So this is the deal, I have a 79 sd. When I jump start the car and its running. I disconnect the leads from the battery. At that point I put a ohm. meter to the positive and negative. It reads 00.0 should it read around 14.0? When I test the battery it ranges from 9.0-12.0 volts.
Would this be the alternator causing this problem of a dead battery?

Thanks!!!

Don't disconnect the leads when you're testing the alternator. Whatever you read off the battery while the car is running is what your alternator is giving you.

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'79 300SD
'82 Chevy Chevette diesel

NEVER disconnect the battery cable(s) with the engine running. That is a good way to fry the alternator and most of the electronic parts in the car.

thesst is right, the voltage at the battery with the engine running is what the alt is putting out. Anything between 13.2 and 14.1 is okay.

Yup, read the voltage across the battery with the engine running and the alternator connected. Let us know what you get.

Ok, Thanks! I'm going to go double check.. so if it reads 13.2 - 14.1 im good and its the battery? I'll go try and see what it says.. I'll let you guys know..

Thanks!

How old is the battery?

Your meter needs to be set on volts and not ohms to get the correct reading. If you are reading ohms, I believe the resistance should be pretty low.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

So when the car was first started it read around 12.5 volts. After I accelerated the engine a little it dropped to about 7.5. The battery is a few years old. I put new brushes in the alternator about two months ago. Could this be related or just time for a new alternator?

Before anything else try to double check your readings. What you have posted that the battery drops to 7.5 volts after starting does not sound right to me. Especially if you had 12.5 volts a minute or two earlier.
I would start the car and disable the glow plug circuit to make sure it is not staying on. It takes quite a bit of current flow to drag a battery down to 7.5 volts that fast. Something just does not sound right.
Did you have the 12.5 volt reading before you started the car with the battery cables hooked up?

My Glow plug light on the dash would go on for a few seconds every now and then. Then it would go out.. Before my current issue with the battery the glow plug light was on more than usual.

Sounds like your alternator. Your battery may or may not be fine. If you have another working car, do the same test with that battery. If the results are the same then it's either your alternator or the wires going to/from it.

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'79 300SD
'82 Chevy Chevette diesel