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Trinary Switch Selection

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  • Trinary Switch Selection

    I am installing an electric fan in my 92 Chevy truck to replace the mechanical fan. It is currently running 134A with a rebuilt system consisting of new o-rings, new compressor, accumulator, orifice tube, parallel flow condenser and a flushed system. I'm looking for a recommendation for a trinary switch to control the fan.

    Instead of using a typical universal switch with wires or spade terminals I would like to find one that has the connector directly attached to the switch and which also includes the matching connector. I've been able to find a few universal ones and some for specific applications but I'm not sure what the correct pressures should be for this setup. I'm more worried about the fan switch closing pressure. It seems like there is a fine line when it should turn on. I don't want the pressure so high that it waits too long to turn on the fan like when at a standstill.

    Any ideas what a good pressure spec would be? Actual part numbers would be a plus! I guess I could base it off the high side pressures while at a standstill and going down the road as a last resort.



  • #2
    A trinary switch is a three function switch, low pressure cut out, high pressure cut-out and fan/shutter. I have only seen them in one pressure setting. You can buy the matching harness plug to go with it.
    IIRC the fan turn on is between 250 and 270 psi and is not adjustable. They are made in NO and NC versions for the fan trip.
    If you just want a fan switch then you have more choices.
    A trinary switch has 4 wires, 2 for the HPCO/LPCO and 2 for the fan/shutter override.
    Last edited by Cornbinder89; 08-03-2018, 09:39 PM.


    • #3
      I wound up with a UAC brand switch, #4085. The fan turn on is at 227 PSI. A typical pressure chart says the high side will be 275-300PSI at 95 ambient. I'm within that range. With those pressures the fan is always running! I can't see when the pressures would ever get low enough to turn off the fan going down the road. Seems like both pressures should be a few PSI above the normal operating range.

      Also, the low cutoff pressure is 28PSI and turn on is 29PSI. That causes the compressor to rapidly cycle, (and locks out the controller) because it doesn't take much time at all for the pressures to cycle 1PSI.

      What am I missing here??!!
      Last edited by 69-er; 08-21-2018, 04:29 PM.


      • #4
        1st, a trinary switch goes in the high side. so no, it will not cycle the compressor. There are only 2 times the high side will get down to 28 psi, 1) it is freeken cold out and most the refrigerant is in liquid form at low temps, and 2) you have lost most all of the refrigerant.
        High pressure is a function of condensing temp, so there is no such thing as "normal operating range" the range is determined by heat load, air temp through the condenser, the efficiency of the condenser.
        Not familure with the one you bought. The RED DOT ones are the most common and cost about $65. On those the fan turns on somewhere in the neighborhood of 250-275 PSI and the HPCO is between 375- 400 PSI.
        They are primarily made for heavy trucks with air operated fan clutches.
        On my stuff, even in 100 deg weather, as long as I am moving over about 25 MPH my fan stays off. I have a big condenser, so even at 100 deg temps, my high side is below 250, It all comes down to condensing temp.
        Remember, these go in the high side, you'll still need your cycling clutch switch in the accumulator for the CCOT system.
        If you want different fan settings ditch the trinary switch and go with separate fan and HPCO switches then you have a lot of choice on psi settings. The LPCO function is covered by the clutch cycling switch.


        • #5
          Duh! How did I miss that? Of course the trinary switch goes on the high side. I “knew” that but in the early planning stages I just planned on replacing the cycling switch with the trinary switch without thinking.

          I’ll reinstall the cycling switch. That always worked fine before with 134a. I found a fan switch in the Sanden catalog that will insert into the HPCO port on the compressor. On at 283PSI and off at 210PSI. Another one’s specs is on at 240PSI and off at 190PSI. Not sure which is best. Don’t want it to turn on unnecessarily early or wait too long either. I also need to decide if I should go the extra step to also install a HPCO. A few pros and cons on that…

          BTW, the reason I changed to an electric fan was because the mechanical fan was too noisy for me and it didn’t pull enough air in stop and go traffic to keep the AC temps cool.



          • #6
            Put the fans switches somewhere else and put the correct HPCO in the compressor if it was designed for one
            Those pressure switches sound like fan pressure settings not HPCO which should be closer to 375-400 psi. HPCO is just to prevent hose burst or compressor clutch from slipping. Under certain conditions the high side may approach the upper 300's and be considered normal, but not ideal. You don't want a HPCO set too low or you'll loose cooling under the worse conditions.
            You don't want to eliminate the HPCO, it serves a valuable function. It can save the system from damage.
            If you have a hose crimper it is easy to add a 10mm or 1/4" flare port in the high pressure line.


            • #7
              Correct, they are fan switches. They are designed to be mounted where the HPCO switch goes. But, since I have decided to install an HPCO, I'll put it where it was originally designed to go and install the fan switch on one of the high side hoses. Since I'll be installing a port for this I have a few more pressure options since I can use just about any fitting size. I just can't decide on the pressure. I guess I could get a few different pressures and see which one works the best for my application.

              Any thoughts?

              Does it really matter if the fan switch is mounted before or after the condenser? I guess pressure is pressure whether it's vapor or liquid.

              I have a bead lock crimper so that will be a piece of cake.

              Appreciate the discussion!


              • #8
                Np it doesn't matter, the pressure will be the same, most are placed after as the measured fluid is cooler and that is easier on seals.