GM Electrically Driven Actuator Motors
Long gone are the cables and rods jutting out of the firewall to operate the heater control valve or a vacuum unit pulling a blend door open. These days, it’s electrically driven actuator motors (modules) accomplishing the job. There are several types of actuator motors used in today’s GM cars. They all function in different ways, but accomplish the same results. Many of the same operational features are very similar to other manufacturers as well. Understanding the fundamentals of the different types of actuator motors used is essential in proper vehicle diagnostics. Hopefully, this will give you a better idea of what is going on when those stubborn doors fail to move.
Two Wire Actuator
The two wire actuator is the simplest of them all. Generally, an 8 to 12 volt signal provides the needed voltage and ground from the control head to the actuator motor. A common use of a two wire actuator would be for a door that only uses two positions, such as a recirculation door that is either fully open or fully closed. The HVAC control head uses its preprogrammed logic and actuator endpoints to position the door correctly. A trouble code may set if the components start to wear and the endpoints drift too far from the pre-programmed positions. Most of these endpoints are calibrated during the relearn procedure.
Three Wire Actuator
The three wire has the same type of power and ground leads as the two wire, with one extra lead which is an input wire from the HVAC control head. These three wire actuators use a preprogrammed logic chip containing the necessary program information to run the actuator in the required position. As the temperature lever is changed, the internal potentiometer inside the HVAC control head sends the appropriate voltage to the actuator. Now, the potentiometer in the actuator compares its internal circuit voltage to the signal from the control head, and when the two match, the circuit voltage is turned off to the motor.
Five Wire Actuator
The five wire, or “tri-state” actuator, uses three of the five wires as a feedback potentiometer circuit. The other two wires are for ignition voltage and control head signals. Once the signal is received from the control head, the logic circuit inside the actuator uses its bidirectional electric motor to move the door to the appropriate position. When a zero voltage is applied to the circuit, the motor rotates in one direction. When 5 volts is applied to the same circuit, the motor rotates in the opposite direction. If, say, 2.5 volts is applied, the motor will stop rotating. This design has sliders moving on resistive paint strips, while the previous two actuators have potentiometers.
The HVAC control head expects to see the actuator move to a certain position with the given signal provided. If the door gets outside of that preprogrammed position, the controller becomes confused and attempts to try to find the correct position by continually searching for it, or it will stop once it reaches its default position. Sometimes this is associated with a ticking sound created when it is trying to find the correct position. (Not to be confused with the ticking sound from a broken plastic gear inside the module.)
If you remove the fuse for a minute or so, the existing information the control head has in its memory will be erased. Then, the control head will have to go into its relearn procedure to locate the door positions.
DTCs B0229, B0414, B0424, and B3770 may set as a result of the door not finding the correct position. A control head reflash may be necessary to expand the acceptable range from 5-250 counts to 0-255 counts, which will allow for a little extra room for the door and the controller to react. If for some reason a door is jammed, the reflash changes the “hold” or crush time from 2 seconds to 0.4 seconds. These changes should eliminate the clicking sound from the door traveling to its farthest limits.