6.0L work truck trim with manual AC (no auto, no passenger). With engine on and fan on AC light blinks three times and does not turn on. AC relay and AC fuse both good. Jumped 30 & 87 pins and compressor kicks on with cold air. Hooked up manifold set to test pressure. 100 degree day with ~50 psi low side and ~150 psi high side. Replaced high pressure sensor. Jumped low pressure sensor and AC compressor didn't kick on. Reset AC holding down fresh air & recirculate, disconnecting battery, and removing AC relay/fuse. All did not work.
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Hello, Dave here, contributing to RepairPal and providing expert advice since 2010.
There is a refrigerant pressure sensor as well as the low pressure sensor. Between those items, the relay and the climate controls themselves, something isn't checking out. You're on the trail, and it's helpful to know that it all works when jumped - but the command needs to get there with good information passing the various tests.
Per the workshop manual -- pressing the A/C button enables the HVAC control module to request A/C compressor engagement and turn on the A/C button LED. The HVAC control module sends a class 2 message to the PCM for A/C compressor engagement. The PCM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil.
HVAC control, wiring, PCM, that relay (even though you say "good") are all areas to look at.
Your engine coolant sensor needs to be reading normally too - if it shows too hot to the PCM it will not let the AC turn on.
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