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Ask Juan Crespo Your Own Question
Juan Crespo
Juan Crespo, Tech Trainer
Category: GMC
Satisfied Customers: 1525
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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No a/c on a Gmc2001 Jimmy, 4wd, 4.3 L we have checked all

Customer Question

No a/c on a Gmc2001 Jimmy, 4wd, 4.3 L
we have checked all fuses, a/c relays are ok, also jumpered high pressure and low pressure switches and compressor still won't come on. It is electronic controls, refrigerant is full charge. When the a/c button is pushed, indicator lights blinked 3 times and then shuts off. Inside fan does come on. Please advise any other ways of checking the issue.
Submitted: 10 months ago.
Category: GMC
Expert:  Juan Crespo replied 10 months ago.

Hi there.

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.

Expert:  Juan Crespo replied 10 months ago.

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 volts
Refrigerant pressure is too low
Refrigerant pressure is too high
Engine coolant temperature (ECT) is too high
Engine is not in RUN
HVAC 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.