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Old 06-23-2018, 07:52 AM   #1
70GMCer
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AC Pressure Switch ?

Still gathering parts to restore my AC in the 83.

Regarding the "Pressure Switch"....I know there is a difference between the R12 & R134 switches.

But, I see on the original R12 switch, there is what looks like an adjustment screw. (see pic)

Has anyone adjusted this switch to work with the R134 conversion & how was it adjusted?

Correct me if I'm wrong, but I believe that the pressure switch would have to be adjusted to a lower setting.....

Or....make it easy......is there a direct replacement for this switch?

One more thing is that the AC system on my truck had been previously changed over to R134.

So, that switch might have been already adjusted..
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Old 06-23-2018, 02:42 PM   #2
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Re: AC Pressure Switch ?

If it's a working LPCO switch you can adjust the cut-in pressure for R134a... if you need to.

Hopefully, there's a HPCO switch in the system now as well.
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Old 06-23-2018, 03:17 PM   #3
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Re: AC Pressure Switch ?

Quote:
Originally Posted by hatzie View Post
If it's a working LPCO switch you can adjust the cut-in pressure for R134a... if you need to.

Hopefully, there's a HPCO switch in the system now as well.
With a 134 conversion, would the low be set at 30psi cut out? Also, I didn't think there was a high pressure switch for this year. If I install one, would I wire it in series with the low switch?
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Old 06-23-2018, 07:52 PM   #4
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Re: AC Pressure Switch ?

I have adjusted them. don't remember the exact pressure though.
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Old 06-25-2018, 10:10 AM   #5
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Re: AC Pressure Switch ?

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Originally Posted by 70GMCer View Post
With a 134 conversion, would the low be set at 30psi cut out? Also, I didn't think there was a high pressure switch for this year. If I install one, would I wire it in series with the low switch?
HPCO should be wired in series between the LPCO & the AC clutch.

The refrigerant charge level, orifice size, and the compressor cycling switch are kindof an interesting dance.

If the charge level is too low the evaporator will freeze. If the charge level is too high the compressor will suffer and vent temps will rise. As outdoor ambient temps rise the compressor head pressures rise.

The LPCO or Compressor Cycling Switch needs to cut out at a high enough pressure that the evaporator doesn't completely freeze and yet not short cycle the clutch. If the pressure is too low and the compressor doesn't cycle the evaporator will freeze regardless of charge level.

R134a should be roughly 35F at 31psi in the evaporator.
I wouldn't set the Cycling Switch any lower til you get some runtime on it and observe vent temps and system pressures at around 85F ambient outdoor temp on MAX AC and max fan.
Good vent temps should be around 40-45F.

At the temperatures we're interested in, 20F-40F R134a runs @ 2PSI lower pressure than R12.

That adjustment screw is sensitive... One eighth turn counterclockwise lowers the cut out pressure approximately 2 PSI and one quarter turn lowers the cut out pressure approximately 4 PSI.

E38.org refrigerant pressure vs temp table
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Old 06-25-2018, 12:43 PM   #6
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Re: AC Pressure Switch ?

Thank you for the input on this.

I'm thinking that your readings are assuming the orifice has not been changed to a R134 orifice.?

I was advised on another thread that the original orifice should be changed when converting to R134. The size recommended was a #61 size.

Keep in mind that I'm NOT going to use the original A4 compressor but will be using a Sanden 508 along with a new evap, condenser, dryer & lines.....
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Old 06-25-2018, 11:09 PM   #7
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Re: AC Pressure Switch ?

A different orifice will have an effect on cooling but it's due to the pressure change across the orifice.
The compressor choice will affect the volume of gas being moved, how much energy it robs from the engine, and how long the compressor itself lasts. The old Harrison R4 is not noted for longevity and it's a fairly small displacement compressor compared to the Sanden 508 & the old Harrison A6. The A6 is old, heavy, and eats a lot more fuel to run than the 508.

You have several different things under your control when you charge the system that affects the temps inside the evaporator and the amount of heat energy the refrigerant moves.
You're charging with a refrigerant that the system was not designed for...
I use the JungleEddy method for charging the system because it seems to yield consistent results no matter what I choose for refrigerant. However, it's not entirely a recipe. There's some skill involved. That's why I called it a dance.

Unless someone suddenly changed the basic gas laws I learned in high school chemistry the published pressure to temp relationship of each of these fluorocarbon refrigerants should be fairly static and follow the table and the graph I plotted in Libre Office Calc as a visual using values from the table.

Boyles' law and Charles' Law are the ones I remember as pertinent...
http://chemistry.bd.psu.edu/jircitano/gases.html
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And please let us know if and how your repairs were successful.

Last edited by hatzie; 06-25-2018 at 11:27 PM.
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Old 06-26-2018, 07:24 AM   #8
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Re: AC Pressure Switch ?

Thanks again.....I'll post back on here after I get this up & going......
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Old 06-28-2018, 06:49 AM   #9
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Re: AC Pressure Switch ?

70gm , I have a 82 and used the oem low pressure switch . When I charged it it seemed to cycle to much . I put just a tad more 134 in it ( not much ) . This solved the cycling and had been working perfect ever since . I am using and ls compressor from a 5.3 truck . So I think your switch would be fine . Just fine tune your 134 charge to your system . I would try this first to see if it would work for you .
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Old 06-28-2018, 06:57 AM   #10
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Re: AC Pressure Switch ?

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70gm , I have a 82 and used the oem low pressure switch . When I charged it it seemed to cycle to much . I put just a tad more 134 in it ( not much ) . This solved the cycling and had been working perfect ever since . I am using and ls compressor from a 5.3 truck . So I think your switch would be fine . Just fine tune your 134 charge to your system . I would try this first to see if it would work for you .
Thanks...I'll give that a try. I'm still in the installation stage of all the parts
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Old 06-30-2018, 07:28 AM   #11
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Re: AC Pressure Switch ?

After doing more research, I found that they make an all in one hi-lo Binary Switch.

They have different pressure ratings though.

Some are rated at 28 off & 29 on low side, 400 off & 398 on hi side, & so on.....

Would anyone know which one to get?

I did look up the part number on the original switch to see if it was a hi-lo one. It just lists for low pressure.

So, I'm thinking that there wasn't a hi pressure switch on these truck ac systems..
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Old 06-30-2018, 04:45 PM   #12
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Re: AC Pressure Switch ?

R12 usually only had a LPCO.
Several AC hose and fitting vendors have liquid side fittings with Schraeder valves inside so the switches can be changed without evacuating the system.
You may be able to get a head for the Sanden 508 that has provision for a HPCO switch... if not you can get braze in sections for the hard lines with an HPCO fitting as described above. The latter is the route I took. HPCO is installed near the high side gauge port on my 76.

You definitely can get a 7 piston Sanden 508 compressor head machined for the same line fittings as the 1983 Harrison R4. https://www.ebay.com/itm/Dirty-Dingo...-/191979228686

The Third Gen F-Body guys have a lot of info on this kind of conversion.
https://www.thirdgen.org/forums/cool...e-systems.html

If you want an engineers perspective look at the E38 BMW boards... the Corvette boards are a friendlier crowd and full of engineers too.
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Old 05-15-2023, 12:53 PM   #13
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Re: AC Pressure Switch ?

Quote:
Originally Posted by hatzie View Post
R12 usually only had a LPCO.
Several AC hose and fitting vendors have liquid side fittings with Schraeder valves inside so the switches can be changed without evacuating the system.
You may be able to get a head for the Sanden 508 that has provision for a HPCO switch... if not you can get braze in sections for the hard lines with an HPCO fitting as described above. The latter is the route I took. HPCO is installed near the high side gauge port on my 76.

You definitely can get a 7 piston Sanden 508 compressor head machined for the same line fittings as the 1983 Harrison R4. https://www.ebay.com/itm/Dirty-Dingo...-/191979228686

The Third Gen F-Body guys have a lot of info on this kind of conversion.
https://www.thirdgen.org/forums/cool...e-systems.html

If you want an engineers perspective look at the E38 BMW boards... the Corvette boards are a friendlier crowd and full of engineers too.

Old thread, but would it be possible to replace the factory LCO swith with an aftermarket Binary switch from Vintage air?

EDIT: Is the switch in the accumulator already abinary switch???

Last edited by Hart_Rod; 05-15-2023 at 01:09 PM.
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Old 05-15-2023, 11:15 PM   #14
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Re: AC Pressure Switch ?

Quote:
Originally Posted by 70GMCer View Post
After doing more research, I found that they make an all in one hi-lo Binary Switch.

They have different pressure ratings though.

Some are rated at 28 off & 29 on low side, 400 off & 398 on hi side, & so on.....

Wire it like factory, use factory low pressure switch.
If you add a high pressure switch, it is a seperate switch, on the high side of the a/c...

High ac pressures can be from too much ac charge, low airflow across the condenser, poor cooling or really high outside temps.. or any combination.

Would anyone know which one to get?

I did look up the part number on the original switch to see if it was a hi-lo one. It just lists for low pressure.

So, I'm thinking that there wasn't a hi pressure switch on these truck ac systems..

Post is old, but..
Jesus christ, the all in one sensor. The hi lo pressure savior switch...
Low side sees 35 psi...
High side sees 250...
If your have this unicorn sensor installed on the low side and sees 250psi, you have more problems than you think..
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Old 05-17-2023, 07:12 PM   #15
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Re: AC Pressure Switch ?

Quote:
Originally Posted by 87chevy.com View Post
Post is old, but..
Jesus christ, the all in one sensor. The hi lo pressure savior switch...
Low side sees 35 psi...
High side sees 250...
If your have this unicorn sensor installed on the low side and sees 250psi, you have more problems than you think..
x2
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Old 05-17-2023, 08:52 PM   #16
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Re: AC Pressure Switch ?

Quote:
Originally Posted by 87chevy.com View Post
Post is old, but..
Jesus christ, the all in one sensor. The hi lo pressure savior switch...
Low side sees 35 psi...
High side sees 250...
If your have this unicorn sensor installed on the low side and sees 250psi, you have more problems than you think..
Quote:
Originally Posted by Corts60 View Post
x2
Ok, not ashamed to admit I’m not the smartest when it comes to AC stuff. I’ve installed 4 or 5 complete Vintage air setups and now I’m doing a hybrid with factory components. I probably should have thought that question through some more, but Vintage Air does offer a binary switch. How does it work? Is it placed on a different location than a factory Setup on a 93 Silverado?
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Old 05-19-2023, 09:12 AM   #17
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Re: AC Pressure Switch ?

Quote:
Originally Posted by Hart_Rod View Post
Ok, not ashamed to admit Im not the smartest when it comes to AC stuff. Ive installed 4 or 5 complete Vintage air setups and now Im doing a hybrid with factory components. I probably should have thought that question through some more, but Vintage Air does offer a binary switch. How does it work? Is it placed on a different location than a factory Setup on a 93 Silverado?
Vintage Air's binary switch (or trinary) is placed in the dryer (high pressure liquid cold side) of their systems and isn't necessarily a cycling switch but a system protection switch. Most of the OEM A/C system of which I am familiar, use a low pressure side switch to control the system pressure.

For me currently I am trying to convert my 74 C10 to a different form of clutch control. I converted to R134a years ago and the system worked but after letting the truck sit while redoing the engine the refrigerant leaked out. I refilled the system again but was having issues with the main inline fuse that kept turning the blower motor off and I believe freezing up the evaporator, at that the capillary cycle switch would shut the compressor down. So I am trying to use a low pressure switch from a later truck or GM car to control the compressor but I am concerned that the R12 might not have the adjustment range needed.
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Old 05-29-2023, 11:48 AM   #18
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Re: AC Pressure Switch ?

Quote:
Originally Posted by 406 Q-ship View Post
Vintage Air's binary switch (or trinary) is placed in the dryer (high pressure liquid cold side) of their systems and isn't necessarily a cycling switch but a system protection switch. Most of the OEM A/C system of which I am familiar, use a low pressure side switch to control the system pressure.

For me currently I am trying to convert my 74 C10 to a different form of clutch control. I converted to R134a years ago and the system worked but after letting the truck sit while redoing the engine the refrigerant leaked out. I refilled the system again but was having issues with the main inline fuse that kept turning the blower motor off and I believe freezing up the evaporator, at that the capillary cycle switch would shut the compressor down. So I am trying to use a low pressure switch from a later truck or GM car to control the compressor but I am concerned that the R12 might not have the adjustment range needed.
Good info. Have you tried a LCO switch from a 93 or newer OBS? I think they were R134A I believe. Just to give you a little background on the system I'm working on. I have a 93 Silverado that I have installed a 2017 L86/8spd LT swap. I used 2017 PWM fans that are controlled by the computer. I replaced all the factory components with OEM stuff (pretty much the same system in a squadrebody) except for the compressor, which is a Sanden SD7. I used the factory AC low cut off switch and installed a 3 wire switch from the LT engine to control the fans. My only mistake was installing that switch in between the condenser and the dryer. I didn't think about the orifice tube being in the outlet of the condenser and therefore my switch wasn't seeing high enough pressures to kick the fans on. This was causing exceedingly high pressures that would not go down until the engine temp kick the fans on. I purchased a fitting that bolts onto the compressor discharge port and moved my fan pressure switch there. Now everything works as it should (fans come on almost immediately) and pressures are perfect. My only other concern is there is no high pressure protection other than the fan speed increasing (pressure based) and the SD7's pressure relief valve. Due the way the PWM fans are controlled, I'm not sure how to wire in a High Cutout switch without causing short cycling of the compressor. Maybe it's not necessary???
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Old Yesterday, 12:12 PM   #19
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Re: AC Pressure Switch ?

I ended up finding a low pressure cycle switch from a 1985 that is made for R134a. The Murray number is 36674.
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