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
Difficulty Scale: 3 of 10
Tools and Supplies Needed
Protective eyewear and clothing
Before you begin, park the car on level ground, with the emergency brake set.
Step 1 - Some heater systems use a vacuum hose
that supplies the control valve on the dash 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. This hose or tube should be intact and in good
working order, if not, repair the hose or tube and recheck system. (After starting
the engine give the system a second to react as vacuum fills the system reservoir.)
Vacuum Tube and Control Servo
Step 2 - If you can hear vacuum escaping (hissing
sound) from near or around the dash while the engine is running, this means the
vacuum supply is escaping. This is a sealed system so if any leak is heard it must
be repaired before the system can hold vacuum and operate properly. Usually the
vacuum mode control (rotary) switch has failed and needs replacing which is located
just behind the temperature control panel.
Step 3 - 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. This failed condition allows vacuum to travel
from the vent control system back into the engine. 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 and needs replacement.
Engine vacuum is used to control the vent position in some vehicles. A simple
control valve and vacuum storage tank is used to operate vacuum controlled servos.
These servos move various air blend doors inside the air management housing or plenum.
Servos are connected to a vacuum source by vacuum hose and utilize a return spring
that forces the servo to return when vacuum is not applied. The main engine vacuum
is supplied to the control valve on or near the dash. A vacuum check valve is used
to stop the flow of vacuum from returning to the engine during low vacuum conditions
for example: engine under heavy load like driving up a steep grade. When the engine
vacuum supply is lost, the vent control system defaults to the "defrost mode" as
a safety precaution. We have listed the most common reasons for this condition below.
Avoid using tape or glue to repair a vacuum line
Written by Ken Lavacot Co-Founder and CEO of 2CarPros.com 35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.
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Article first published 2009-08-05 (Updated 2015-04-16)