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Thread: Low pressure switch - cutoff pressure for R134a

  1. #1
    Senior Member 82DMC12's Avatar
    Join Date:  May 2011

    Location:  Olathe, KS

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    Low pressure switch - cutoff pressure for R134a

    I just completed a 1,500+ mile round trip from Kansas City to Kentucky Bourbon Country. The car performed beautifully, which is confidence-inspiring after my frame off which I completed a year ago. Even in driving rain, no leaks - and that was the first time my car saw rain in over 10 years!

    The only issue I had (aside from a wiper blade malfunction) was I found on an overcast day in rain (humid, so needed the AC on but low outside ambient temp) my A/C would gradually have less air volume for a given fan speed. I assumed I was experiencing evaporator frost-up. When I parked at the destination, sure enough my accumulator had frost on it. On the drive back home (9 hours) even though it was hot out I had to turn the mode switch to Vent for about 10 minutes every two hours to defrost the evaporator and get my air speed back up. High ambient temp on the way home did not allow the accumulator to frost but you could tell it was frosting inside the HVAC box.

    When I got home I put gauges on it and found my charge and operating pressures to be correct but the low pressure switch wasn't cutting out until about 15 PSI. Of course that would cause the compressor to cycle less often, if at all, given certain ambient temps and steady engine speed.

    I have it adjusted now to 20 PSI which, so far, is working great and still giving 38F vent temps. I would have to take a long road trip to really test it again though.

    Wondering if 20 PSI is sufficient or if I will continue to get freeze-up? I see 18-21 PSI is "normal" for R134a but one vendor on a different post mentioned 25 PSI which I think would result in short cycling on all but the hottest days.
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!
    Total frame-off restoration completed 2021-2023

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Kansas City

  2. #2
    TNDMC Founder JBaker4981's Avatar
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    Following.

    Ironically enough, my car is doing the same thing from the same trip. Still have not followed your advice as I've not even LOOKED at the car since I unloaded it on Sunday... but I'm going to tackle this before Friday night if I am going to an AACA car show on Saturday

    I can let you know of the behaviors after the adjustment
    Jesse Baker
    VIN 628
    Black Interior, Automatic
    TNDMC: TN DeLorean Motor Club

  3. #3
    Senior Member 82DMC12's Avatar
    Join Date:  May 2011

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    I haven't heard anything from anyone else regarding this topic, but so far so good at 20 PSI cutoff. Air conditioning is working very well and no signs of freeze-up since adjusting the switch. I haven't been on another long trip yet but maybe I'll go on a weekend afternoon highway cruise to give it another test. Another plug for ceramic tint, too - the cabin cools down a lot faster and stays comfortable, so nice that I usually only have the A/C on fan speed 1 or 2 once the cabin has cooled, and even then, the blend knob only around 2 o'clock. My A/C is highly effective!
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!
    Total frame-off restoration completed 2021-2023

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Kansas City

  4. #4
    Senior Member
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    Quote Originally Posted by 82DMC12 View Post
    I haven't heard anything from anyone else regarding this topic, but so far so good at 20 PSI cutoff. Air conditioning is working very well and no signs of freeze-up since adjusting the switch. I haven't been on another long trip yet but maybe I'll go on a weekend afternoon highway cruise to give it another test. Another plug for ceramic tint, too - the cabin cools down a lot faster and stays comfortable, so nice that I usually only have the A/C on fan speed 1 or 2 once the cabin has cooled, and even then, the blend knob only around 2 o'clock. My A/C is highly effective!
    I have mine set to about 20-21PSI as well, haven't had any issues and the A/C works really well.

  5. #5
    Senior Member
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    Four Seasons 36674, or NAPA 207883 is a direct replacement for the original switch, but is calibrated for R134a

  6. #6
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by Ethan Rode View Post
    Four Seasons 36674, or NAPA 207883 is a direct replacement for the original switch, but is calibrated for R134a
    Do you happen to know exactly what the cutoff pressure is?

    Sent from my Pixel 6a using Tapatalk
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!
    Total frame-off restoration completed 2021-2023

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Kansas City

  7. #7
    Senior Member
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    The best way to set up for -134 is to measure the correct amount into the system. It is 10% less than the amount of R-12. Then you set the pressure switch to cycle the compressor with the motor at 1,000 RPM's and the fan set to high. This is assuming the evap coil and the condenser coil are nice and clean and you have a fan blowing at the front of the car while you do it (and it is not TOO hot) where you are doing it. It's all about temperature.
    David Teitelbaum

  8. #8
    Motors about after dark Michael's Avatar
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    I just worked on my system and need to adjust my switch and this is good info. I believe I had it set to 25-26ish and it always worked well but if I can get away with 20, I'll try it.
    http://dmctalk.org/image.php?type=sigpic&userid=90&dateline=161808992  9

  9. #9
    Junior Member
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    Quote Originally Posted by 82DMC12 View Post
    Do you happen to know exactly what the cutoff pressure is?

    Sent from my Pixel 6a using Tapatalk
    according to rockauto "off" is 25, and "on" is 49. Although rockauto also shows this switch fitting a 78-93 chevrolet, which makes me think its an r12 switch since r134a wasnt used (at least on chevy pickups) until 1994 models. However the pressures are in line with r134a.
    on the subject, the delorean was originally filled with 2.7lbs of r12, then they later released a bulletin stating to reduce it to 2.2lbs of r12. A 1998 chevy pickup factory spec takes 2.0-2.2lbs of r134a, the condenser is larger compared to a delorean, but theres far less hose on the pickup, im convinced a delorean needs 2.0-2.2lbs of r134a, I certainly wouldnt run any less. Both the pickup and delorean use an accumulator, and a sanden style compressor. this is how i set up my car when i did the conversion and it seems to work great.

  10. #10
    Senior Member 82DMC12's Avatar
    Join Date:  May 2011

    Location:  Olathe, KS

    Posts:    1,725

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    Quote Originally Posted by The_J View Post
    according to rockauto "off" is 25, and "on" is 49. Although rockauto also shows this switch fitting a 78-93 chevrolet, which makes me think its an r12 switch since r134a wasnt used (at least on chevy pickups) until 1994 models. However the pressures are in line with r134a.
    on the subject, the delorean was originally filled with 2.7lbs of r12, then they later released a bulletin stating to reduce it to 2.2lbs of r12. A 1998 chevy pickup factory spec takes 2.0-2.2lbs of r134a, the condenser is larger compared to a delorean, but theres far less hose on the pickup, im convinced a delorean needs 2.0-2.2lbs of r134a, I certainly wouldnt run any less. Both the pickup and delorean use an accumulator, and a sanden style compressor. this is how i set up my car when i did the conversion and it seems to work great.
    Yeah but a Chevy truck doesn't have 20 feet of refrigerant lines going front to back each side.

    Sent from my Pixel 6a using Tapatalk
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!
    Total frame-off restoration completed 2021-2023

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Kansas City

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