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Discussion Starter · #1 ·
Truck: 2002 Silverado 4.8L

A few times this weekend in addition to my fuckin SES light for fuel pressure, my battery light has come on a few times and my gauge has dipped below 14v to about 12v. With the truck off I metered the battery and it reads about 12.8 volts. I metered it with the truck running and the voltage does not go up. I metered at the alternator itself and it still reads 12.8v. This shows me the alternator is not charging the battery.

* My question is, does the alternator constantly charge the battery, or does the pcm control when it outputs current?

* Does this sound like the alternator is taking a shit? It appears to be the original delphi unit?
 

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GMFS slowest truck :read:
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pcm has a pinout to turn it on. get the alt tested at a parts store or something. shouldn't take 15 mins to pull and the test is free.
 

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Discussion Starter · #4 ·
just got back from autozone, they tested it and it passed perfectly. i noticed that when the truck is running the pcm is not turning the alternator on to charge the battery. what the hell could be causing this. it was running fine, this just came out of nowhere. thanks
 

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A pcm turns the alt on and off? is that a nbs thing? or does everything do that?

I don't think my obs or grand prix do anythng like that, always constant 14v and both have a big cable from the alt directly to the battery.
 

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It constantly charges. While the truck is running, disconnect your battery and see if your truck will still run. If not, then the alt isnt charging. Check your cables from the alt to battery, battery to ground, and whatnot.
 

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Discussion Starter · #7 ·
I was under the impression that the "generator" aka alternator was constantly spinning, but was activated to charge the battery by the pcm. the pcm activates the alternator with a small amount of current like 5v via the plug at the top of the alternator to engage it.
 
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