ho alt burned up any thoughts on why?

Okay, I have a 1996 ford ranger ext cab. 3.0l v6 5spd. I bought a 250 amp HO alt off ebay. It worked great for the last year, but today i went to start up my truck, and my alternator literally burned up. Smoke poured out of it, and now it tests bad, and dont put out anything. i was working on a big 3 and then some upgrade. i am putting a 3000 rms amp in soon, and had added a 1/0 power wire to the alt. i forgot to hook it up to the battery terminal. Is this what caused it to burn up? It wasnt shorting to the frame, and the b terminal on the alt didnt get fried, or the wire to the battery didnt get fried either, or even hot. Any thoughts on what caused it, or what to check would be helpful, i dont wana put another HO into it, just for it to burn up right away.

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Disturbin' Tha Peace

It's common for rewound HO alternators to fry, but that's when in use.   


So you started the engine (and alternator) without the battery attached to B+ (it output)?

And is it internally regulated?
Or externally regulated and you didn't have a good regulator ground?


The only simple test is a diode test across the B+ to GND, not that that is conclusive.
Your DMM should measure (say) between 0.3V to 1.4V in both directions (swap + & - probes across B+ & GND).
If either is ~0V, you have at least one shorted diode. (However I doubt that in your case.)
If either measures open circuit, that's open diodes or windings.

But measuring the expected typical 0.6V-1.4V only means at least one (of 3) diodes appears ok - other diodes or windings can be open.
And it does not test the integrity of the copper blob... I mean, the windings and insulation (except to GND).


Usually a test (with DMM or diode tester) will only indicate that it is bad. An ok test does not mean that it is ok or capable of full output (maybe only 1 of its 3 phases is working).


But clarifying the battery or load connections at smoko time as well as its regulator location and wiring, and if it smoked upon some connection or engine-cranking & starting should narrow down the likely failure mode.

so i guess what im asking, is when you dont have an alternator battery wire hooked up, and it aint shorting, especially with a HO alt, will the alternator burn up?

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Disturbin' Tha Peace

Yes it can, but hence my questions...


With an internal regulator, it shouldn't.

But some alternators have weak power diodes that can't handle short-duration or longer high loads (eg, my experiences with Bosch alternators after disconnecting a jumper battery, or driving with a collapsed battery).

And some can lose diodes from spikes or surges when the battery is disconnected.   


PS - it could also have been a bad ground, especially with an external regulator.


POST EDIT - ignore the last line below - I reread that you have used it successfully for the past year. But I'll keep it as a warning for others....
But it could be the alternator was faulty from the start. EG - it may have had shorted diodes or shorted windings.