What is a blend door?

Blend door and blend door actuator

A blend door moves to vary the amount of air passing through the vehicle’s heater core. The blend door is controlled by a blend door actuator, which can be a cable or an electric motor. The blend door is also referred to by some car makers as an air mix door. Both terms mean the same thing.

How cabin temperature control works in a car

Here’s a diagram of a typical heating system in a vehicle. The Recirculation Door moves to allow outside air in to be heater or cooled, or allow the re circulation of inside air. All air passes through the AC evaporator. If the AC is turned on, the temperature of the cold air can be altered with the temperature control. That control commands the blend door actuator to open or close partially or fully to allow air from the heater core to mix with the cold air from the AC evaporator.

When the AC isn’t in use, the air still flows through the evaporator, but is not chilled. It then flows through the heater core where it is heated. The blend door can route fully heated air to the ducts or can move to allow outside or recirculated air to mix with the heated air.

The Mode doors direct the flow of air to the floor, dash vents and defroster. Each door is operated by either a vacuum or electric motor.

How heat controls work in a vehicle

Blend door actuator

The blend door is controlled by the vehicle’s heater controls. If the actuator is an electric motor, the driver operates a knob to change from cold to hot. The knob tells the heater control “head” the preferred temperature setting and the control head commands blend door motor operation. The heater control head learns the actual position of the blend door through feedback mechanisms built into the blend door actuator.

The feedback can be in the form of digital data or variable voltage. If battery power is removed, these electric actuators may have to be re-calibrated so the heater control head learns their current position. In some vehicles, this calibration is done automatically by commanding the motor to full closed position. The control head monitors current flow to the motor and detects an amperage spike when the door reaches fully closed position. It then sets that position as the “home” position. Other vehicles require a scan tool recalibration.

How does a blend door actuator fail?

A blend door actuator can fail mechanically, where the worm gear that drives the pinion gear wears and no longer rotates the blend door open or closed. Or, the motor or feedback electronics can fail.

What happens when the blend door actuator fails?

When a blend door actuator fails, you’ll no longer have control over the temperature coming out of your vents. In a dual zone setup, one blend door actuator can fail while the other side works just fine. In that case you may have heat on the driver’s side but cold air on the passenger side.

Some heating systems are set up to have a default “fail” position –either full cold or full hot.

Cost to replace a blend door actuator

Blend door actuator motors are relatively inexpensive, usually less than $100. Labor times can vary depending on year, make and model. In some cases, the blend door actuator can be accessed by removing the glove box. Replacement can be as simple as removing three screws and swapping the motor. However, other vehicles are more complicated and require the removal of the console and portions of the dash.

Other causes of no heat

Some makes and models have blend door design failures where the blend door itself breaks or distorts, causing hot air to enter the cabin all the time. Or the door can stick and bind, causing the actuator to fail repeatedly. In those instances the car maker may issue a service bulletin and a redesigned blend door. Installing that modified part may require substantial dash disassembly.

©, 2018 Rick Muscoplat