What Is a Blend Door Actuator?

Built-in heating systems have pretty much been standard equipment on most automobiles since the 1950s and 1960s. In the early days of motoring, a heater was an option and most likely protruded quite noticeably from the bottom of the dashboard. One thing that has remained consistent over the years is that the heat source for an automobile heater comes from the engine coolant in most vehicles. The coolant is routed from the engine though a heater core -- which looks like a small radiator -- mounted within the heater assembly inside the passenger cabin. A blower motor forces hot air from the core to the passenger cabin. The temperature of the heat in modern vehicles is regulated by a blend door controlled by an actuator. A blend door may also divert some or all of the heat to the windshield defroster.

Early automobile air conditioners were add-on units that were likely installed by the dealership, not at the factory. These air conditioners were totally separate from the heater, and like early heaters, were mounted conspicuously under the dashboard. As of 2012, the heating and air conditioning systems in most vehicles share many parts, including the temperature control on the face of the dashboard and the duct-work behind the dashboard. With both systems being combined, actuator-controlled blend doors regulate the temperature of the conditioned air and route it to its desired location.

A blend door is mounted within the heating and air conditioning system and pivots to divert warm or cool air into different passages within the system to keep the passenger area at the desired temperature. When a vehicle is cold, you may desire maximum heat to enter the passenger compartment. In this case, the blend door may be moved to divert all of the heated air into the cabin. Once the vehicle warms up, the blend door may be moved to divert only part of the heated air into the cabin. When cool air is desired, a blend door is closes so that no heat enters the passenger area. Blend doors move by way of a mechanical device referred to as an actuator.

In some automobiles made prior to the 1980s, the blend door may have been actuated by a metal cable that was connected directly to the temperature control lever on the dashboard. A sure way to move the blend door, some systems took more than a little effort to operate. As autos had more and more labor-saving devices added as standard equipment, heater and air conditioning controls could be operated with less effort when automakers started installing blend door actuators. An actuator moves the blend door automatically when the temperature control is moved. An actuator can be powered by vacuum or by a small motor mounted within the heat and air assembly.

Like any mechanical device, a blend door actuator will eventually fail. Replacement parts to fix the problem may be as little as a few dollars, but the labor charges to replace it may go into the hundreds. This is due to the actuator being buried within the heat and air assembly behind the dashboard. When some autos are assembled at the factory, the entire dashboard, including the heater and air assembly, may be put together as an entire unit before being installed into the car. To replace a part within this complex system requires removing many other parts first. While this may be a major job, it can be accomplished by a backyard mechanic with a shop manual particular to the vehicle and regular automotive tools.