79-wiring
Fleet Mechanic
Joined: Jan 2012
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From: Saskatchewan
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Senior User
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thanx i have alot of faith in what i read here. new to site ,owned this truck over 25yrs, origanol 302 went yrs ago with the 4x2- now 351m 4x4 rolling into my paint shop[ soon as weather breaks]
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
Because you have 12 volts at the 'I' terminal with the key in RUN, the ignition switch is doing everything it needs to do and is not related to the problem. Here are the next things to check:
1) The ALT light is on, but is the battery actually failing to charge? What voltage do you measure across the battery terminals with the engine idling out of gear with no accessories (like lights) on?
2) If you unplug the regulator and turn the key to RUN, does the ALT light shut off?
3) Verify that you have a good connection between the F terminal of the regulator and the FLD stud of the alternator.
4) Verify that you have a good connection between the S terminal of the regulator and the STA stud of the alternator.
Lastly, make sure the regulator is properly grounded (bolted to clean sheetmetal, which makes its way back to the engine block with a ground strap).
1) The ALT light is on, but is the battery actually failing to charge? What voltage do you measure across the battery terminals with the engine idling out of gear with no accessories (like lights) on?
2) If you unplug the regulator and turn the key to RUN, does the ALT light shut off?
3) Verify that you have a good connection between the F terminal of the regulator and the FLD stud of the alternator.
4) Verify that you have a good connection between the S terminal of the regulator and the STA stud of the alternator.
Lastly, make sure the regulator is properly grounded (bolted to clean sheetmetal, which makes its way back to the engine block with a ground strap).
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79 wiring
1] battery volts= 14.39 at idle and nothing on, had battery tester with load tester and charge system tester all in one-every thing was good 2]regulater unhooked, key in run =no light 3,4] at reg. plug the wires seem alittle loose in the plastic holder when unhooked from reg.- cleanned them best i could on both ends last-reg. is bolted to painted surface, will remove it and clean surface in mornin
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
If you're getting 14.4 volts at idle, then your battery is charging perfectly. The light has to be stuck on because of a short, or because the regulator is partially bad. If disconnecting the regulator makes the light go off, then there is no short and something is wrong with the regulator. It might be working just enough to work the alternator, but the mechanism to shut off the light could be stuck.
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Senior User
Joined: Feb 2013
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If you're getting 14.4 volts at idle, then your battery is charging perfectly. The light has to be stuck on because of a short, or because the regulator is partially bad. If disconnecting the regulator makes the light go off, then there is no short and something is wrong with the regulator. It might be working just enough to work the alternator, but the mechanism to shut off the light could be stuck.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
The light goes off with the regulator removed from the fender because that's the ALT light's path to ground. It will shut the ALT light off but the battery will not charge either. That's because the regulator needs a clean ground reference to gauge the voltage presented to the 'A' terminal (which is the sense input). The regulator is supposed to provide a path to ground through the 'I' terminal, which lights up the ALT light. After it gets a signal at the 'S' input, it's supposed to open this path which effectively shuts off the ALT light.
Everything you've shown me seems to suggest that there is a short in your voltage regulator that continues to ground out the ALT light but does not seem to affect charging. I haven't heard of that happening but I guess it could be possible; I'd recommend swapping out for a different voltage regulator.
Everything you've shown me seems to suggest that there is a short in your voltage regulator that continues to ground out the ALT light but does not seem to affect charging. I haven't heard of that happening but I guess it could be possible; I'd recommend swapping out for a different voltage regulator.
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