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AC blows warm on one side, cold on the other

AC blows warm on one side, cold on the other

In a dual-zone vehicle, the AC blows warm on one side, cold on the other

When AC blows warm on one side, cold on the other, the cause is usually a bad blend door actuator. In fact, this is a fairly common complaint on vehicles with dual-zone temperature controls. To diagnose the cause, you must first do some research to learn whether your particular year, make, model vehicle uses a single AC evaporator with two blend doors and actuators, or two separate evaporators. Most vehicles have a single evaporator up front with two blend doors and actuators to vary the temperature for the driver and passenger. However, some luxury vehicles do have two separate evaporators, one on each side of the vehicle.

dual zone ac system

Why a blend door in a dual-zone AC system

In older vehicles carmakers used a heater control valve to vary how much hot engine coolant ran through the heater core. To vary AC temp, the temp knob would open the heater control valve to heat up the heater core and in turn, heat up the newly chilled air coming off the evaporator. Carmakers switched away from that design because once the heater control valve was closed, engine coolant would sit in the heater core all summer. That caused sediment to accumulate and clog the heater core.

Newer vehicles run engine coolant through the heater core at all times to prevent clogging and they vary AC temperature with a “blend door.” The blend door can be completely closed, where chilled air flows from the evaporator to the ducts, or partially open where some of the chilled air flows across the heater core to pick up some heat. In other words, in a blend door system, the blend door mixes cold air with warm air coming off the heater core to get you the desired temperature.

A Blend door actuator moves the blend door

A blend door actuator is a variable movement motor that opens or closes the blend door according to the driver or passenger temperature preference. The blend door motor is a stepper motor or servo motor with position feedback. When a blend door actuator fails, you can no longer control the air temperature.

Replace a blend door actuator

Replacing a blend door actuator is not hard. It’s usually held in place by just three or four small screws. Getting your hands up under the dash to remove those screws, however, can be quite a problem. In some cases, it helps to remove the glove box, but in other cases, you must remove portions of the dash and console.

Test a blend door actuator

As you can see in the illustration above, you’ll have to locate the blend door actuator on the heater box. Then change the temperature for that zone while observing the actuator. If the actuator moves, then the problem may be a stuck blend door. But if the actuator doesn’t move, it has usually failed and must be replaced. That’s the cause of an AC blows warm on one side, cold on the other condition.

©, 2019 Rick Muscoplat

Posted on by Rick Muscoplat

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