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Old 05-02-2011, 11:56 AM   #1
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Default A/C question - do I need a low pressure switch?

I am using a 1999 Camaro A/C compressor on my swap. The A/C clutch is not controlled by the PCM (because the PCM is newer and the A/C request goes through serial data.) I plan on using the OEM high pressure switch to turn on the fans via the PCM.

Right now, there is a +12V wire from the car's climate control that engages the A/C compressor clutch. I originally planned on hooking that wire directly to the A/C compressor. Do I need a low pressure switch in that circuit to keep from damaging the compressor? Also, can that switch be installed inline, or do I need a relay? Where can I get one of these switches?

I looked at the A/C compressor wiring diagram for the 1999 Camaro and I did not see a low pressure switch. However, I am using a 2006 Sierra 2500 PCM and harness, and that A/C compressor wiring diagram does show a low pressure switch.

BTW, I don't know much about A/C systems. If I'm missing something else that's important please let me know!
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Old 05-02-2011, 12:56 PM   #2
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You need both a high and low pressure switch. It's called a binary switch. It's open below a certain pressure to prevent the compressor running without refridgerant (and oil). In addition it's also open above a certain pressure to prevent the pressure from getting too high. This is what cycles the compressor.
The low/high setpoints for positive displacement compressor are pretty much standardized. Your f body is a variable displacement and has slightly different operating pressures. Check with an auto AC shop for a binary switch, they'll have one for you.
btw, you can get a trinary switch too to control the fan as well as compressor.
Compressors are typically run directly through the switch. If you use the trinary switch to control the fan, run that through a relay.
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Old 05-02-2011, 01:23 PM   #3
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Let just say you can get away without them if you really wanted to.. you just have to wire everything up directly off of the control switches and panels. But why would you chance not having the switches there that are designed to add a safety factor into the system to keep it from burning up or exploding !
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Old 05-02-2011, 01:34 PM   #4
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Thanks for the info guys. I want to wire it up correctly with the proper protection. I just don't know enough about A/C to know what I don't know... ya know?

The binary switch goes on the "low side", right? And the A/C pressure sensor that runs to the PCM goes on the "high side"? Is the high side also where your fan controls come from?

Also, I noticed that the 2006 Sierra only has a low pressure switch (not a binary switch) and the Camaro has no switch at all. Why is that?
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Old 05-02-2011, 01:49 PM   #5
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IIRC the camaro runs the compressor through the ecu. A pressure sensor sends a signal to the ecu to operate as a binary switch. That controls off at hi and low pressures thru the ecu. Cant remember what effect AC has on fans.
Trucks run the same kind of sensor that cycles the compressor AND have the additional safety switch that interupts the AC control signal.
Without running the stock sensor and ecu controls, you need a binary switch.
In addition, temp controls use a thermostat for the 12 V AC on signal, thru the binary, then to the compressor.
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Old 05-02-2011, 02:36 PM   #6
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Quote:
Originally Posted by garys 68 View Post
You need both a high and low pressure switch. It's called a binary switch. It's open below a certain pressure to prevent the compressor running without refridgerant (and oil). In addition it's also open above a certain pressure to prevent the pressure from getting too high. This is what cycles the compressor.
The low/high setpoints for positive displacement compressor are pretty much standardized. Your f body is a variable displacement and has slightly different operating pressures. Check with an auto AC shop for a binary switch, they'll have one for you.
btw, you can get a trinary switch too to control the fan as well as compressor.
Compressors are typically run directly through the switch. If you use the trinary switch to control the fan, run that through a relay.
So through using a trinary switch, the fans would be on any time the AC is on, or just on when the compressor is on? Maybe have the tri switch control the fan relays? Details please
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Old 05-02-2011, 02:42 PM   #7
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I think the trinary switch is a binary switch with a third circuit for fan controls. For instance, a binary switch will act as a low pressure safety (will not turn on compressor unless there is a minimum 30psi line pressure) and a high pressure safety (turn off compressor if line pressure exceeds 400psi) and a fan switch (turn on fan relays if pressure goes over 250psi).

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So through using a trinary switch, the fans would be on any time the AC is on, or just on when the compressor is on? Maybe have the tri switch control the fan relays? Details please
On the the LSx vehicles, instead of having a trinary switch, there is a pressure sending unit. The pressure sending unit measures the line pressure and sends its reading back to the PCM. Then the PCM decides what it wants to do... if it wants the compressor on or off, or if it wants the fans on or off.

It looks like the Sierra 2500 had the pressure sending unit AND a separate low pressure switch. I wonder if that low pressure switch was on the high side or low side??
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Old 05-02-2011, 02:51 PM   #8
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That pretty much summarizes it as far as I know.
Details on trinary switch operation:
Fan on at 227 PSIG, off at 185 PSIG. Low opens at 28 PSIG closes at 29 PSIG. High opens at 454 PSIG closes at 369 PSIG

As for the additional switch on trucks...the switch interferes with the dash control....maybe it's there to prevent something other than the compressor from working....not really sure.
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Old 05-02-2011, 02:58 PM   #9
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The trinary switch is on the high side, right?

Quote:
Originally Posted by garys 68 View Post
That pretty much summarizes it as far as I know.
Details on trinary switch operation:
Fan on at 227 PSIG, off at 185 PSIG. Low opens at 28 PSIG closes at 29 PSIG. High opens at 454 PSIG closes at 369 PSIG

As for the additional switch on trucks...the switch interferes with the dash control....maybe it's there to prevent something other than the compressor from working....not really sure.
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Old 05-02-2011, 03:03 PM   #10
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Yep, typically on the dryer or a crimp on inline fitting after the condenser.
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Old 05-02-2011, 03:17 PM   #11
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Okay cool.

Even though my Camaro compressor is variable displacement, will I still need a binary switch on the high side to cycle the compressor on and off?

Also, is there ever any type of switch on the low side, such as a low pressure switch?

It seems to me like the high pressure switch would be on the high side, and the low pressure switch would be on the low side.
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Old 05-02-2011, 08:06 PM   #12
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I'm no expert on the variable displacement compressors, but I do know several here have used them with the standard binary switches. My understanding is that the compressor pressures are different so the efficiency is decreased.
I would check with an AC shop and use a binary switch on the high side (AC units have been doing that for 40 years) that will hopefully work with the variable displacement compressor.
I'm guessing you're using a condenser, evaporator, and expansion valve from a positive displacement system, so it's going to be a hybrid anyway.
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Old 05-02-2011, 10:03 PM   #13
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Thanks for the info. I'll need to find a good A/C shop to get the nitty gritty questions answered. I think I have enough now to run the appropriate wires through my wiring harness.

So far, it looks like I'll be able to use all of the parts from the Camaro except for the evaporator and the expansion valve. I might even be able to take out the expansion valve and use an orifice tube if it would make the system work better.
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Old 05-02-2011, 10:03 PM
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