Wiring A/C to work without PCM 98 7.4L K3500

[SpencerW]

nws, my apologies for taking this long. Unexpected trip out of the U.S. I'll post these 2 schematics. 1 is original wiring tied into the ecm. 2nd is how I did mine. Since I have a Cummins engine, and am not using the original ac compressor, I used a binary switch in place of the GM high pressure cutout switch. It does the work of both the Hi and Low cutout switches, as my compressor does not have a port for a separate Low cutoff switch. With that in mind you may be able to use the switches that you already have in the factory setup. To determine if you can do that you will need to ohm the Low cutoff switch that is in the back of your compressor. Unplug the wire terminal and Check for continuity across the switch in the compressor. If that switch shows closed (reads continuity) then just wire your 3 switches in series....like I have done with my 2 switches. If that switch is "open", no co tinuity, then you'll need to buy the binary switch and wire 2 switches in series like I have done. Any questions at all, ask away.

I started this project couple years ago, had a baby and stopped. She's now 2, loves riding in the truck, but it's too hot.
I've cut some of my wires. Have a low pressure switch, high pressure, factory fuse block, and I'm struggling to get the AC to work. I had one wire cut to the high pressure and same on low pressure, not getting power to my compressor when jumping the switches.
What wires were you referring to that you had cut?

 

[yevgenievich]

Stock ecu controls the relay for the clutch. Need to take the control wire from hvac panel(should show 12v when ac panel ac switch is pressed) and run it through low pressure cycling switch followed by high pressure cut off switch and then to the relay skipping the ecu.
Just shorting the wires does not work without ecu as they go to ecu in factory application.

 

[Jgze99]

nws, my apologies for taking this long. Unexpected trip out of the U.S. I'll post these 2 schematics. 1 is original wiring tied into the ecm. 2nd is how I did mine. Since I have a Cummins engine, and am not using the original ac compressor, I used a binary switch in place of the GM high pressure cutout switch. It does the work of both the Hi and Low cutout switches, as my compressor does not have a port for a separate Low cutoff switch. With that in mind you may be able to use the switches that you already have in the factory setup. To determine if you can do that you will need to ohm the Low cutoff switch that is in the back of your compressor. Unplug the wire terminal and Check for continuity across the switch in the compressor. If that switch shows closed (reads continuity) then just wire your 3 switches in series....like I have done with my 2 switches. If that switch is "open", no co tinuity, then you'll need to buy the binary switch and wire 2 switches in series like I have done. Any questions at all, ask away

NWS.....this is just a temp answer to bump this back to top. I'll write a detailed procedure when I'm off this weekend.
Briefly though, I used the lime green wire that comes from the AC button on the heater/ac control dash module. I think it originally went to the low pressure cutoff sensor on the back of the ac compressor. I used that as the trigger to the relay that already exists for the AC. That relay is the one that is in the Relay Center box under the hood. Since I used a non GM compressor that did not have the low pressure cutout switch/sensor those wire were freed up for me. To still have a low pressure cutout, i bought a binary switch that was both the low and high pressure switch. I bought it from Cold Hose in Florida. It, along with a wiring connector was 30 bucks delivered. I placed that switch in the high side line that goes to the condensor. I wired the binary switch and the compressor cycling switch in series.
I'll draw a couple schematics and get them posted here this weekend.

Thanks a lot, I used your diagram and got my ac system wired up and blowing cold air. Right in time before the real heat comes lol