The HVAC system defrost function is meant to remove fog or condensation from the windshield in cold temperatures and relies on the A/C’s natural tendency to dehumidify the air for the defrost to work best. When you select defrost, the A/C compressor kicks on automatically. When activated, defrost mode is designed to direct air through the defrost ducts toward the windshield.
Dehumidified air that has passed through the cold evaporator will clear the condensation and fog to clear your field of view.
If you’re wondering why air only blows through the defrost vents, it’s because the HVAC components that control defrost mode can malfunction. The A/C can get stuck in defrost mode and make the cabin hot and uncomfortable.
In this article, we’ll discuss possible ways you can address your HVAC system when it gets stuck in defrost mode and how the defrost function works.
Here are some possible reasons why your car’s air conditioner is stuck on defrost mode:
When defrost mode is engaged, a part called a “mode door actuator” is responsible for redirecting air to the windshield vents. This door closes off the airflow to the register and redirects it to the defrost duct. On most newer vehicles, it is operated by an actuator with plastic gears and an electric motor, usually with a feedback potentiometer built in.
If the mode door actuator fails on defrost, the passengers will get little to no air from the HVAC vents, but at least the driver will never have a problem with condensation or fog.
To fix this issue, you’ll need to replace the faulty actuator and perform any relearn procedures. Sometimes a scan tool is required. The control head needs to know where the open and closed positions are on the actuator and will usually do a sweep of all actuators during the learning process.
To fix a mode door actuator failing to defrost, you’ll need to replace the faulty actuator and perform any relearn procedures. Sometimes a scan tool is required. The control head needs to know where the open and closed positions are on the actuator and will usually do a sweep of all actuators during the learning process.
–Richard McCuistian, ASE Certified Master Automobile Technician
Vehicles can have several door actuators placed in various locations in your vehicle. You’ll need to find the faulty actuator before you can replace it.
The defrost door actuator in older vehicles relies on vacuum hose for it to move. Vacuum hoses can become brittle and break after several years of being exposed to the engine’s heat and actuator diaphragms fail. When the hose or actuator diaphragm fails, the defrost door actuator is spring loaded so it will default to the defrost mode. Fixing the issue involves finding the broken vacuum hose, leaking reservoir, or bad actuator. your vehicle’s intake manifold.
Sometimes, the HVAC system can get confused and get stuck in defrost mode. This issue can occur after you’ve replaced your vehicle’s battery or if your vehicle has an electrical surge. On some vehicles, resetting the climate control system involves removing the defroster/heater fuse for five minutes.
The exact location of the defroster/heater fuse can vary depending on your vehicle model. The defroster/heater fuse might be located in your vehicle’s fuse box. However, you should still refer to your vehicle’s owner’s manual to find its exact location.
Your vehicle’s climate control computer is located somewhere on your vehicle’s dashboard. Sometimes, moisture, vibration, or just normal usage can cause it to malfunction. When this happens, it might cause the A/C to get stuck in defrost mode. In a modern vehicle, it’s possible to do a controller area network (CAN) scan to properly diagnose the issue, but not that this is something most DIYers will not be equipped to do. Meanwhile, the climate computer found in older vehicles must be tested separately using a wiring or vacuum diagram.
The HVAC system is designed with several fail-safe features that ensure that the driver will always have the defrost feature even when certain components fail. However, this comes at the cost of proper airflow through the passenger vents. The next time you wonder “Why is my A/C only blowing out the defrost vents,” it’s probably because a component like the defrost door actuator or vacuum lines has failed and your HVAC system defaulted to the defrost setting.
Fixing or replacing these components can be a challenge, especially for someone who isn’t experienced in automotive repairs. If you think you can’t fix the issue yourself, don’t be afraid to leave the task to your trusted mechanic.
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