by Thomas West
You may not notice that the heat has stopped working in your vehicle until the first cold morning when you move the heater temperature control to its maximum setting. The thought of trying to troubleshoot the mass of cables, hoses, and conduit under your dash to fix the problem can be overwhelming. A somewhat common problem in vehicles that are a few years old, your vehicle’s heating and air system can be the victim of a faulty mix door actuator.
Built-in heating systems have been standard equipment in most automobiles since the 1950s and 1960s. In the early days of motorsports, a heater was an option and probably protruded noticeably from the underside of the dash. One thing that has remained constant over the years is that the heat source for a car heater comes from the engine coolant in most vehicles. The coolant exits the engine through a heater core, which looks like a small radiator, mounted within the heater assembly inside the passenger cabin. A fan motor forces hot air from the core into the passenger cabin. The heat temperature in modern vehicles is regulated by a mixed door controlled by an actuator. A mix door can also divert some or all of the heat to the windshield defroster.
The first automobile air conditioners were add-on units that were likely installed by the dealer, not the factory. These air conditioners were completely separate from the heater and, like the first heaters, were visibly positioned under the dash. As of 2012, the heating and air conditioning systems in most vehicles share many parts, including the temperature control in the front of the dash and the ducts behind the dash. With the combination of both systems, actuator controlled mixing doors regulate the temperature of the air conditioner and direct it to the desired location.
A mix door is mounted within the heating and air conditioning system and pivots to divert hot or cold air to different passages within the system to keep the passenger area at the desired temperature. When a vehicle is cold, you may want maximum heat to enter the passenger compartment. In this case, the mixing door can be moved to divert all the hot air into the cabin. Once the vehicle warms up, the mix door can be moved to divert only a portion of the hot air into the cabin. When cool air is desired, a mixing door is closed to keep heat out of the passenger area. The mixing gates are moved by a mechanical device called an actuator.
Mix gate actuator
In some cars made before the 1980s, the mix door may have been actuated by a metal wire that was connected directly to the temperature control lever on the dash. A safe way to move the mixing door, some systems required more than a little effort to operate. As automobiles increasingly had labor-saving devices added as standard equipment, heating and air conditioning controls could be operated with less effort when automakers began installing dual-door actuators. An actuator moves the mixing door automatically when the temperature control is moved. An actuator can be driven by vacuum or by a small motor mounted within the air and heat assembly.
Like any mechanical device, a mix gate actuator will eventually fail. Replacement parts to fix the problem can cost as little as a few dollars, but the labor costs to replace them can run into the hundreds. This is because the actuator is buried within the heat and air assembly behind the dash. When some cars are assembled at the factory, the entire dash, including the heater and air assembly, can be assembled as a complete unit before being installed in the car. Replacing one part within this complex system requires removing many other parts first. While this can be an important job, it can be done by a backyard mechanic with a vehicle-specific shop manual and regular automotive tools.