there is no actual "adjustment" that I'm aware of. Supposedly what you're supposed to do, is to unhook the cables near the glove box, and at the heater valve. Then you're supposed to manually operate the valve, the connection at the glove box, and the temperature knob until they are "closed" then you reconnect the cables. If you're still getting warm air, it's possible that the heater valve is leaking internally past the flap, even if it's closed.