I have this problem in my Yukon. I changed alternators to a 250a DB electrical and it still drops below 14v with the AC on and I have an optima yellow top...

I'm about to unplug it because even with the meter reading below 12v, my lights don't dim at all. I hate this stock gauge.

 

I saw this thread on EFI Live forum, if you were logging the PID volts, then it is PCM voltage and not the actual voltage output by the alternator/battery. I am not sure volvo's solution will work but worth a shot, didn't you switch out the entire harness?

http://forum.efilive.com/showthread.php?t=11490

 

Yeah it's definitely battery voltage, not PCM voltage. My dash lights dim, and I have a voltmeter on my radar detector that shows the drop as well. I still haven't messed around with the grounds yet...still thinking that I might have a problem there. Just too damn hot to make myself go crawl around in the garage.

And yes, I did swap engine harnesses.

 

did brian have them send a electric blower?? lol

have you tried to run a power wire strait from the alternator to the battery? I ran a 1/0 and I havn't had a problem since.

 

Yup I have 1/0 from alt to battery, 1/0 from battery to frame. Need to put in a 1/0 from engine to frame still.

 

I'm having the same problem with my alternator now after my TBSS intake swap. In my logs as soon as I go WOT the voltage drops to battery voltage. Guess I'll check all my battery and ground connections after work tonight.

Did you ever figure out the problem?

 

No I haven't worked on it yet. Too busy and it's too hot outside.

 

I still can't figure this one out. I'm thinking of having them warranty out my alternator just to make sure that isn't the problem. Seems to be something with these 145 amp alts.

 

did you ever get your problem fixed mine has never done it before untill after i used the hptuner and moved the rev limit to 6500 right about 5400 the light dim and the guage was dropping now the gauge is pegged to the right along with my temp gauge bad motors in gauge i guess cuz hptuner live readout says everything is good just wanted to know if you found anything out

 

My problem was a cheap POS rebuilt alternator from Autozone. Apparently the rebuilt alts aren't built well enough to handle high rpm. I found an OEM take-off alt for a better price anyway and got my money back from Autozone.

If your gauges stopped working all of the sudden I would check all the fuses and relays. If the regulator on your alt went bad you may have had a voltage spike and fried a few things. When my stock alt went bad it fried one of the relays to my e-fans.