A thermistor in the airstream just after the evaporator is used to disengage the AC compressor when the evaporator gets too close to freezing. The set point for disengagement is supposed to be about 38 degrees and re-engagement at about 42 degrees. Early SSRs had a “blue” signal conditioning case attached to the thermistor and there was a service bulletin to replace it with a “green” cased part. The blue cased part is known to cause compressor “short cycling” and warmer-than-expected AC air. You can reference GM Technical Service Bulletin # PIT3626B (Document ID: 1701094).
According to GM Service procedures, replacement of the “Temperature Switch” (thermistor and signal conditioner) requires 16 labor hours. This is to cover the complete removal of the dash and air conditioning unit out of the SSR and splitting the case open to replace the sensor. OMG…. All that labor ($1600 or more) to replace a low dollar component.
This kit provides a new (green case) 89018791 Temperature Switch assembled onto an external mounting bracket and a precision drill guide. The included instructions direct you to simply drill a hole in the HVAC case and attach the new bracket with switch. Drilling the hole in the HVAC case is accomplished simply by placing the drill guide onto the case under the glove box area and using a 1/4″ drill. No need to remove the dash or HVAC unit. Once the new Temperature Switch has been installed, you simply move the electrical connection onto the new switch unit. The old sensor and switch box are merely abandoned and left in place. Total time for replacement is about 30 minutes, due to the need to remove the glove box to access the electrical connector.
Detailed instructions with photographs are included.