I'm having the exact same problem. Mine is climate control. Is yours?
I suspect that the problem is with the temprature sensor
Check your vaccum hoses, they may be loose or not getting vacuum to it.
Heater control valve may be stuck closed. Also you may have a clogg in the
heater core, you can take it into one of those quick lube places and have
them do a reverse pressure flush on the cooling system. Good luck.
>I have a 1990 Nissan 300zx and I love driving it until it gets cold
>outside. When I try to turn my heat on it blows luke warm air but it
>isn't hot enough to defrost my windows or keep the car warm. I turn it
>up all the way to 90 but I get the same old luke warm air. My A/C
>works perfectly though. If anyone has some advice It would be greatly
>appreciated. Thanks.