Hey guys my ranger is a 2008 fx4 4.0 v6 with the 5 speed manual transmission (it’s from Canada) so today was the first hot day of the year and I go to turn on the ac and nothing but hot air came out. Ran to harbor freight and got a cheap can of refrigerant to charge it but when I went to charge it I realized the compressor clutch was not engaging (yes I had ac turned on in the truck) checked fuse 25 in the engine bay and it was all good but replaced it anyways. Still nothing. I pulled the relay out for the ac clutch and jumped it with a wire and the clutch engaged just fine and ac blew cold. I’m Hoping it was a low pressure sensor I got the compressor spinning from jumping it and topped it off with refrigerant. When I put the relay back in still nothing. I tried replacing the relay and still nothing. At this point I assumed I had a bad pressure sensor so I tried pulling the sensor connector and bridging the prongs on the connector with a wire but still nothing…tried the high and low pressure sensors and got nothing…the only way I can get the clutch to engage is if I bridge the two prongs on the relay and then it blows Ice cold just fine. Anybody have any ideas why the compressor won’t engage on its own? I’m out of ideas and about to wire a button from my dash to the relay 😭

 

Check fuse 27 in the cab fuse box, 10amp

In 2008 the PCM(computer) controls the AC Clutch Relay

In 2008 when you turn the AC Controller to ANY position but OFF, so compressor should run YEAR ROUND, thats normal
Contro;er sends 12volts out to the first pressure switch, then to the 2nd and then to the PCM
PCM then GROUNDS the AC Clutch relay to activate it so compressor comes on

Fuse 27----------------controller---------------pressure switch----------pressure switch------------------------------PCM

So use a volt meter or test light to see if you have 12v at each pressure switch, key on/AC on

Ground----------PCM-------------------------AC Clutch relay--------------------PCM Relay--------12volt key on

 

thanks! I’ll try checking for 12v at the sensors tomorrow idk how to check the pcm…I’ll also try maybe tracing the wires to check if they disconnected anywhere…I forgot to mention in the post up top that I also replaced fuse 27 in the cab…i hate that they switched the fuse panel over to the passenger floor.

 

The 12v from the controller comes out on a Green/orange wire and runs to first pressure switch, so I would check both pressure switch for that wire color and test it with key on/AC on

Then the 12v runs on a Yellow wire to the next switch, test for 12v there with previous switch plugged in
Then a green/blue wire to PCM pin 15 on C175B

Pull out AC clutch relay, key off
Test each slot in relay base for 12v, only one should have 12v, full time power from fuse 25, remember it
Turn key on, there should now be a 2nd slot with 12v, from PCM relay
If so then all that's missing is the GROUND from PCM to activate relay

PCM has 2 "overrides" for not grounding AC clutch relay
One is overheating, if ECT sensor shows coolant temp higher than 240degF(3/4 on temp gauge) it will shut off AC Clutch relay
Other is WOT(wide open throttle), if TPS(throttle sensor) shows WOT PCM will shut off AC clutch relay until it doesn't show WOT, this gives best power for passing

I did read/help someone else with similar issue and it did turn out to be PCM issue, bad internal relay, they ended up adding an external relay bypassing the PCM so AC worked as normal, just no overheating or WOT shut off

 

okay so I just tested it and the first sensor with the green/orange wire reads no voltage through the plug but if I test the green/orange in the plug to ground that wire has power but the other sensor has no power at all (makes sense if the yellow wire wasn’t giving any power at the first sensor) would that be a break in the yellow wire? It’s ran through loom all through the engine bay I really hope I don’t have to dig through all of that😭.

 

Test the pressure switch with OHM meter, should be 0 ohms if its closed
If its not closed then the 12volts can't go thru it to the yellow wire and next switch

Switch closed
12v-----green/orange----------switch//-----------yellow wire----12v

Switch open
12v-----green/orange----------switch/ /-----------yellow wire----0v

Replace switch or pressure is too low

"New" switch means "NEVER EVER TESTED" switch, not "it works", new parts are no longer tested, they have a warranty, "you test it and then return it if it doesn't work"

 

I’ll test it in about 30 minutes and get back to you but I already bridged the gap on both pressure sensor plugs with a wire and it wouldn’t turn on wouldn’t that mean that the issue isn’t the sensors?

 

Yes, if you jumped the connectors on both switches and HAVE 12v on the green/blue wire going to computer then that would take the switches out of the circuit
But if you don't have 12v on the green/blue wire then yes, yellow wire could be the problem

You can try using a jumper wire from green/orange wire to the green/blue wire bypassing the switches and the yellow wire to see if AC compressor comes on
If it does that would confirm switch or wire issue in that part of the circuit

Generally speaking unless a wire is easy to trace you just run a new wire, no need to disassemble a full harness

Never run AC system without pressure switches for any length of time, you WILL burn out compressor or blow off hoses, there is no "best case" scenario with that, lol

 

I ended up running a new wire in place of the yellow wire between the sensors and it’s working great so that was the problem…it engages, builds pressure, and shuts off all as it should thanks for your help saved me a lot of time, sweat and money emphasis on the sweat 😂

 

Good work

Thanks for updating the thread with THE FIX