Free Car Repair Advice by Professional Mechanics
Troubleshooting
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Why Do My Vents Stay in Defrost Mode?
Why Do My Vents Stay or Go to 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 Guide - READ COMPLETELY BEFORE BEGINNING
- Step 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 (rotator) switch has failed and needs replacing.
- Step 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.
For exact specific repair procedures for any of the steps above visit our
online repair
manual page.
Troubleshooting
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Why Do My Vents Stay in Defrost Mode?
Additional Resources
Check Manufacturer Specific Car
Defroster Questions and Answers
Related Subjects
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