Wiring AC compressor on '72 C-10 LS swap - The 1947 - Present Chevrolet & GMC Truck Message Board Network

406Weiand142t56ina65 · 08-03-2025, 12:50 PM

No hurry on this…but let me know if you can give me some direction and better understanding if my plan is good on hooking up the AC on my ’72 C-10 FACTORY AIR gold/white truck with 2004 Tahoe 5.3 and ECM. I have already vacuumed down and charged the system with the appropriate amount of refrigerant.

Issue: wiring AC using factory C-10 AC demand switch, to activate LS engine AC compressor, cooling fan and heater control valve. I’ve looked online but it doesn’t seem any two people do it alike.

Objective: turn on factory dash AC “plunger type” switch, have the high/low pressure switch complete/control the circuit, have AC compressor & electric fan receive "turn on” via relays due to high amp load and VINTAGE AIR electric heater control valve to receive "activate" signals to close heater control valve… and also wondering if the PCM needs to be involved or controlling this for idle, etc purposes…as I was planning on NOT involving the ECM as that is where my confusion comes in.

Plan A:

Materials: Planning to use Single post 4 way power distribution block, two relays, four 14 gauge wire & inline fuses, 10 gauge for pins 30 & 87.

Wiring: power from factory AC dash switch (already confirmed good) to/thru high/low pressure combo switch, then to distribution block via inline fuse, split 3 ways there, all three fused inline... the 1st to supply first relay pin 86 for triggering AC clutch via 87, the 2nd supply wire to 2nd relay relay pin 86 for triggering electric fan via pin 87… and finally the 3rd wire to the heater control valve, no relay required. Of course, 10 gauge battery to pin 30, 10 gauge from pin 87 to cooling fan and AC clutch individually.

Plan B: Do most of the above but interrupt the circuit between high/low switch & distribution block (or before high/low switch) and send it to the ECM fuse box for the ECM to be aware of AC demand. I think it involves tying in different pins…maybe between the high/low combo pressure switch and running it thru two spots in the ECM so it can control circuit…but I’m cloudy on that and there seems to be a variety of opinions online.

Thanks much

weq92f · 08-03-2025, 03:33 PM

.

Your pressure switch should be used to trigger the condenser fan for activation (best if fan is dedicated to condenser). Install a trinary switch. This will cycle the fan during operation rather than just keeping it on 100%.

Feed your block with something big like 6 or 4AWG and keep it close to the BATT.

My VA shutoff valve is controlled by a rotary dial and has a few different open positions. This allows combining heat and A/C in increments. The input to the valve is 5 leads. I'm sure they have something more like you want, just make sure before ordering.

I'm not familiar with the ECM in question or how to inform it. It is likely to increase the idle rpm slightly. You can live without that at first just leave an extra trigger in there for use later. If you use the pressure switch to control the fan, initial Demand On will only fire the clutch and shutoff valve. If you don't, you'll be demanding torque from the compressor and the alternator simultaneously which may cause a dip in RPM or even a stall depending on your idle settings. The ECM trigger, if available, would help with that.

EDIT: Not sure if I would want the ECM controlling the circuit but I can see where it might be useful for example under WOT the ECM could/may cut off the AC pump. It may also do that under other conditions like overheating. My very simplistic ECM takes as input a ground to show A/C demand...all it does is increase idle...not control the circuit.

Send it.

-Kevin

**A1971Blazer** · 08-04-2025, 08:05 AM

these might be of some help

https://67-72chevytrucks.com/vboard/...d.php?t=858132
https://www.lt1swap.com/aircondition...WaYNseqoWVxrUg

406Weiand142t56ina65 · 08-04-2025, 10:20 AM

-kevin[/quote]
thanks again kevin for your detailed response and help!

406Weiand142t56ina65 · 08-04-2025, 10:22 AM

Hey Kevin, Thanks much for your detailed reply...I'm going to reply below in all caps so I can see each section. Thanks, Geven

Your pressure switch should be used to trigger the condenser fan for activation (best if fan is dedicated to condenser). Install a trinary switch. This will cycle the fan during operation rather than just keeping it on 100%.
GREAT SUGGESTION...I'M GLAD YOU MENTIONED THAT.

Feed your block with something big like 6 or 4AWG and keep it close to the BATT. I CAN FEED THE BLOCK WITH A 6 OR 4 GAUGE...BUT KEEP IN MIND THIS WOULD JUST BE FOR THE SUPPLY WIRES THAT TURN ON THE THE RELAY...NOT THE PIN 30 OR 87 THAT WOULD BE CARRYING THE AMPERAGE.

My VA shutoff valve is controlled by a rotary dial and has a few different open positions. This allows combining heat and A/C in increments. The input to the valve is 5 leads. I'm sure they have something more like you want, just make sure before ordering.
MINE IS A GEN2 I THINK...IT ONLY TURNS ON OR OFF...I DID SEE THAT VARIABLE ONE ON THE WEBSITE AND MIGHT BE ABLE TO UTILIZE THAT AS AN ADD ON...IF I CAN'T FIGURE OUT HOW TO BLEND TEMP OR MAKE THIS 1987 SILVERADO EVAP IN MY '72 CYCLE RIGHT. I'M TRYING TO FIGURE OUT HOW THE '87'S WORKED AS FAR AS CYCLING.

I'm not familiar with the ECM in question or how to inform it. It is likely to increase the idle rpm slightly. You can live without that at first just leave an extra trigger in there for use later. If you use the pressure switch to control the fan, initial Demand On will only fire the clutch and shutoff valve. If you don't, you'll be demanding torque from the compressor and the alternator simultaneously which may cause a dip in RPM or even a stall depending on your idle settings. The ECM trigger, if available, would help with that.
I AGREE...AND EVEN THOUGH IT IS A STOCK CAMMED ENGINE (WELL AN LQ9 CAM IN A 5.3), I WAS HOPING IT WOULDN'T BE AN ISSUE BUT WOULD PREFER TO FIGURE OUT THE PINS TO TIE IT IN. I THINK YOU TIE INTO THE FUSE BLOCK IN TWO PLACES...AND DO SO WITH THE PRESSURE SWITCH WIRE...BESIDES IDLE, I'M JUST NOT SURE WHAT ELSE THE ECU WOULD DO.

EDIT: Not sure if I would want the ECM controlling the circuit but I can see where it might be useful for example under WOT the ECM could/may cut off the AC pump. It may also do that under other conditions like overheating. My very simplistic ECM takes as input a ground to show A/C demand...all it does is increase idle...not control the circuit.

Send it.

-Kevin[/QUOTE]
THANKS AGAIN KEVIN FOR YOUR DETAILED RESPONSE AND HELP!

08-03-2025, 12:50 PM	#1
406Weiand142t56ina65 Registered User Join Date: Mar 2007 Location: Dawsonville, Georgia Posts: 17	Wiring AC compressor on '72 C-10 LS swap No hurry on this…but let me know if you can give me some direction and better understanding if my plan is good on hooking up the AC on my ’72 C-10 FACTORY AIR gold/white truck with 2004 Tahoe 5.3 and ECM. I have already vacuumed down and charged the system with the appropriate amount of refrigerant. Issue: wiring AC using factory C-10 AC demand switch, to activate LS engine AC compressor, cooling fan and heater control valve. I’ve looked online but it doesn’t seem any two people do it alike. Objective: turn on factory dash AC “plunger type” switch, have the high/low pressure switch complete/control the circuit, have AC compressor & electric fan receive "turn on” via relays due to high amp load and VINTAGE AIR electric heater control valve to receive "activate" signals to close heater control valve… and also wondering if the PCM needs to be involved or controlling this for idle, etc purposes…as I was planning on NOT involving the ECM as that is where my confusion comes in. Plan A: Materials: Planning to use Single post 4 way power distribution block, two relays, four 14 gauge wire & inline fuses, 10 gauge for pins 30 & 87. Wiring: power from factory AC dash switch (already confirmed good) to/thru high/low pressure combo switch, then to distribution block via inline fuse, split 3 ways there, all three fused inline... the 1st to supply first relay pin 86 for triggering AC clutch via 87, the 2nd supply wire to 2nd relay relay pin 86 for triggering electric fan via pin 87… and finally the 3rd wire to the heater control valve, no relay required. Of course, 10 gauge battery to pin 30, 10 gauge from pin 87 to cooling fan and AC clutch individually. Plan B: Do most of the above but interrupt the circuit between high/low switch & distribution block (or before high/low switch) and send it to the ECM fuse box for the ECM to be aware of AC demand. I think it involves tying in different pins…maybe between the high/low combo pressure switch and running it thru two spots in the ECM so it can control circuit…but I’m cloudy on that and there seems to be a variety of opinions online. Thanks much Last edited by 406Weiand142t56ina65; 08-03-2025 at 02:14 PM. Reason: EDIT

08-03-2025, 03:33 PM	#2
weq92f Registered User Join Date: Oct 2016 Location: Dallas Texas Posts: 3,850	Re: Wiring AC compressor on '72 C-10 LS swap . Your pressure switch should be used to trigger the condenser fan for activation (best if fan is dedicated to condenser). Install a trinary switch. This will cycle the fan during operation rather than just keeping it on 100%. Feed your block with something big like 6 or 4AWG and keep it close to the BATT. My VA shutoff valve is controlled by a rotary dial and has a few different open positions. This allows combining heat and A/C in increments. The input to the valve is 5 leads. I'm sure they have something more like you want, just make sure before ordering. I'm not familiar with the ECM in question or how to inform it. It is likely to increase the idle rpm slightly. You can live without that at first just leave an extra trigger in there for use later. If you use the pressure switch to control the fan, initial Demand On will only fire the clutch and shutoff valve. If you don't, you'll be demanding torque from the compressor and the alternator simultaneously which may cause a dip in RPM or even a stall depending on your idle settings. The ECM trigger, if available, would help with that. EDIT: Not sure if I would want the ECM controlling the circuit but I can see where it might be useful for example under WOT the ECM could/may cut off the AC pump. It may also do that under other conditions like overheating. My very simplistic ECM takes as input a ground to show A/C demand...all it does is increase idle...not control the circuit. Send it. -Kevin __________________ 67 C10 fleet fuel injected '70 402, 700r4, 3.73 posi 07 335 sport turbo 6sp 94 Trans Am GT LT1 6sp posi -- sold after 22yrs 99 540 sport V8 6sp -- sold 73 240z L24 4sp -- given to friend 68 C10 step 350/350 3.73 open -- sold Last edited by weq92f; 08-03-2025 at 03:49 PM. Reason: ECM comment added

08-04-2025, 08:05 AM	#3
A1971Blazer Senior Member Join Date: Nov 2013 Location: East Tennessee Posts: 2,408	Re: Wiring AC compressor on '72 C-10 LS swap these might be of some help https://67-72chevytrucks.com/vboard/...d.php?t=858132 https://www.lt1swap.com/aircondition...WaYNseqoWVxrUg __________________ 1967 C10 Step side 1968 C10 Step side 1970 Chevrolet K/5 Blazer .............

08-04-2025, 10:20 AM	#4
406Weiand142t56ina65 Registered User Join Date: Mar 2007 Location: Dawsonville, Georgia Posts: 17	Re: Wiring AC compressor on '72 C-10 LS swap -kevin[/quote] thanks again kevin for your detailed response and help! Last edited by 406Weiand142t56ina65; 08-04-2025 at 10:23 AM. Reason: EDIT

08-04-2025, 10:22 AM	#5
406Weiand142t56ina65 Registered User Join Date: Mar 2007 Location: Dawsonville, Georgia Posts: 17	Re: Wiring AC compressor on '72 C-10 LS swap Hey Kevin, Thanks much for your detailed reply...I'm going to reply below in all caps so I can see each section. Thanks, Geven Your pressure switch should be used to trigger the condenser fan for activation (best if fan is dedicated to condenser). Install a trinary switch. This will cycle the fan during operation rather than just keeping it on 100%. GREAT SUGGESTION...I'M GLAD YOU MENTIONED THAT. Feed your block with something big like 6 or 4AWG and keep it close to the BATT. I CAN FEED THE BLOCK WITH A 6 OR 4 GAUGE...BUT KEEP IN MIND THIS WOULD JUST BE FOR THE SUPPLY WIRES THAT TURN ON THE THE RELAY...NOT THE PIN 30 OR 87 THAT WOULD BE CARRYING THE AMPERAGE. My VA shutoff valve is controlled by a rotary dial and has a few different open positions. This allows combining heat and A/C in increments. The input to the valve is 5 leads. I'm sure they have something more like you want, just make sure before ordering. MINE IS A GEN2 I THINK...IT ONLY TURNS ON OR OFF...I DID SEE THAT VARIABLE ONE ON THE WEBSITE AND MIGHT BE ABLE TO UTILIZE THAT AS AN ADD ON...IF I CAN'T FIGURE OUT HOW TO BLEND TEMP OR MAKE THIS 1987 SILVERADO EVAP IN MY '72 CYCLE RIGHT. I'M TRYING TO FIGURE OUT HOW THE '87'S WORKED AS FAR AS CYCLING. I'm not familiar with the ECM in question or how to inform it. It is likely to increase the idle rpm slightly. You can live without that at first just leave an extra trigger in there for use later. If you use the pressure switch to control the fan, initial Demand On will only fire the clutch and shutoff valve. If you don't, you'll be demanding torque from the compressor and the alternator simultaneously which may cause a dip in RPM or even a stall depending on your idle settings. The ECM trigger, if available, would help with that. I AGREE...AND EVEN THOUGH IT IS A STOCK CAMMED ENGINE (WELL AN LQ9 CAM IN A 5.3), I WAS HOPING IT WOULDN'T BE AN ISSUE BUT WOULD PREFER TO FIGURE OUT THE PINS TO TIE IT IN. I THINK YOU TIE INTO THE FUSE BLOCK IN TWO PLACES...AND DO SO WITH THE PRESSURE SWITCH WIRE...BESIDES IDLE, I'M JUST NOT SURE WHAT ELSE THE ECU WOULD DO. EDIT: Not sure if I would want the ECM controlling the circuit but I can see where it might be useful for example under WOT the ECM could/may cut off the AC pump. It may also do that under other conditions like overheating. My very simplistic ECM takes as input a ground to show A/C demand...all it does is increase idle...not control the circuit. Send it. -Kevin[/QUOTE] THANKS AGAIN KEVIN FOR YOUR DETAILED RESPONSE AND HELP!



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