Rick's Free Auto Repair Advice

No heat in car or Heat is always on

No heat in car – Heat always on

If you have an no heat in car condition or the opposite where you have a heat always on situation, you might have a bad blend door actuator. Most late model vehicles use a blend door system to regulate heat.

blend door motor, blend door actuator.

An electronic blend door actuator opens and closes a blend door to regulate the air temperature in your car

Hot engine coolant always flows to the heater core when the engine is running. An electronic motor (blend door actuator) rotates a shaft on a door in the heater box to regulate hot much air passes across the heater core. If you have the temperature set to low, only a small portion of the airflow passes across the heater core. The rest bypasses around the heater core. That’s why this system is called a blend door style. It blends a certain amount of indoor air with hot air from the heater core.

Blend doors are opened and closed with either a vacuum motor or electronic “flat-pack” motor. Vacuum motors are usually spring loaded to default to a full heat position. If you don’t have a heater control valve on your vehicle and your heat is always on, check for vacuum to the blend door actuator. If the vacuum motor isn’t getting full vacuum, consult a shop manual and conduct the troubleshooting procedure to determine why vacuum isn’t reaching the vacuum motor. The problem may be as simple as a cracked vacuum hose, broken vacuum switch, or faulty electronics.

Test the vacuum motor

If the vacuum motor is receiving vacuum but not operating the blend door,

blend door actuator, blend door motor

Vacuum operated blend door actuator

the problem may be a cracked diaphragm or a binding blend door. Remove the vacuum motor and move the blend door linkage by hand. If the blend door moves freely, replace the vacuum motor

If your vehicle uses an electronic motor to open and close the blend door, it may fail in any position—fully open or closed, or partially open or closed. Blend door actuators contain a small motor, gears, and a feedback position sensor. The sensor tells the heater control the position of the blend door. In a vehicle with automatic climate control, a temperature feedback sensor constantly monitors the inside temperature of the vehicle and maintains that temperature by commanding the blend door actuator to regulate the door position. Since the blend door actuators in these vehicles operate more often, they are more prone to failure.

Test an electronic blend door actuator

Locate the blend door actuator and observe the motor while adjusting the heat setting (key in RUN position).

blend door actuator, blend door motor

Refer to a shop manual to locate the blend door actuator

The motor should rotate and open and close the blend door. If the motor doesn’t move, unscrew it from the heater box and try moving the blend door by hand. If the door moves easily, then the problem is in the blend door actuator or the heater control head. Consult a shop manual for the electrical tests to determine whether the motor or the control is at fault.

blend door actuator, blend door motor

Check blend door actuator movement

If the door binds and cannot close properly, or the actuator fails, the door may remain open. Depending on the design of the heater box, that could cause a heat always on condition. If the motor fails when the door is closed, that could cause a no heat in car condition.

During air conditioning season, a bum blend door actuator can fail in a full or partially open position. In that case it heats the newly cooled air from the air conditioning evaporator coil, cancelling out the cooling effect of the air conditioning.


© 2014 Rick Muscoplat

Posted on by Rick Muscoplat

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