GMC Troubleshooting Questions? Ask a Mechanic for Answers
The blinking and shutting off of the auto HVAC control module mean it is detecting a problem and therefore not turning the compressor on. The module will set and store codes that can be retrieved with a capable scan tool.
The A/C light is designed to flash and disallow A/C operation if any of the following conditions are present:Ambient air temperature (AAT) is too cold (35 degrees or colder)Vehicle system voltage is less than 9.5 voltsRefrigerant pressure is too lowRefrigerant pressure is too highEngine coolant temperature (ECT) is too highEngine is not in RUNHVAC Control Head is in the OFF position
A common cause of no A/C request is a wrong signal from the ambient temp sensor (see attached graphic). If the circuit becomes open (sensor disconnected, broken wire), the HVAC control module will take that as a very cold temperature (-40F) and won't send an A/C request to the PCM. Jumpering the sensor will fool the HVAC module into thinking it is 80F outside, which will then allow it to send the request.
Please let me know if you need furhter assistance with this issue.