I am running out of things to try. I get the 4th gear hunting and it throws a P0123 but only when I turn the A/C on. I have changed A/C compressor, alterator, TPS, cleaned all grounds and battery posts, bypassed the ECM added 4 extra grounds, put tin foil on alternator ground. Spent three hours on dealer diagnosis machine was told it was PCM. so I got a rebuilt PCM still the same. Does anyone have a idea why it would only happen when I turn on the A/C?
Open for suggestions besides not turning A/C on.
It is a 2002 2500 auto 2X4

 

Drove it to work and back without turning the A/C on and ran fine 50 miles each way. On the second day when I was leavng work check gauge light came on and I noticed the alternator gauge was not working truck was still running fine. Stopped at store checked codes and got a p1682 code. Started it back up no check gauge light and gauge worked fine.

 

I had a similar problem with my truck got the same code 1682 and I changed the alternator but it looks like you have done this already. I would take the alternator and get it tested again maybe a bad one.

 

check this thread, may help some


https://www.dieseltruckresource.com/...&highlight=cps

 

I did notice another symptom when I have the a/c on it will not downshift out of overdrive. With a/c on it will down shift from 3rd to 2nd. With a/c off it will downshift all the time. Since the a/c seems to be the common factor I will check a/c wiring tomorrow. I didn't give the original alternator back for the core charge since I thought it was still good so I will take it down and have it checked. Let you know what I find. Thanks for the tips.

 

Took to dealer mechcanic fixed the problem by disconnected all connections and reconnected them. He disconnected them to run testsand everything was fine he reconnected everything and problem went away. Has been 2 months still fine.

 

Probably just moved the bare wire off the area it was shorting to, if it comes back look at the wires going to the compressor.