I JUST BOUGHT A 1992 CADILLAC DEVILLE AND THE HEATER DOES NOT WORK.
I REPLACED THE THERMOSTAT, FILLED THE RADIATOR, AND STILL NOTHING.
WHEN THE FAN IS OFF THERE IS A LITTLE HOT AIR COMMING THROUGH, BUT SHORTLY AFTER THE FAN IS TURNED ON, THE COOL AIR IS BACK.
THERE IS NO SENT OR SIGN OF AN ANTI-FREEZE LEAK IN SIDE THE CAR.
ALSO BOTH HOSES GOING TO THE HEATER CORE ARE HOT.
PLEASE HELP!!!

 

Hey twisty: Heating and Air Con can be fun. Your problem could be air mixture door stuck in AIr mode. It could be the heater water valve stuck. or the heater water valve vacuum motor. Could also be the Ambient air temp. sensor on the electrical side of things. Go into the glove box and pull out the liner. You will find a black plastic cover for the air mix chamber. put the heater through it's modes and see if the mix door is doing it's thing. You will then know if it's the programmer , the door is sticking, or one of the other issues above.