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Discussion Starter #1
Put in a new HO alternator the other day and added in some 4/0 wiring with appropriate fuse. I kept the stock alternator wire attached as well. Started it up and even with the glow plugs cycling it was putting out 14.79 volts. It's a 390 amp alt which was tested and hit 410 amps, and is internally regulated at 14.8 volts, so it's definitely a good alt. I also have two brand new batteries, installed Friday. All week long everything has been fine. No issues at all.

This afternoon I started it up at work and the battery light came on. First thing I checked was to be sure the belt was ok. Next I plugged in the adapter for Torque and the voltage was at 14.7. I got home (just a couple miles) and checked with a meter. Voltage was 14.78. I turned the truck off. Resting voltage was 12.85. Pulled the wires off the alt and checked voltage individually. They each had battery voltage. Reconnected and then checked the plug on the alt. Still firmly attached. Started truck up and the battery light was no longer on. Checked voltage anyway and it was 14.78. Wiggled the connector on the alt while wife watched the dash and the light didn't come on.

I know the PCM triggers the light (or at least I think it does), but with good voltage it makes no sense as to why it was triggered. Can it go off if the voltage was briefly too high? Ideas?
 

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Might be on with too high of voltage, I've seen it with other cars, try turning everything on to lower the voltage and see if it goes off if nobody give the correct answer
 

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check the plug on the back of the alternator for tightness. loose plug will trigger the battery light but still charge.
 

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I know the glow plugs can't take extremely high voltages, but I don't recall the upper limit. I'll have to check on that.

I also don't know if that is part of the strategy that throws the battery light (ie is there an upper voltage limit specifically for when the glow plugs operate).
 

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Discussion Starter #5
Well the connector was snug and clipped in, and everything else checked out 100%. The only thing I can think of (unless we can find proof of an over voltage setting it off) is perhaps the key return was not quite as smooth as it should have been, and battery voltage wasn't present at just the right time. I know I'm reaching, but nothing else really makes sense. Oh, and it hasn't done it since. Just the one time.
 
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