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 AC voltage from the alternator 
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Joined: Sun May 12, 2013 5:21 am
Posts: 319
Location: Michigan
How much and what will AC volts from the alternator damage?

HEI, starter, gauges, ignition coil, battery?

Tuner said I should have .1, I've read the upper limit is .5 and looks like I got 30-40, but car seems fine with a MSD HEI module and it wouldn't run with a Mallory programmable dist.

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Sat Jun 11, 2016 7:18 pm
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Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1704
It isn't likely to damage the items you mentioned, but it will definitely cause microelectronics to act crazy. This is why the timing computer and rev limiter in the Mallory is acting stupid. It probably didn't damage it, or it wouldn't work at all. After you fix the alternator and try it again you will find out. If you have been using a wideband O2 gauge it will have been giving you inaccurate numbers.

This happens to alternators when the output is disconnected from the battery when the engine is running. That is a no-no. The usual culprit is an improperly wired battery shut off switch. The alternator must never-ever be disconnected from the battery when the engine is running ... never-ever. If you want to have some fun, try to program an ECU in a car with a bad alternator.


Sun Jun 12, 2016 4:03 am
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Joined: Sun May 12, 2013 5:21 am
Posts: 319
Location: Michigan
Good info, and I've herd of something like that, that a cut off switch can be bad for alts but didn't know exactly why. Pretty sure it wasn't unhooked and I don't have any disconnects.

All I know is after I replaced the bad block ground the volts got real high, then I slapped a larger underdrive pulley on and it was fine. That was before I even knew alts were suppose to be around 14ish and no higher than 14.8. The parts store test showed 14.6 and I've tested 14.7 with my multimeter up to 3500rpm.

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Sun Jun 12, 2016 4:34 am
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Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1704
It can have a bad diode and still have the correct DC Voltage. There should be no AC Voltage, not over .1 Volt AC from a Delco alternator.


Sun Jun 12, 2016 5:29 am
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