Automotive Air Conditioning Information Forum

AC compressor will not turn on. Here is what we have checked

coolant is fine
AC works if you pull the relay and jump it.
relay has been swapped with a known good relay.
climate control head has been changed.
pcu/ecu has been changed
plugging a diagnostic computer in to the truck never shows the AC button being pressed
ac button does light up
fuses are all good

i am thinking that it must be a wire somewhere, but where do I go from here? I know the wiring from the underhood fusebox to the AC compressor is good because if we jump the relay it turns on and blows cold air

Have you checked the pressure switch? I think the A/C command goes from control panel to pressure switch to PCM. You should get a schematic of the truck and probe to see where the signal is being lost rather than parts swapping.

How do you know "coolant" (refrigerant) is fine? Is static pressure above 47 psi? If so, and relay is not energizing, then problem is most likely an input to the ECM is not happy. So the two primary inputs are the low pressure cycling (evap freeze protection) switch on accumulator, and the high pressure transducer at the high side output line near the compressor. You can "jumper" the connector at the accumulator, and wiggle, or probe with DVM the three wire high side transducer- look for ground, 5 volt input, and 0.5-1.0 volt reference (depending on static pressure)- while connector is connected of course. Have seen the wires break on the high side transducer a few times.