I had a blown head gasket on my 99 deville N* engine so i replaced it with a N* engine from a 2000 deville but the problem is i can't get the ac to work correctly every time i try to put it in auto it switches to econ. i had the mechanic that installed the engine check the system and it was full of freon but the ac clutch is not getting any power going to it. Do I need to replace the 99 PCM to a 2000 PCM because of the new engine so everything is recognized works correctly or is there something else that needs to be done?

 

My first thought is there are A/C fault codes in the BCM. Enter diagnostics and erase all codes. report back

 

I cleared all the codes and the ac still does not work

 

You will need to check the basics, something disconnected, fuse(s), etc. There are alot of electrical items under black plastic cover near left front strut tower. I would start there.