History and Root Cause of Blend Door Failures

On earlier automobiles there was generally a slide lever on the HVAC control panel (remember?). This slide lever was connected by a cable to the blend door and you had the control to slide the lever to full heat, full AC, or anything in between. In the 90?s most automobile manufacturers moved from manual blend door control to mechanized computer control.

Generically, the HVAC system operates by providing air flow that can be directed through the AC evaporator and the Heater Core. The AC evaporator is cooled by the engine driven compressor and a flow of refrigerant, and the Heater Core is heated by a flow of hot radiator fluid through the core. The air flow is usually directed through the evaporator core and to the blend door where the air can be directed down two different paths to the vent system. One path goes directly to the vents, and the other path is directed through the heater core.

For AC, the compressor is on and air is directed by the blend door directly into the vent system. For heat, the compressor is off and air is diverted through the heater core. These are the two extremes, and the system can split the amount of air flowing down the two paths and control on/off of the compressor to achieve any desired temperature between the two extremes. When the blend door fails, control over which path the air flow takes is lost and dependent on the design of the system and where the broken door falls, you will lose heat, AC, or both.

When the manufacturers went to electronic control, different variations of a stepper motor were deployed to provide fine control over the position of the blend door. Some have variable resistance sensors built into the motor, or time measurements to gauge how long to run the motor to get to a desired position, or counting commutator clicks. Independent of the method used for fine control, the methodology requires that the system “knows” the extents of movement. The system has to measure where the door stops against the two end points of movement. The computer does this by driving the motor to a stall point and measuring a voltage surge or lack of change in the position monitor input. The DC motors are geared to move slowly with a great deal of force. The root cause of most blend door problems is the over-stress of this calibration process, and the plastic doors break over time like bending a coat hanger back and forth. The computer system generally goes through this calibration routine every 20 times that the car is started. Time-to-failure is not a traditional mileage, age, or driving habit function, but just how many times the car is started. A pizza delivery guy that stops the car for each delivery will fail much faster than someone commuting to work once a day.

Automobiles were not built with this failure mechanism in mind and generally the process of replacing the blend door is involved and expensive. The usual dealer fix is to remove the steering column, pull the dash panel, evacuate and disconnect the AC system, drain and disconnect the radiator, and then remove the HVAC plenum (heater box). Once the box is out, it’s fairly simple to open it and replace the cheap plastic door, and then put the whole thing back together hoping you don’t screw up something else. HeaterTreater has developed processes for replacing the blend door without having to remove the dash and heater box. Cuts are made into the box in non visible areas with a Dremel tool, and the blend door is replaced with precision machined metal components. The process results in an easy, cheap fix and replacement components that are designed to outlast the automobile (much stronger than OEM plastic).

To date, thousands of our HeaterTreater repair kits have been installed in multiple different models with 100% installation success and correct operation. We are intent on maintaining superior customer service and would appreciate your consideration of our products.

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