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How to test high side pressure switch

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  • How to test high side pressure switch

    I have a 1992 Rexhall airex motorhome built on a Ford F53 chassis with a 7.5 litre (460 cid) V8 engine. I've had it for a year and a half, and this year I am trying to get the AC to work. At some point it was retrofitted from R12 to 134a. But the writing on the retrofit sticker is no longer visible, so I don't know when the retrofit was performed, or the amount of refrigerant the system holds. These photos are of the high side pressure switch. On the side of the switch it used to say "R12 R134a." Listing both refrigerants makes me think the switch could work with either system. Also, it has two sets of contacts, possibly one set to use for R12, and the other for 134a. My compressor does not kick in, but I can get it to operate by taking 12 volts directly to the compressor clutch. I am wondering about this pressure switch. Is there a way to test it without a fully pressurized system? How does this switch operate? does it kick the compressor on when a certain pressure is reached? Or does it kick the compressor off when a set high pressure is reached? Thanks for the help!

  • #2
    That is called a "trinary" switch. It preforms 3 functions. 1) is low pressure cut out that shuts off the compressor when the system is low. 2) high pressure cut-out, shuts off the compressor if the high side is too high to prevent blowing a hose or opening the blow-off valve on the compressor if so equipped, and 3) it controls a fan for the condenser if the high side pressure rises to about 275 PSI.
    The compressor is controlled by one set of terminals and the fan by the other.
    If it has the factory plug, the compressor wires are black and black with a tracer and the fan is yellow and orange.
    The same switch is used for R12 and 134a. The low pressure cut out is around 35-50 psi and the high pressure is around 350- 375 PSI
    Most likely you are low on refrigerant, so connect a set of gauges and let us know what the pressure readings are.
    Last edited by Cornbinder89; 06-27-2017, 10:03 PM.

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    • #3
      Thank you for the explanation. That helps. It only had one set of wires on it, one of which runs to the compressor clutch. The fan appears to have been re-wired to a switch on the dash. Your explanation will help me get things put back to how they should be. Thank you. Is there any way to test this switch off the vehicle?

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      • #4
        Yes, but it is not easy, you need to apply pressure and test for continuity between the terminals. There needs to be enough pressure to "turn on" the compressor. Much easyer to test on the vehicle with a set of gauges on the A/C system. If the A/C system is below 50 psi with the compressor not running, Then you are low on refrigerant and that switch is doing its job and keeping the compressor from running and burning it up.
        I've had the switches fail, but more common is low refrigerant.

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        • #5
          I have another question....Still trying to figure things out here....The receiver/drier has some fittings on the "out" side that include these spring loaded valves or something. Two of them, right in a row, on the same side, as pictured. What purpose do these spring loaded valves serve? Do they need to be there? And why two of them...?

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          • #6
            I seam to have a problem with the site, so not sure this will be readable. I think those are "quick connect" fittings used during mfg so the system can be connected while it (or part of it) is already charged. I would replace with a drier that takes std O ring fittings and eliminate the quick connects.

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            • #7
              Thank you. I will follow that advice.

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              • #8
                As I mentioned, this 1992 motorhome was retrofitted from R12 to 134a at some point. The sticker is completely unreadable. Must have used disappearing ink! As I'm getting ready to re-charge the system, one question remains: How can I know what type of oil to use in the system and how much, and also, how much refrigerant to use?

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                • #9
                  Oil: ester or PAG. If you are draining and starting from scratch, it doesn't matter if you are adding, better not to mix type. Amount: depends on the compressor, What compressor does it have? I don't know about amount, without a sticker, try and contact the Mfg, otherwise its going to be trial and error.

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                  • #10
                    Trial, yes, but hopefully not too much error! Ha! Thanks

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                    • #11
                      Well, ultimately I have not had success. The trinary pressure switch is working as it should. System is nice and tight, holding vacuuming perfectly. Compressor kicks in as it should. I charged the system with 44 oz of refrigerant (2.75lbs)....BUT NO COOLING! High pressure side would rise to 250, and then the fan would kick in, as it should. Low pressure side, however, was reading about 50 or so, and staying right there. I was expecting it to read 25-35. So there seems to be a problem somewhere, with the low side reading too high....fully charged, but no cooling. Any ideas? Thank you.

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                      • #12
                        try spraying the condenser with water while watching the pressures and vent temp. Also could be a stuck Tx (thermostatic expansion valve) that is stuck (if your system uses this)
                        How did you come up with the amount? last I knew that was an open question because the sticker was missing.
                        edit: not a stuck Tx with high low side, Early in the am and I am not thinking clearly. Overcharged? doubt it with that amount, what does the high side pressure do when the fan cuts in? Did you add oil or replace the drier?
                        Last edited by Cornbinder89; 07-07-2017, 09:36 AM.

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                        • #13
                          Thanks again for helping me with this. I added about an ounce of oil. Figured there was already some in the system. I came up with the 2.75lbs of refrigerant through a web search. Seemed like all the Ford trucks of this type were anywhere between 2.0lbs and 2.75 lbs. I had no way to verify the reliability of this information. Ford did not even provide the a/c systems on these motorhome chassis. It was up to the motorhome manufacturer, which is now out of business. When the fan kicks on, the high side pressure slowly falls. That seems right to me. And yes, I did replace the drier.

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                          • #14


                            From what I can tell, this has none of the "Ford" body and likely nothing of a Ford truck would apply except what compressor was used and what oil is in that compressor.
                            The #1 reason for compressor failure is lack of lube. When a compressor fails it can send debris though out the system and be very expensive to fix. I would Drain and refill the compressor so you KNOW you have enough oil.
                            How many evaporators does the system have, I.E is there front and rear A/C? are they both run off the engine?
                            The quick connects on the old drier suggest that nothing beyond the drier is "stock" Ford but all Rexhall. I would get on the .net and find an owners group for the Rexhall and get some hard facts on capacity and system type ( Tx or CCOT).
                            Compressors can take from mid 4's in oz to over 10 oz of oil. 1 oz wouldn't do much for a system that takes at least 4x that amount. Until you do that, you are playing a very expensive game of roulette!
                            The Ford truck system was likely CCOT and they use less refrigerant than a Tx generally. Also CCOT don't use a drier on the high side, so that is why I suspect a Tx system.
                            If we are going to have to guess at the amount, we are going to need a lot more info on the system.
                            1st order of business is to make sure there is enough oil in the system.
                            2nd is to try and get an amount of refrigerant from a Rexhall owners group.
                            Once those two are know to be correct, we can go about diagnosing faults.

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                            • #15
                              Well, it's definitely an expansion valve system. Front a/c only, so just the one evaporator. I have not had much luck finding much info online about the hvac system rexhall used. I had an equally difficult time when I needed to replace the heater core. I had to get creative, as the company which made the system, not to mention the RV manufacturer, is no longer in business.

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