Heater Control Valve

The heater core is the main component of the passenger heater system and uses engine coolant to provide heat to the passenger compartment. A heater control valve controls the amount of coolant flowing through the heater core. It is not used in all heater systems as some manufacturers allow coolant to flow through the heater core any time the engine is running.

The heater control valve allows the flow of coolant through the heater core to be controlled and switched on and off without effecting the operation of the rest of the engines coolant system. It may be located on the inlet or the outlet port of the heater core and activated by a cable , electronic control, or a vacuum signal. Its also important to note that the valve may be normally open allowing coolant to flow until activated or normally closed allowing coolant to flow only when activated.

The cable operated heater control uses a cable to drive a flap located inside the valve. As the occupant moves the lever in the dash a cable moves a flap in the valve that either allows of blocks the flow of coolant into the heater core.

The vacuum valve does the same thing with a pintle and diaphragm except this type of valve is normally open. This is in case of malfunction, heat may still be provided to the occupants by the heating system. Vacuum is supplied by the engine through a switch in the dash.

Electronic control can be PWM (pulse width modulated) or by a simple on/off solenoid. Pulse width modulation offers more control and is used is a variety of systems found through out the vehicle. The number of on an off pulses sent from the computer in a certain amount of time determines the position of the solenoid. The control valve may also be either opened or closed by a signal from the BCM.

It's also important to note that a poorly operating thermostat will effect the operation of the vehicles heating system. If the heater control valve is stuck closed depriving the heater core of heated coolant the inlet hose to the heater core will be hot and the outlet hose much cooler. This may also be caused by a clogged heater core. A leaking heater core will typically leak on to the floor board and result in a sweet humid odor coming from the vents while in operation.