Electric Blend and Recirculation Doors
Electric blend and recirculation doors are used with (EATC) electronic automatic temperature control systems. A stuck blend door will result in the same blend of cold and hot air being mixed in the case. As seen in the illustration, the blend doors function is to mix cool and heated air in the case to achieve the desired selection. These motors are reversed by changing the polarity of the wires. Applying voltage to one wire and grounding the other moves the door in one direction. Reversing the polarity of these wires moves the door in the other direction. These five wire motors contain a potentiometer indicating the blend door’s position to the controller. A two wire motor either opens or closes a door; it does not have the variable door positions of a three or five wire motor.
When testing these circuits, use a high-impedance multimeter and not a standard test light. This is because a test light may draw too much current, resulting in computer or circuit board damage. Digital multimeters have little impact on the circuit being tested. There was a day when a test light could be used throughout the vehicle, but today's vehicles contain many onboard computers, and the air conditioning system is no exception. Use care and the proper tool when diagnosing sensitive automotive circuits.