How does a blend door work?

Updated: 10/25/2022
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14y ago

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A blend door is short for Blend Air Door. There are other names such as temperature door, etc.. The blend door, blends or mixes the hot air with the cold air. There usually is an actuator motor that opens and closes the blend door a little at a time to adjust how much hot air and cold air to mix for the temperature setting you have chosen. If your blend door or doors are broken sometimes you will only get cold air coming thru the vents or maybe just hot air with no air conditioning. For more information on the fault codes, etc for the Jeep Grand Cherokee blend door problems, checkout the Blen Dor Store on Ebay. There is an easy blend door replacement kit with only two parts to install. No Dashboard Removal. The Blen Dor kit is guaranteed for as long as you own the vehicle. For more information try a search for Blen Dor .

Close, but not exactly. On most modern vehicles, air is constantly blown through the AC evaporator core and then the blend door can divert some or all of the air through the heater core. In full AC mode, the blend door is closed against the heater core inlet and all air flows around the heater core and directly into the vent system. In heater mode, the compressor is deactivated and the air flowing through the evaporator core has no temperature change. As before, the blend door will now direct some or all of the air flow through the heater core and into the vent system.

In the old days, the blend door was controlled by a slide lever on the AC panel which was connected to a cable that ran to the blend door. In the mid 90's most automotive companies went to computer control over the blend door, using a pseudo stepper motor to control the door. A common design flaw in these systems is the end point detection for door movement. For a computer algorithm to control the door, it has to "know" when the door is closed against the heater core or blocking the free flow of air through the vent system. This is done by driving the motor and door to a stall point and detecting a voltage surge from the motor as a digital signal indicating the end extent of movement. The catch is that over time the plastic doors cannot withstand the over-stress of the calibration routine and will break. When this happens, all control over heat and AC is lost.

The problem is common on multiple vehicles and HeaterTreater has built a line of products for the most severe cases. We are always looking for additional applications for our repair technology and would be happy to discuss how we can provide a solution for your particular issue.

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Q: How does a blend door work?
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