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Hi all, newly registered but have appreciated much help from previously covered forum posts over the past couple of months.

Bought a sorta neglected '76 115 chassis 300D a few months ago, and have been doing my best to keep it on the road. I'd rate myself advanced beginner/intermediate with working on cars in general, *absolute beginner* with diesels. So hope you'll bear with some stupid questions...

Car left me stranded in a parking lot in the rain today. Second time. After first time a few weeks ago:

Battery -> New installed
Alternator and voltage regulator -> New installed (old one wasn't getting more than 8 volts to the battery)

Got jump start today, drove home, power available was so low it'd barely turn the windshield wipers. Also noticed a new buzz coming from a silver box relay under the steering column. Don't know if these two things (car won't start and buzzing box) are related. From picking thru some online parts catalogs, I'm thinking this box might be a glow plug relay. Has numbers: SK 9160, 000 545 9032, and "12V max 60A." It's bolted with a bracket under the steering column area. And last piece of evidence, it was warm to the touch -- not hot, just warm.

So the questions;

Is there anything other than a short that would cause this good battery in conjunction with new alternator/voltage regulator, to lose it's charge?

Where do I begin to look for a short? (i'm afraid of the answers to that one...) Is this [glow plug relay?] a suspect?

Thanks for any help.
 

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In general do the following.

Put a volt meter across the battery terminals. You should have full battery voltage. Turn key to start. Voltage reading should drop slightly & car should start. A significant drop means that your battery is discharged. If so, Charge & take it to a parts store for testing. The machine can tell if all cells are good. New does not always = good. If car starts, you should have 14+ volts across the terminals. If not make sure all connections between alternator & battery (including battery terminals) are clean. If that doesn't fix it, have alternator bench tested. Parts store test for free but watch process so you're comfortable with quality of test. (Idiot may be behind counter.) A good wiring diagram would help. I'm still looking for 1 for my 85.
 

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If above checks out, now you will need a diagram. Disconnect negative battery terminal & put a 12 volt test light between terminal & post. With everything off, test light should be off. (volt meter will always show voltage because of clock & possibly other items) Don't forget to close the doors so interior light is off. If not, pull fuses until light goes out. That is the circuit with the problem. Start at far end of circuit & disconnect loads until you find the culprit.
 
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