A/C compressor won’t engage

[Fuelsmech]

So I have a 2000 Ranger 2wd, 3.0. I recently swapped the cab and switched over everything from my old truck to the new cab. Everything works perfectly except the air cond. I vacuumed the system and charged it but my compressor won’t kick on.
List of items replaced
Accumulator
Low side pressure switch
Swapped the switch assembly in the dash with a spare I had
Most of the seals

I also checked the relay and it’s getting power from battery and power to actuate the relay. The thing is, the compressor will kick on if pull the relay and put in a jumper wire and it will blow cold. But without jumping it, nothing.
it’s been a while since I’ve work AC issues but this one has me scratching my head. If anybody had any insight to toss my way I’d be very grateful.

 

[oldgeek]

My 94 uses the same model relay for the a.c, fuel pump and ecu. It's an easy swap to see if a relay is bad.

 

[Fuelsmech]

I have about 6 spare relays that all check good and I’ve swapped in every one.

 

[RonD]

Welcome to TRS

Fuse 10, 7.5amp, in the cab fuse box powers the AC "ON" switch in the cab
That 12volts goes to the Engine Computer(PCM), pin 41, called "AC demand" signal
Check this fuse first, pull it out and with key ON, test if its slot has 12v, maybe a wire didn't get hooked up?

Next circuit involved
Pressure switches are the GROUND signal to PCM
Ground-----------low pressure switch-------------high pressure switch-----------------PCM(pin 86)

Inside the PCM is a Relay, pin 69 is the out from this realy to AC clutch relay in engine bay, it GROUNDS the AC Clutch relay to activate it using the Pressure switches Ground
AC clutch relay should have 12v in one slot key off, then 12v in a 2nd slot with key on, that doesn't activate the relay, grounding activates the relay

So test the pressure switches, simple way is to unplug both and put a jumper wire between the 2 wires on each
Test if these are a GROUND, i.e. test light or volt meter from battery positive to these jumper wires, should show 12v, means a good ground, if no ground then a wire is unhooked.
Then see if compressor comes on with AC ON, if so un-jumper one connector and plug it back in, if AC Compressor doesn't come back on replace that pressure switch
Then do the same for next switch
New" doesn't mean "tested and works" anymore, new now means YOU TEST IT and we will replace it if it doesn't work, so heads up on any "new" part

If pressure switches test as OK, so good ground to PCM that it can pass that on to the AC clutch relay then could be PCM is not getting the "AC Demand" signal from cab controller
Its a Purple wire at pin 41 on the PCM connector, in engine bay, ground test light or volt meter and see if you can get a voltage reading from the back side if connector without removing it
Key on and AC selected in cab

1999/2000 used the same setup

 

[Fuelsmech]

That diagram should help. I did check for ground at the pressure switches and the high pressure side has ground continuity but the low side has 12v going to it. I must’ve messed up somewhere if they are both supposed to be ground.

 

[RonD]

Yes, they are the ground path for the AC Clutch relay, to activate it, test with Key OFF

 

[Fuelsmech]

Looks like something shorted out the relay on the pcm. My high side pressure switch goes to ground but my low side does not. I also don’t have continuity on the red/yellow from one to the other. So I ran a ground straight to the low side pressure switch ball/yellow and still nothing. Ran a ground to the pink/yellow at the pcm and the clutch actuates. Basically the relay in the pcm isn’t sending the ground to the relay in the power distro box If I’m troubleshooting this correctly. Wondering if it’s advisable if I fix the wiring from the pressure switches, and bypass the relay in the pcm to send the ground straight to the PDB relay or would that cause more problems?

 

[Fuelsmech]

And thank you again for the diagram and your extensive knowledge.

 

[Fuelsmech]

Yes, they are the ground path for the AC Clutch relay, to activate it, test with Key OFF

I got it. Thanks again. It was the connector that connects the two sides of the switch harness in the engine bay (drivers side about a foot behind the battery) needed to be reseated. Runs cold as ice and engages properly. I couldn’t have done it without your assistance.

 

[RonD]

Good work

Thanks for the update, it will help someone else down the road