smokey

I'm looking for ideas as to fixing a heater/AC problem. It's been going on for four years, ever since I got the car. I've tried four different used control units. I've cleaned all the connections I could get to easily. It's not the clips. I've tested all the temperature control units, and they test within spec as per the manual. I replaced the ground wire under the dash. Everything works fine on the street, but when I get it on the track and everything heats up, the heater turns on, and continues to malfunction. The heater valve stays open, and I get hot air. I can stop the hot air from coming into the cockpit by disconnecting the mixing flap motor when the flap is closed, which is what I did yesterday at the track. I could just disconnect the heater, but I refuse to do that on principle. Any ideas?