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Trinary Switch Theory of Operation

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  • Trinary Switch Theory of Operation

    I understand how a trinary switch operates. My problem is understanding the pressure specs needed to operate an R134a system properly.

    The universal switch I am using now has a low cutoff of 28PSI and a turn on at 29PSI. The fan turn on is 227PSI and off at 185PSI.

    The 28 and 29 PSI settings are too close IMO. Once the pressure gets down to 28PSI it takes no time at all for it to get to 29PSI. This results in rapid compressor cycling and causes the controller to lock out the compressor circuit. The factory switch specs are open at 24PSI and off at 44PSI which seems logical to me.

    The fan switch PSI is too low. My high side is typically is around 275-300PSI. The fan turns on at 227PSI and stays on. I don’t see how the pressure could ever get lower than 185 to turn off the fan. It seems like both pressures should be a few PSI above the normal operating range.

    Am I looking at this correctly?

  • #2
    92 Chevy Truck: OK - It's a CCOT set up and OE came with just a LPCO should have been adjustable just FYI and warning you doing so it may leak! Screw is inside that OE if OE still there.

    HPCO required to retrofits just wanted to cut out before it blew out most didn't both just a place for a leak. If you want now all electric fan do remember you are cooling the engine is more important look for a spot to add a switch for return coolant.

    Just off the top of my head thing of 134a as 80% of the "caloric" heat exchange power of R-12 so pressures and efficiency adjust to about the 20% you need to make up and pressures run higher about that give or take.

    It's your venture can be done with a lot of thought and right choice of fan I'd probably go for a two speed you need it also not using A/C don't forget that. No engine, no cooling for you is not a good trade off.

    There may be pre designed kits already for GM trucks especially let you do the searching they are NOT at this site nor the old one this came from.

    If determined I'm personally sorry never did this nor would either! The layout is already more than adequate if you choose the right fan clutch and OE fan or hunt down better.

    If you don't mind seasonal switching fan clutches the non thermostatic one pull lots more air than thermostatic ones without regard to heat coming at it. Done that and swapped back just for off season.

    Always your call what to take on. Almost doesn't matter but think a real strong fan clutch when you don't need it both A/C and engine is really sucking up some horse power but so is an alternator or electric fan(s) probably about equal but can shut totally off if not needed by speed of vehicle alone the key advantage,
    MetroWest, Boston


    • #3
      see your other post.