Wierd Alternator question

budhayes3 · March 15, 2005

Hi GM Tech, got a kinda wierd scenario for you. I've replaced the my original PCM in my 99 with a PCM from an 02, and the truck runs night and day with the new PCM. (If you check my sig you'll notice that I have some performance mods, one of which is a camshaft and valvesprings...the 99 PCM's don't like the increased valvetrain harmonics and allow for 7.9* of KR most of the time, while the 02 is not only base-tuned better, but doesn't seem to be affected by false knock)

After performing the PCM swap, the truck runs great, but the battery light flashes. Turns out that the 02's have an extra wire that runs from pinC of the alternator to pin 52 Red of the PCM. A couple of guys have run this wire and it cured their blinking battery light...but one guy had an upgraded 145a alternator, while the other has a 2500 6.0, which may have a beefier 130a alternator. Right now, I'm using the factory 105a alternator (until the 145er that I won on ebay shows up at my door)

Just tryin to give you a little history before I build up to the question:

There are now 2 wires in the connector to my alternator (used to be just one brown wire)

The brown wire (terminal B) has 12v when I probe it (vehicle running at idle)

The added wire (terminal C) has 0 volts (vehicle running at idle)

What should these readings actually be? I'm trying to figure out if the new alternator that I luckily picked up will cure my blinking lt, or if I have a loose connection somewhere (I checked the connections repeatedly already)

Any help would be greatly appreciated

GM-Tech · March 15, 2005

I'll have to sit down and look at a couple schematics tomorrow and see if I can figure out what's going on.

facn8me · March 15, 2005

I'll have to sit down and look at a couple schematics tomorrow and see if I can figure out what's going on.

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i know I 've said it before but YOU ROCK DUDE. Awesome of you to help people out.

GM-Tech · March 15, 2005

i know I 've said it before but YOU ROCK DUDE. Awesome of you to help people out.

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Hey, I said I'd look, I didn't say I'd figure it out!

budhayes3 · March 15, 2005

Thanks for your help man, I really appreciate it...I don't want to ask too much, but if you have access to a charging system schematic for an 02 that you could post or email to me, that would be great. If that's a problem, no worries

GM-Tech · March 15, 2005

Thanks for your help man, I really appreciate it...I don't want to ask too much, but if you have access to a charging system schematic for an 02 that you could post or email to me, that would be great. If that's a problem, no worries

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Here's both the 2002 (first) and the 1999 schematics. You'll note they're quite a bit different, with the pcm directly controlling the alternator via duty cycle in the '02. I'm surprised your setup even works at all.

budhayes3 · March 15, 2005

Thanks a ton GM-Tech...that's just the info that I needed. From the looks of those schematics, both the alternator and PCM are different...I don't understand how those other guys got their flashing batt lights to go off just by running the wire. Hopefully my new alternator will cure this light when it arrives. (03 145 amp)

The 99 to 02 PCM swap has become fairly common amongst the crowd over at www.ls1tech.com...it really makes a difference in the way the truck runs, like night and day.

mcreyaufmiller · March 15, 2005

Bud, are you aware that the upgraded alternator also uses a different bracket? At least that is what was done on the OBS.

budhayes3 · March 16, 2005

Bud, are you aware that the upgraded alternator also uses a different bracket? At least that is what was done on the OBS.

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I thought that it did, but one of the guys over at ls1tech got a 145 amp alt and it bolted right in. (then again, some of the guys over there were able to run a wire and turn their blinking batt lt off...no such luck for me) I guess that I'll find out when I go to bolt it in, if it needs a different bracket, I'll have to do more research. I already ordered a longer belt. Thanks for the heads up, just in case

Here's the link to creederado's write-up on the alternator and e-fan upgrade (large pics):

http://www.ls1tech.com/forums//showthread.php?t=282637