interesting phenomenon last night testing electrical

we were doing some ground testing around the car- still chasing down Coyote issues...

my son used a multimeter between the battery ground and the alternator case - it measured ~15v ?!
we tested the wires to the alternator and the pin read 12v from battery

we disconnected the battery, even disconnected our extra fuse panel, tripped the 150A breaker between the alternator and the battery - and eventually the 'charge' went away

we started the car a few more times, but never saw that 'charge' on the alternator case again

what could cause such a thing to happen?

That is strange. One would think the alternator case to be grounded (common) to the battery negative, assuming the battery negative to be connected.
Your negative cable was connected, engine grounds connected, and the negative battery cable connected to a point on the frame that is bare steel?

One thing about a DMM, it will detect voltages that have little to no current potential. It can sometimes detect "float" that really isn't there.

With the metal case of the alternator mounted to the metal alternator bracket which in turn is mounted to the metal engine block which in turn is grounded to the frame via a flexible wire strap then it (the alternator case) has to be grounded (provided there is clean bare metal to metal contact at each juncture of course).

Earl

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Originally Posted by BRRT

That is strange. One would think the alternator case to be grounded (common) to the battery negative, assuming the battery negative to be connected.
Your negative cable was connected, engine grounds connected, and the negative battery cable connected to a point on the frame that is bare steel?

One thing about a DMM, it will detect voltages that have little to no current potential. It can sometimes detect "float" that really isn't there.

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Originally Posted by narly1

With the metal case of the alternator mounted to the metal alternator bracket which in turn is mounted to the metal engine block which in turn is grounded to the frame via a flexible wire strap then it (the alternator case) has to be grounded (provided there is clean bare metal to metal contact at each juncture of course).

Earl

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agreed - it's the M-8600-M50ALTA that most everyone uses on the Coyote, and yes, it's mounted directly to the engine and 4 grounds on my engine to frame...

really wondering how the casing had voltage... the alternator is putting out 14.3-14.8v according to the ECU monitoring

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Originally Posted by toadster

agreed - it's the M-8600-M50ALTA that most everyone uses on the Coyote, and yes, it's mounted directly to the engine and 4 grounds on my engine to frame...

really wondering how the casing had voltage... the alternator is putting out 14.3-14.8v according to the ECU monitoring

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It would be interesting to check the potential to ground at each of the "junctures" I mentioned previously to see if one of those "connections" is poor/faulty.

Earl

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Originally Posted by narly1

It would be interesting to check the potential to ground at each of the "junctures" I mentioned previously to see if one of those "connections" is poor/faulty.

Earl

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we measured resistance from battery (-) to each of the 4 grounds and got zero ohms

I like to use the continuity check, rather than ohm resistance. I would check continuity on the alt case to engine and grounds with key off, on and engine running.
Are you Catholic? Just joking. I figure you need a laugh. My liquor bill would be skyrocketing.

Grounding aside, the most likely sources of 12V to the alt case is the stator and rotor. Induction or an unwanted short? Less likely but possible is an internal circuit board to housing short.
Trouble shooting would require disassembly.
jim

ok I think problem averted, if you guys ever see a little battery light on your multimeter, change the battery LOL

we were seeing 4.4 volts in mid air nothing connected... I've said it before, I'll say it again - Multimeters are the devil!

Been there, done that!

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Originally Posted by toadster

ok I think problem averted, if you guys ever see a little battery light on your multimeter, change the battery LOL

we were seeing 4.4 volts in mid air nothing connected... I've said it before, I'll say it again - Multimeters are the devil!

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Maybe there is just too much pent up "energy" in your air over there!

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Originally Posted by Ted G

Maybe there is just too much pent up "energy" in your air over there!

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yeah we're looking for an aerial dewitcher :)