Even when I don't have the heater turned on or when I have the air conditioning on, as soon as I turn the control to heat, it makes a thumping sound behind the controller. The air conditioning works, but the heater just blows out cold air. Does anyone know what the problem is?
starting with the heater control sound there are various boxes that are vacuum driven and when switching the positions etc the vacuum closes them and opens them. these use gates or doors. now on to heater non operation there is a valve in the engine compartment that has heater lines going to it. this valve also has a cable and often a vacuum line. the purpose of this valve is to regulate how much hot coolant passes into the heater system. the cable works like your kitchen faucet slowly opening a passage either fast or slowly. this part is called the heater control valve. it likely has frozen inside and not allowing water to flow. ps you might wish to go to an autoparts store and see what it looks like for your car so you can locate it. it is on the passenger side of the engine area.
