Your car's defroster is needed to keep the inside of the windshield from
icing up in cold weather. It can be a safety problem if the defroster is not working or
quite uncomfortable if the vents are stuck
in the defrost position. The defroster is part of the
air conditioner system which is housed inside of the heater plenum.
The vent control system for the defroster in integrated into the operation of
all temperature controls and uses forced air provided by the
blower motor. On newer cars the vent direction and temperature controls are
operated by a series of 3 or 4 electrical actuator motors which are adjusted by
the climate control module. On older cars with manual controls these vent and
temperature controls are operated by vacuum servos. Vacuum is provided by
the engine while it is running.
What Goes Wrong?
Heat, extreme cold and vibration of the engine and car's suspension can cause
component of the defroster to fail. In some cases an electrical motor can burn
out or the gears inside the actuator can break causing the defroster door to get
stuck. A climate control system can stop working after a battery
replacement as the control prompt of the system is lost and needs to be rebooted
by removing the defroster/heater fuse for a period of time and then reinserted.
On older cars a vacuum line can crack allowing vacuum to escape rendering the
vacuum vent control servos non-operative. A control
switch on the dash can fall apart allowing the vacuum to escape and not reach
the servos. A defroster can also fade when traveling up a hill but then return
once the grade is overcome which is a signal the vacuum check valve has failed.
Let's Get Started
If you are not sure which kind of system you have either vacuum or electronic
any car made after about 1995 has an electronic system with no vacuum controlled
vent door servos. We will go over the more modern system first.
1. Actuator Replacement
The climate system is responsible for controlling the position of the
defroster actuator which is connected to the air position door in the heater
plenum. This actuator controls defrost, middle and lower air vents. When it
fails it can do so in the defrost position and must be replaced. Each car design
is a little different but this guide on
how to replace the defrost actuator will
show you what you are in for when doing the job. If you need to know the
location of the defrost actuator in your car please
ask one of our online mechanics
and they will get it for you.
Watch the Video!
2. Reset Climate Control
When the logic of the climate control gets confused it can lock up much like
a desktop computer. This can happens due to a surge or spike in the cars
electrical system. Before going too far with any repairs it can be a good idea to
remove the climate control fuse and allow the system to power down. When the
fuse is reinserted it will allow the system to reset and regain its logic and
start working again. Locate the climate control or
fuse and remove it by using the owner's manual. This step is easy to do and doesn't cost you.
3. Replace Climate Controller
The climate control computer is located in the panel itself and is subjected
to moisture and vibration which can cause it to go out. Older models will have
the controller somewhere under the dash which uses the control panel as a
switching station. These systems must be tested separately using a wiring
4. Check Vacuum Feed Line
If you have an older car then the defroster system is controlled by using
vacuum supplied by the engine. Lift the hood and inspect the vacuum hose or tube
that leads from the intake manifold to the passenger's side firewall. The image
below shows a typical line break which causes the vents to stay in the defrost
If a broken line is found use the rubber vacuum tube to insert
it over the broken part of the plastic tube.
Then locate the other end of the broken vacuum
tube and insert in into the rubber line.
After both ends have been inserted into the rubber line, double check for
other line breaks. This is a common problem with defroster issues.
Vacuum lines travel from the vacuum control switch on the dash to vacuum servos
under the dash. These servos move the air doors which turn on and off the
defroster. If you can hear a hissing sound when the car is running from under
the dash then vacuum is escaping. This kind of system also uses a check valve
which will allow the vent system to engage the defroster when travelling up a
hill when engine vacuum is low. This is a sign that the check valve is bad and
replacement is needed.
Got Any Question?
If you have any
defroster questions please visit our forum. If you need
repair advice, please ask our community of mechanics is happy to help you and
it is always 100% free.
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