I have a 2006 3500 that I use with a 53 foot wedge trailer to haul cars. The other day I cut my air on, and it blew out hot air then my temp shot up! then it would go to normal, and up again. I open the hood and felt the hose and it appeared the thermostat was faulty. I replaced, and the air still isnt working, and when I climb hills the temp shoots back up, but when I get to 6th gear it goes to normal. HELP

 

Sounds like your electronic clutch fan has quit. It's too bad someone doesn't come up with a way to "pin" the fan so it is locked so you can at least keep moving. Sure the fuel economy would suffer, but sure beats getting stranded.

 

Thanks J,
Will the fault codes show this?

 

Might show up as a speed sensor error as that is what usually goes on them. Not much of a "test" on that fan, but it can be monitored with a DRB. I usually turn the a/c on and then monitor the fan duty cycle and the high side a/c psi. Once the psi goes up, the ecm will command the fan to go 95% duty cycle which at that point you can usually hear the fan start pulling air and the a/c psi will go down.