I replaced the servo with a new one and it moved to the same position and does not respond to the heater controls. I moved both servos by hand and they both moved back to the original position. Pulled the hvac control ***** and everything looks fine on the circuit boards (not that I could tell if it wasn't something obvious). Tested the resistance of the thermistor and is seemed fine (1.5k reducing down to about 1k as it heated up). I'm stuck, any ideas?

 

Manual or automatic climate controls?

 

Manual

 

Does the motor plug have 5 wires? Pink/blue, pink, purple, black/white, green.

Toyota has nothing on testing the thermistor.

Per Toyota, when "the temperature control SW is pressed, the ECU in the A/C control assembly sends a signal to the air mix control servo motor. This signal drives the motor to reach the temperature set by the temperature control SW, and moves the film
damper."

You may need an oscilloscope to check for any signals. The servo motor goes right to the control switch board circuit.

 

Yes, that is the motor. I found a resistance graph for the thermistor in the service manual and it seems to adhere to that although I am eyeballing it. I don't know if the thermistor actually interacts with the switch though. It does shut off the compressor if I disconnect it. I found the diagram you are referring to with the control switch circuit board and it looks like it controls the motor by changing resistance but I don't have an oscilliscope or any way to test the actual switch circuit board. I ordered a used control panel in hopes that will do the trick.

 

The HVAC system is supposed to have its own diagnostic that outputs via the control lights. Youtube may have videos on this.

Would assume one sets the temp. The "computer" then activates the servo while getting a signal from the thermistor. When the temp is at the set point the computer stops sending a signal.

Don't know how the servo is driven, perhaps a stepper motor. Toyota changed over the HVAC blower to a stepper motor instead of resistance.

 

Turns out it was the switch panel. Popped in the replacement and everything is back to normal.

 

Great!

Thanks for posting back with the fix!