What is a mode door?
A mode door is used to direct the flow of air in a vehicle. A typical heater box may use several mode doors and mode door actuators direct the flow of air to the windshield defroster, dash vents or floor vents. The mode door actuators on late-model vehicles are electric motors with position sensors that tell the HVAC control head the exact position of the mode door. On older vehicles, carmakers relied on vacuum motors to change the mode doors. On still older vehicles, the mode doors were moved by cables operated by the driver.
Mode door failure symptoms
When a mode door or mode door actuator fails, you may see any of the following systems:
- Air only flows through one outlet and does not change when you change the setting. That can be a completely failed electric actuator or a vacuum leak to a vacuum motor actuator. Vacuum actuators are spring operated and default to a certain position when they lose vacuum.
- Air flows through multiple outlets when you choose only one outlet, like the “floor.”
- A clicking sound is heard when you change air direction. That’s the sound of stripped gears inside an electric mode door actuator or broken linkage between the actuator and the door.
- Snapping sound when changing air direction. Mode doors contain foam gasket material that can deteriorate and cause the door to stick. When the door finally moves and slams open or shut, you hear a snapping sound.
How does a mode door actuator work?
Electric mode door actuators contain small motors, pinion gears, and variable resistors or Hall effect sensors to report actuator position back to the control head.
Vacuum motor actuators incorporate a diaphragm and plunger that moves linkage to move the mode door. Most have a spring that forces the actuator into a default position if it loses its vacuum supply
How to calibrate a mode actuator?
On some older vehicles, you can force the HVAC head into performing a calibration routine by simultaneously pressing several buttons. But most late-model vehicles can only be calibrated with the use of a professional-grade scan tool.
Repair cost for mode door actuator
The repair cost for mode door actuator varies widely depending on the year, make and model. Most mode door actuators are held in place by three screws so the actual replacement isn’t difficult. But getting to the actuator can be quite difficult. In the worst examples, the technician must remove a portion of the dash to gain access to the mode door actuator. Others can be accessed from below the glove box. The cost for the actuator itself is fairly reasonable, usually costing around $100.
©, 2020 Rick Muscoplat
Posted on by Rick Muscoplat