Alternator issue got me stumped

[TonyM]

My alternator stopped charging on my ls swap. Tried a new one same issue. With the exciter wire unplugged I have 12v at the output post on the alt. When I plug the exciter in it drops to 5v. Exciter wire has 10v running thru it when the key is on and truck is running. Ohm’d the wires going back to the ecm and they’re good so it appears it’s telling the alt to charge. I tried running a wire from ign to alt with resistor and get nothing. All signs say it should charge but nothing. Any ideas?

 

[Schurkey]

WHAT VEHICLE???

WHAT ALTERNATOR??? What alternator did that vehicle come with? Did you swap in something different?

Did the alternator work properly before the failure, but after the swap?

 

[TonyM]

I guess a little back story could help. 1990 truck, 5.3 from a 2005, stock style alternator for the 2005 which was a new reman installed at time of engine swap. GBCM deleted and ecm repinned for it. It worked like a champ for a couple months, charged fine, alt light on dash functioned as it should. I was just driving home the other day and noticed voltage was low. Got home and shut it off. When I restarted it the alt light was on.

 

[Schurkey]

Is that the sort of ECM-alternator system where the voltage regulation is in the ECM?

First Guess: This thread should be in the LSx + Carb Swaps sub-forum since it's dealing with newer LS-series accessories and ECM.

Second Guess: You need an '05 service manual.

 

[TonyM]

First Guess: This thread should be in the LSx + Carb Swaps sub-forum since it's dealing with newer LS-series accessories and ECM.

Second Guess: You need an '05 service manual

Thanks to this top notch advice, the issue is fixed. Not sure how I would've accomplished it without your vast wisdom

 

[GoToGuy]

Uh, you gave us a problem and result but didn't share the " fix answer" ?

 

[Schurkey]

Oh, I'm sure there's plenty more sarcasm where that "fix" came from.

 

[AuroraGirl]

OP does your alternator have a plug with 2 or 4 wires

if its 4 pinyou have a failed regulator or rectifier most likely
If its 2 pin your alternator needs to be PWM controlled so does your PCM work as "stock" 2005 and does your battery have a sensor utilized