis, "ups & downs") parts for sale
2003 TJ with Hemi (to go anywhere, quickly) sold
2001 Excursion Powerstroke (to go dependably)
1970 Mustang 428SCJ (to go fast)
1962 Corvette LS1 (to go in style)
2001 Schwinn Grape Krate 10spd (if all else fails)

Thanks for the feedback everyone. I haven't investigated the diodes or voltage regulator and brushes, but to answer some questions:

The light with the battery symbol comes on, if that's what you mean.

Good call. The positive clamp was a bit rusty and corroded on the inside so I cleaned that, as well as the battery terminals. The cable seems a bit corroded where it meets the clamp, and I suspect there might be more corrosion in hard-to-clean areas.

Interesting idea...the vacuum shutoff doesn't work, so is it possible there is something electrical tied to the shutoff system that prevents the glowplugs from turning off?

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1985 190D 2.2L 717.411 5-speed
1983 300D Euro OM617.950