I have a 95 3500 4x4 with a cummins. Rebuilt tranny around 3 yrs. ago. Total after market. The problem now is when everything is cold shifts great but when hot shifts in and out of OD. Also my idle is low, cc does not work, oil pressure gauge bounces (new sending unit) and ac comes and goes. Could this be the ECC, PCM or even not releated.

 

Its the solenoid, I'm not sure but I think it is something like the tcc solenoid, not positive though. There is a little spring in it that will go bad causing it not to shift from third to fourth, my father had this problem in his 98 and just rebuilt the solenoid with a kit from the transmission shop and has shifted like a top since.

 

as far as the cc and the ac not working its probably a vac line. check all the vac lines by spraying w/ carb cleaner. if the idel goes up or down when carb cleaner is applied, thats where the line is broke/cracked. A common spot is under the battery tray for the vac line to go bad. (try drivers side cause im going off of a gas truck w/ one battery) Thats where themost likely place is.