Dash Air Vents Stay in Defrost Position (Older Cars)

Step by step guide on how to repair air vents that stay in the defrost position, this article pertains to older vehicles using a vacuum operated heater/air conditioner systems.

Difficulty Scale: 3 of 10

Tools and Supplies Needed

  • Flashlight
  • Protective eye wear and clothing
  • Rubber vacuum line
Before you begin, park the car on level ground, with the emergency brake set, engine "OFF"

Step 1 - Older heater/defroster systems use a vacuum hose that supplies a control valve in the climate control panel with vacuum, this hose can break due to the extreme heat of the engine compartment.

Lift your hood and inspect the vacuum hose or tube that leads from the engine intake manifold or vacuum supply manifold to the passenger's side firewall.

Broken Heater System Vacuum Line
Step 2
- Use the rubber vacuum tube to insert it over the broken part of the plastic tube.

Inserting Rubber Vacuum Line
Step 3 - Locate the other end of the broken vacuum tube and insert in into the rubber line.

Insert Broken Plastic Tube
Step 4
- Once both ends have been inserted into the rubber line double check for any other line breaks.

Vacuum Line Repaired
Step 4
- These vacuum lines continue from the engine to under the dash and onto the heater box plenum and should be intact and in good working order, (After starting the engine give the system a second to react as vacuum fills the system reservoir).

Vacuum Tube and Control Servo
Helpful Information

If you can hear vacuum escaping (hissing sound) from near or around the dash while the engine is running, vacuum is escaping.

If the vents move to the defrost position when you are traveling up a hill or under heavy throttle the vacuum check valve has failed and needs replacement. To check for this condition locate the system check valve which is located under the hood and remove it. Once the valve has been removed try to blowing through it in both directions, you should be able to only blow through it in one direction, if air flows in both directions the valve has failed.

Engine vacuum is used to control the vent position using a simple control valve and vacuum storage tank to operate vacuum controlled servos which move various air blend doors inside the air management housing or plenum.

Servos utilize a return spring that forces the return when vacuum is not applied.

When the engine vacuum supply is lost, the vent control system defaults to the "defrost mode" as a safety precaution.

Best Practices

  • Avoid using tape or glue to repair a vacuum line
Article first published