I ahve a 76 300D 115, I meaured the alternator putting out around 12.8 with just the ipod and one interior light drawing. The moment I turn the headlights go on I have 0 nada zip. I put a new voltage regulator in today, and it didnt help at all. Do you just think a new 55 amp alternator will do the trick? It seems like a bad alternator to me but I figured I should double check. I am going to be running silverstar headlights, ipod/FM transmitter, and maybe the fan. DO you feel the 55 amp is adequate or is there a upgrade? Thanks much

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1976 300D 115 Blue Grey exterior 95K miles


2004 Dodge Ram 2500 Cummins Silver exterior 5"stacks etc...

were you measuring at idle only? do the same test and rev the engine. a lot of the older diesels i have seen put out virually nothing at idle

Check the wiring on your headlight switch. If you look at a wiring diagram, you'll find that the main power feed for the whole car goes to the headlight switch. I've also seen corroded headlight switches cause all kinds of odd electrical glitches.
This could also be a grounding issue.

Where was the voltmeter hooked? Something is wrong with this picture. Voltage should never go to 0.

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84 500 SEL (307,xxx miles)

I had the voltometer hooked up to the positive lead that I disconnected from the battery and to the negative lead that was still connected to the battery while it was running. It was at low idle when I last measured. Today I will try it at a higher rpm.

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1976 300D 115 Blue Grey exterior 95K miles


2004 Dodge Ram 2500 Cummins Silver exterior 5"stacks etc...

Leave the cables connected to the battery during the test.

1) Run the vehicle at idle

2) Read the voltage at the battery.

3) With the vehicle running at idle, turn on the headlights and read the voltage at the battery.

4) Increase the engine speed to 2000 rpm with the headlights on and read the voltage at the battery.


Report back with three separate numbers.