I have a good(tested) alternator(internal regulator), and a new battery. I've put the tester on the battery after I start the car and the voltage remains the same. 12.4v thereabouts.

The idiot light comes on before I start it, then goes off after the car is running.

Any ideas why I might not be getting charge from the alternator?

Isn't that suppose to be what should happen - light goes off after starting?

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if you are at 12.4 after start, what's it sit on before the start? how long after the start? the glow plugs can run for 3 minutes on some cars, that would drop the alternator voltage.
my bet is the alternator is not working or wired correctly.

Just tried it again...12.6 before startup, 12.50 after startup, and 12.54 after I shut it off again. This is after running it for 4-5 minutes.

well, if you verified the alternator was working, then it's got to be wired wrong.

Just took a five mile cruise....got home and it was 12.48 while running, and then after I shut it off, it went to 12.54. How could it be wired wrong, when it's the same plugin for the alternator? And a few months ago it worked fine for years?

Thanks for your input!

d

could be a bad wire, could be a bad alternator, even though it was tested. the batt light on the dash is coming on, so that's there, but something is not working, I'd have the alternator tested on the car.

how was the alternator tested? You mention regulator in your post, but there also brushed that wear out on the alternator which will cause it to stop producing electricity. Sometimes the brushed wear out and get stuck, i would try to band on the alternator and see if that does anything. Another thing you can try to test the alternator is that once you start the car you can disconnect the positive terminal on your battery and see if you still have power in the car(check something like the lights, or something power electronically). The engine will still be running even if there is no power in the car.

sorry for the typos in the previous post, but i meant to say to band with a hammer on the alternator.
Also on the last part. With the car running and the positive terminal disconnected and if there is power then you can conclude that the alternator is working properly.

in both previous posts, did you mean to Bang on the alternator?

make sure your ground strap is tightened. Take a wire and run it from the engine to the - post on the battery and see if that fixes it

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1981 300SD 512k OM603

yes i meant bang. Sorry for the typing error. Havent been getting much sleep lately. Also the banging on the alternator would be a temporary fix. not something i would rely on for extended period of time.

There is more than a few things that can be checked.

Pull the Alternator Connector/Plug off. The 2 large slots in the Connector fed by the 2 fat Read Wires should have the same voltage as your Battery has all the time.

With your key off the smaller slot in the Connector should have no Voltage. When you turn the key on you should get slightly less than Battery Voltage. The Blue Wire circuit is the one that goes to the Instrument Cluster No Charging Light.

After that Remove the 2 Screws that hold on the Voltage Regulator and pull the Regulator out. The Alternator Brush Holder is attatched to it. You can inspect the Alternator Brushes. Minimum protruding length for the Brushes is 2mm.
The importance of the Brushes is that I have read on this forum that when one Brush is worn the No Charging light will act like it is charging but actually the Alternator is not charging.
Also there can be no Oil on the Brushes or the Slip Rings inside of the Alternator.

Next your Voltage Regulator uses the ground to control the charging on the Alternator. If you have a poor ground it causes problems. While the Engine is running try connecting 1 Jumper Cable from the negative Battery Terminal and the other firmly touching the outer Aluminum casing of the Alternator Housing and see if it starts chargine. If it does you will need to clean all the Battery Ground connections Including the Engine to Chassie Connection.

See if any of the above turns up a problem area.

An other option is to remove the Alternator and take it to a parts place for a free testing. This would eliminate any grounding or wiring issues as the problem.

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X2 on the battery ground connection.
I would expect that by now your battery would be dead if there was no charging at all.
Have you disconnected the battery with the motor running? Its a quick way to destroy the reg.
Most alts will run without problems for years & then fail, just because it worked before, isnt a reason to say its not faulty now.

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Thanks everybody, for the info. I'll be trying those things. One thing, I was wondering where the 'engine to chassis' ground is? I only find the one from the negative post, to the fenderwall.