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

[nws649]

Wondering how to wire the compressor to work without PCM. Currently have a Holley Terminator max so most of the PCM isn't plugged into anything anymore and it's causing the compressor to not cycle just stay on when the A/C is switched on. CumminsBronco

 

[yevgenievich]

On mine, I took a signal from the ac control unit first through low pressure cycling switch, then high pressure cut off switch and in to a relay. Relay then energizes ac clutch

 

[CumminsBronco]

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.

 

[nws649]

Thank you. The schematics would be very helpful

 

[CumminsBronco]

NWS649, I haven't forgotten you. I have most of the schematic drawn up, but couldn't remember a couple wire colors on the bottom side of the underhood relay/fuse center that I had to cut and splice. Just been busy and haven't had time to disconnect batteries to get to those wires to complete the schematic. I will get to it though and post it.

 

[nws649]

Alrighty mostly just concerned with it now so the defrost will work in the winter

 

[CumminsBronco]

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.

 

[CumminsBronco]

nws...if any of that isn't clear to you, you can give me a call at 812 989-6059. If I don't answer leave a message and I'll call you back soon as I can.

 

[nws649]

Thank you I appreciate it. I should be able to make it work based off your drawing

 

[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.