Possesed Auto AC Unit

[Vicompc]

My HVAC is driving me mad, the fan speed will be low when it's over 90 outside with the interior registering over 100 degrees. Logically the fan should be on high, I can manually adjust the fan speed to high but the Auto AC seems to think it's cooler in the cabin than it really is. I can set a temperature with the auto AC button and it normally works for a minute or two, then the fan speed will drop down to medium or low (it seems to not like being on high ). I have installed a new outside temp sensor, and a new blower motor resistor and blower motor. I have searched through the threads but i haven't seen any problems like this one. BTW the outside sensor is working correctly, and I can feel the air being pulled into the B-Pillar temperature sensor.

 

[BRomanJr]

My HVAC is driving me mad, the fan speed will be low when it's over 90 outside with the interior registering over 100 degrees. Logically the fan should be on high, I can manually adjust the fan speed to high but the Auto AC seems to think it's cooler in the cabin than it really is. I can set a temperature with the auto AC button and it normally works for a minute or two, then the fan speed will drop down to medium or low (it seems to not like being on high ). I have installed a new outside temp sensor, and a new blower motor resistor and blower motor. I have searched through the threads but i haven't seen any problems like this one. BTW the outside sensor is working correctly, and I can feel the air being pulled into the B-Pillar temperature sensor.

Very strange behavior, There are also multiple sensors in the duct work under the dash. At this point it may require troubleshooting with a high end scan tool capable of reading all the sensors.

There is also a sun load sensor up on top of the dash, be sure you are not blocking it.
Try blowing some air into the B-pillar sensor to be sure the thermistor is not dirty (covered with dust).

It is acting like either the outside temp sensor or the B-pillar sensor is inaccurate.

 

[Mooseman]

You could reset the system and make it do a calibration by pulling the HVAC-B fuse for a minute. Put it back in, start the truck and leave it alone for a couple of minutes while it does its thing.