I have a 1985 300ZX, and he cotrols for the heater / air conditioner seems to be stuck on defrost. No matter what button you select on the controls you only get air from the defrost vent. Has anyone had a similar experience? Is it the control module that needs to be replaced?

 

Check your vacuum hoses, if you have a vacuum leak that will cause your heater controls to malfunction. I've had that happen waaaayyy too many times. Dod you also have problems with your air temp being always warm unless you turn it all the way to cold? If so, it's your vacuum lines for sure =)

 

The same sort of thing happened to my 85 300zx and I found it was a small hose that had come off the vaccum pump motor which is behind the right headlight I put it back on and all the heater controls worked like new again.