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Discussion Starter #1
Backround - 69 Cruiser with GM Crate small block, painless wiring kit, optima battery, 3 wire 90 amp alternator.

OK I determined the last alternator was bad, so I snagged this 90 amp unit from a caddy in the local yard. Works great, charges like a mad demon on crack. Yet when I shut her off for the night and come back in the morning, the battery is dead. WTF?? ASSumption? The alternator is sucking the battery dead while the motor is off.

My buddy tells me that I can get a plug with a diode that will not let the power go back into the alternator. OK that's great, but I don't really want to buy a whole plug and use it because I already had to tear the other one off to fit the mounting....(long story)

So my question is this, how much "diode" do I need to handle this set up? Which wire does it go on? Anyone??
 

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again, a long shot.
A diode is like a one way valve for electricity. About .50 at radioshack?
 

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dr_claw said:
again, a long shot.
A diode is like a one way valve for electricity. About .50 at radioshack?
Yeah that makes sense to me. But how much "diode" do I need?? The nerds at radio shack talked circles around me and kept saying "Oh man, a diode for a car charging system, that thing has got to be huge....."

ah fawk it, i'll go buy the plug and cut the diode out of it. hahaha
 

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It is hard to imagine what kind of external diode you would need: With the key off, there should be no current to the any of the alternator terminals, the B+ terminal, the field F terminal or to the idiot light terminal (L?). If you "punched through one of the internal diodes for your stator, that would be a big drain. You can check the drain from all of these wires by just hooking an ammeter is series with them. If you need a diode in the field circuit, it will need to hadle at least 10A with a PIV (peak inverse voltage) of at least 30V. More won't hurt you or cost much. The internal diodes handle about 60A each and have a PIV of about 300V.
 

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Glo -- do you have a decent meter? i.e. FLUKE, ect. Might want to measure the current draining from the battery to get an idea. Check this when the engine is not running. Oh, most meters are rated for 10 Amps... with a fuse inside, yes I've popped more than a few fuses. :)

I've had diodes (internal in the alternator) fail in a variety of ways. Common is to find a "leaky" diode, but the leakage is something like 3 Amps. Not a dead short, nor an open, just drains the battery down overnight.

Had one alternator that worked fine... but discovered when it was running it was HOTTER than hell! One of the windings was shorted out to the case. So when energized it would dump that current right to ground. Finally got hot enough to melt the solder (1100F) off the wires, then I learned why it was so damn hot to touch!

Don't own a current meter? Well, make up a cheap one from a LIGHT BULB! Just use a tail light bulb (twin filament). Hook up both filaments. They take around 2 Amps of power to light up.

Disconnect the battery cable. Hook the light in series. With everything OFF it should not glow at all. If it does, you have a current drain someplace. Nice thing about the bulb... it limits current, so you can trace out wiring problems without popping fuses or burning up a wiring harness (my brother kept popping fuses - so in his wisdom - wrapped a fuse in tin foil so it wouldn't burn out. I should have let his car burn to the ground, would have saved me lots of time!).

Tom :usa:
 

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Discussion Starter #7
Thanks everyone for the advice. I stopped and got me a nice meter on the way home last night, and a couple diodes too. Just in case.

After installing the diode and checking the current drain, as described above it was at zero. Then this morning I was able to fire her right up. :D Hell I would have drive her to work if I had a windshield. :D WHOO HOOO
 
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