Mystery Electrical Problem - Mercedes-Benz Forum

 
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post #1 of 5 (permalink) Old 05-30-2008, 05:32 PM Thread Starter
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Angry Mystery Electrical Problem

I have a 1981 300TD which I just bought sight unseen, but I had a mechanic do a inspection and he said it was solid. Long story short, the car doesn't seem to charge the battery unless I'm on the highway, if I drive a few days around town, especially with the lights on, it will get weak and then won't start w/out a jump. I replaced the alternator, the a battery (which apparently wasn't right for a diesel) and still, after every jump start, like clockwork a week later, rrr rrr and it won't start. VERY aggravating. The mechanic, says that the wiring is aftermarket and that it's the cause. He also says the new battery isn't good enough, but the one he wants is 750cca, whereas the one I have is 700, so I don't think so. Is it the wiring really? Why does it only charge at high speed? Thanks for the help. -Alex
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post #2 of 5 (permalink) Old 05-30-2008, 06:33 PM
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What wiring is aftermarket? The wiring harness? The battery cables? Are you sure the alternator you bought is the correct one for the car? I don't have any experience with diesel batteries, but the amperage would have to do with starting power I would think, not whether it would keep a charge. The electrical system uses the battery to start the car then generates the power to keep everything working--that's why it sounds like the alternator isn't putting out enough charge to power everything and the electrics in the car are draining the battery because there isn't enough power coming out of the charging system. That's the reason for my alternator question.

"Don't pick a fight with an old man. If he is too old to fight, he'll just kill you."--John Steinbeck
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post #3 of 5 (permalink) Old 05-30-2008, 06:56 PM Thread Starter
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Alternator..

Well, actually the old alternator wasn't original and I had it rebuilt and replaced it. The mechanic checked that it was generating enough power though so that's why I didn't think it was the cause. Could it be wiring, or is that really unlikely. Oh, and I think it's the a battery cables that are aftermarket. I agree that all the signs point to the alternator: slow loss of charge, only charges on the highway, etc... I would just hate to fix that AGAIN and still have the same problem. Thanks for your input.

-Alex
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post #4 of 5 (permalink) Old 05-30-2008, 07:09 PM
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If it's not the charging system, then you could have a closed switch or a short somewhere that's draining the system when the car is supposed to be asleep. That will be a scavenger hunt, I suppose, and I don't have any idea where to suggest you start. Maybe if you turned the car off, gave it a while to down-cycle and then listened for running fans or hunt for live circuits. I wish I could be more help.

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post #5 of 5 (permalink) Old 05-31-2008, 10:14 AM
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To begin with...

I am wondering if the battery was ever fully charged.

Try getting a trickle charger and bring the battery up to a full charge and then see how things go. The alternator on modern cars is not really intended to bring the battery up a full charge from "dead".

Also, is the alternator the correct one for the car? What is the rating [look in the owner's manual under specifications] and compare that with the information from your mechanic.

Lastly, you can check for a drain on the electrical system. With the ignition off, connect a VOM/DVM in series with the battery and the positive cable; then one by one, remove the fuses and then the current flow drops to zero have have found the circuit in question, then the detective work begins...

If your are unclear on any of this, then let the tech deal with it... no sense messing up the electrical system of your car.

Good luck,

JR
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