Air Vents Stay in the Defrost Position
Engine vacuum is used to control the vent position in most vehicles, a simple
valve and vacuum storage tank is used to operate
vacuum controlled servos. These
servos move various 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 like when
under heavy throttle or towing up a hill. When engine vacuum supply is lost to the
vent control system it defaults to "defrost mode" as a safety precaution. We have
listed the most common reasons for this condition below.Car Repair GuideStep 1 - Most 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.
Step 2 - If you can hear vacuum escaping (hissing sound) from near or around the dash while the engine is running, this means vacuum is escaping. This system is sealed 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 replacingStep 3 - If you have noticed the vents go to the defrost position when you are traveling up a hill or under heavy throttle you might have a vacuum check valve that has failed. This allows vacuum to travel from the heater system back into the engine. To check for this condition find the system check valve and remove it. Once the valve has been removed try to blowing through it both ways. You should be able to only blow through it one way, if you can blow through it both ways it has failed, replace it with a new unit and recheck system.Step 4 - (GM only) If you have a late model GM car or truck and the system is controlled electronically the blend door actuator becomes weak. When this situation occurs it will not move the air direction door inside of the heater/air conditioner plenum. Replace the actuator with new and recheck the system.If further technical assistance is needed, our certified car repair technicians are ready to answer your car questions.Related Car Repair Information
Step 2 - If you can hear vacuum escaping (hissing sound) from near or around the dash while the engine is running, this means vacuum is escaping. This system is sealed 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 replacingStep 3 - If you have noticed the vents go to the defrost position when you are traveling up a hill or under heavy throttle you might have a vacuum check valve that has failed. This allows vacuum to travel from the heater system back into the engine. To check for this condition find the system check valve and remove it. Once the valve has been removed try to blowing through it both ways. You should be able to only blow through it one way, if you can blow through it both ways it has failed, replace it with a new unit and recheck system.Step 4 - (GM only) If you have a late model GM car or truck and the system is controlled electronically the blend door actuator becomes weak. When this situation occurs it will not move the air direction door inside of the heater/air conditioner plenum. Replace the actuator with new and recheck the system.If further technical assistance is needed, our certified car repair technicians are ready to answer your car questions.Related Car Repair Information