A/C not working

MIKESENS

MEMBER
2006 FORD F-150
4.2L
V6
2WD
MANUAL
120,000 MILES

My A/C compressor will not come on. The compressor, condenser, orifice tube, dryer, and low pressure switch are all new. The system is newly charged with 34 ounces of r134. The compressor came pre-charged with 7 ounces of pag46 oil. The compressor will not come on when the A/C is on, even when the MAX AC is selected. I have checked the compressor relay and it is good. I have checked the associated fuses (#11, 12, 13, 14, and 32) and the PCM relay, they are all good. I have checked the A/C compressor diode and it shows good on my multi-meter. I can force the compressor to come on by jumping the relay prongs (1 and 2) (85 and 86)) but there is no power signal to the relay from the HVAC control module when MAX A/C is selected. I get a -12 reading on my multi-meter rather than the expected 5 volts. I have tried 3 used HVAC control modules to make sure my control module isn't bad and the same problem persists. I assume I need to check the power to the A/C switch circuitry and the wire from the A/C switch to the A/C compressor relay for continuity? I need a wiring diagram for the HVAC control module. The truck has manual controls, no automatic climate control.

5 Replies

KASEKENNY

MECHANIC
18,907 POSTS

On the A/C clutch relay what is the voltage on position 2? This is the signal from the PCM. Basically the high pressure switch and the cycling switch need to be closed for the PCM to ground the relay allowing power to go to the compressor. If you don't have voltage here, then either switch or the PCM is the issue.

I assume this is the switch that you are talking about where you have no power from the HVAC control module, but I just want to confirm. Thanks

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Thursday, April 15th, 2021 AT 2:33 PM

MIKESENS

MEMBER
3 POSTS

I have no signal power to the compressor relay. I believe this should be 5v coming from the HVAC control module A/C switch to the compressor relay on the control side. Instead of 5v on the incoming signal to the compressor relay I'm getting a reading of -12. I've replaced the low pressure switch (cycling switch). I will research how to check it. I'm also hoping to check the circuitry wiring for the signal to the compressor relay all the way back to its source (through the HVAC control module). Can you provide me a wiring diagram / pin out diagram for the HVAC control module? I can't find one online.

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Thursday, April 15th, 2021 AT 2:33 PM

KASEKENNY

MECHANIC
18,907 POSTS

I just want to make sure we are on the same page because you are saying the signal wire but this is not a signal wire from the relay to the PCM. So in the attached diagram where it is circled is where you are getting -12 volts? If so, switch your leads and tell me what the voltage is.

This wire is not a 5 volt reference wire. It is actually a control wire where the PCM is grounding the relay which energizes the relay that pulls it closed. So if you have 12 volts then that proves out all the way back from the control module through the two switches, through the PCM and to the relay. You need 12 volts here when the AC is on to close the relay and turn on the compressor.

I also attached the connector views for the manual HVAC and the wiring diagrams.

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Thursday, April 15th, 2021 AT 2:33 PM

MIKESENS

MEMBER
3 POSTS

Yes, the location you circled in red. I thought it was direct from the HVAC control module and the ground side was from the PCM. I can now see in the diagrams I was incorrect. I'm getting -12 at that location on the compressor relay with the MAX AC switch on, the system charged, and a new low pressure (cycling) switch. I'm going to work on the truck tomorrow with the diagrams you have provided. I thought the relay only needed 5v to close but I see you are saying it needs 12v.

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Thursday, April 15th, 2021 AT 2:33 PM

KASEKENNY