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Old 10-19-2020, 08:37 PM   #1
truckster
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AC Principles of Operation - Electrical

I posted this on the 88-98 board, but things seem kind of dead over there:

I'm installing the AC components from a 98 Silverado in my 72 Blazer with a 5.7 Vortec. I'm using the serpentine belt from 98 as well as all the components for the AC (all new). I want to make sure I'm wiring it right. If I understand correctly, one side of the compressor clutch harness goes to ground. The positive side runs from the relay (via the AC switch) through the low pressure switch as well as through a cycling switch and then to the other side of the clutch harness. One switch is on the accumulator and the other is mounted on the manifold at the rear of the compressor. Is this right?

Thanks in advance.
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Old 10-20-2020, 10:25 PM   #2
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Thumbs up Re: AC Principles of Operation - Electrical

That sounds right.
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Get out and drive the truck this summer and have some fun!
It sucks not being able to hear!

LWB trucks rule, if you don't think so measure your SWB!
After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.
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Old 10-21-2020, 12:38 PM   #3
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Re: AC Principles of Operation - Electrical

Thanks. I'll wire it up that way, and if it catches fire I'll share the blame...

Seriously, though, I appreciate the second opinion.
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Old 10-21-2020, 01:31 PM   #4
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Re: AC Principles of Operation - Electrical

In '98 the A/C compressor was computer controlled. The cycling switch and both high and low pressure switches reported to the PCM and the PCM controlled the relay in the fuse box.
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Old 10-21-2020, 02:50 PM   #5
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Re: AC Principles of Operation - Electrical

Quote:
Originally Posted by Bigdav160 View Post
In '98 the A/C compressor was computer controlled. The cycling switch and both high and low pressure switches reported to the PCM and the PCM controlled the relay in the fuse box.
I was afraid of that. I'll have to rethink my plan, I guess. Is there an earlier year that would use the same parts without a PCM?
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Last edited by truckster; 10-21-2020 at 03:06 PM.
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Old 10-21-2020, 03:11 PM   #6
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Re: AC Principles of Operation - Electrical

I am a commercial ac tech, but have not played with car stuff.
But can you buy a harness to connect into your system?
What about going to pick a part to see how they have done it.

I always thought one side if the ac coil was grounded, and the control side. Or 12V power was routed through a low pressure switch, and relay. If you use a single pole double throw. You can connect the idle speed coil too.
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Old 10-21-2020, 04:08 PM   #7
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Re: AC Principles of Operation - Electrical

My concern is that the system in the 72 was designed with a POA and a continuous-run compressor. I'm not sure if a newer compressor (it's a Sanden style) will be damaged by running continuously, or if the orifice tube will freeze over.

I could build a harness, but I don't want to add a PCM to control it. My theory is (was) that the cycling switch would shut off the compressor when the line pressure gets too high and the low pressure switch would simply keep it from running if it's too low on refrigerant (as a safety measure).

I still have the POA valve from the original system, so maybe I can adapt it somehow to fit into the new system and remove the cycling switch. I'm definitely going to have to do some more research.
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Old 10-22-2020, 07:22 PM   #8
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Thumbs up Re: AC Principles of Operation - Electrical

It will work. My 71 is hooked up as I said to begin with. Yes you can have the ECU control it if you want. But you don't have to. It's a matter of how hard are simple you want it. I wanted mine so I could work on it and wired it to cycle as it should.
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1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver
Seeing the USA in a 71


Upstate SC GM Truck Club
2013,14 and 2016 Hot Rod Pour Tour


http://upstategmtrucks.com/



Get out and drive the truck this summer and have some fun!
It sucks not being able to hear!

LWB trucks rule, if you don't think so measure your SWB!
After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.
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Old 10-22-2020, 11:19 PM   #9
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Re: AC Principles of Operation - Electrical

Thanks, Andy. So, just to be clear, you wired it the way I described and it works without having to have an ECU?
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Old 10-23-2020, 01:14 PM   #10
Andy4639
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Thumbs up Re: AC Principles of Operation - Electrical

Mine is wired to work off the fan switch in the dash. When I turn the fan switch on it goes through the 71 AC harness to the relay I mounted out in the engine bay to the compressor and low freon and cycling switch that is mounted to the accumulator. I have the orifice tube in the line going to the condenser under the battery tray.
Here is a link to some of my AC wiring stuff.
I also have the one fan running off a adjustable thermastat.



http://67-72chevytrucks.com/vboard/s...=705388&page=2
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1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver
Seeing the USA in a 71


Upstate SC GM Truck Club
2013,14 and 2016 Hot Rod Pour Tour


http://upstategmtrucks.com/



Get out and drive the truck this summer and have some fun!
It sucks not being able to hear!

LWB trucks rule, if you don't think so measure your SWB!
After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.
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Old 10-23-2020, 02:24 PM   #11
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Re: AC Principles of Operation - Electrical

That's good news. Thanks.
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