I have a 97 F150 4.3L... NO HEAT... full air though, like I can turn the air all the way on full blast it works, but no heat... changed thermostat, flushed out the heater core, getting heat through the hoses, but nothing through the heater... not sure if maybe the door for when you turn the *** from hot to cold is working or not, maybe it wont open up. PLEASE help me with this, sold the truck to a friend and he cant drive it like this... BTW i live in damn canada, minus 25c EVERYDAY. Any help would be a gift from henry ford the ******* that started this all! lol

 

your on the right track. when you turn the **** from hot to cold, you are sending an electrical voltage to a small motor to move the blend door. The door mixes hot and cold air to get the right temp. Either the heater **** is slipping on the shaft, or the motor is bad, or the cam that moves the blend door is broken. THe door is under the radio, above the transmission hump. Pull the heater **** off and look to make sure it is moving the shaft, then remove the plasic panel above the hump and look at the motor area.