HVAC Control Head

[Donny]

Hello All, I drive a 91 grand marquis that has had some HVAC problems for a while now. Replaced the heater box (someone cracked it to replace a heater core), replaced the blend door actuator, and checked for continuity between the control head and the actuator. I have had 3 temp control modules which all seem to be bad. The blower speed and vent selector all work, but when changing the temperature, the blend door actuator doesn't switch between a/c and heat. I have confirmed that it doesn't move in the slightest because i still have the glove box out and it is visible. I have tested and confirmed that the actuator works. My father has helped me diagnose this problem, and as he is a current ASE certified technician, I do trust his work. After having had 3 faulty of the same part, though, we are both somewhat confused. If anyone has any knowledge on this or even theories, whether it could be a bad ambient temp sensor or outside temp sensor causing the entire temp control module to quit, please respond! Having done such a big job on the car I don't want to leave it so unfinished. Thank you!

[gadget73]

I'd go for the temp sensor. Seems unlikely that 3 heads would have the same problem. Should be able to read resistance across it. Not sure what the specs are but it ought to be in the EVTM. If the resistance is not in range i don't know what the control unit actually does, but "nothing" is a possibility.