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Technical AC system dryer question

Discussion in 'Traditional Hot Rods' started by AGELE55, Jul 9, 2021.

  1. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    I'm currently upgrading the AC in my 55 Chevy. I need a new dryer. I see some dryers have a Schrader valve. Would this valve become the high pressure side connection for testing? I already have a high side schrader so just double checking it would provide no additional function.
    Ps- I am NOT an AC proficient kind of guy..
     
  2. tim troutman
    Joined: Aug 6, 2012
    Posts: 375

    tim troutman
    ALLIANCE MEMBER

    Schrader on the drier would be for a switch highside bianary or trianery to make the compressor cycle or a fan come on if to high pressure or if no Freon keep the compressor from running
     
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  3. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    So... In essence..is it needed? My old system did not have one.
     
  4. onetrickpony
    Joined: Sep 21, 2010
    Posts: 536

    onetrickpony
    Member
    from Texas

    It's not necessary if you have one somewhere else and don't need a pressure switch.
     
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  5. Ziggster
    Joined: Aug 27, 2018
    Posts: 922

    Ziggster
    Member

    Every AC system requires a high-pressure switch to cycle the compressor off for safety reasons in addition to a pressure relief valve. If you have none mounted (switch) on the compressor discharge fitting, or high-pressure side of the system already, you’ll need a port on the filter/drier. This is assuming you have a liquid type filter/drier and not the GM low-side (vapour) style accumulator (filter/drier).
     
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  6. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    The 55 will be running an R4 compressor, as it has for the past 20 years. It never had high or low pressure switches and ran fine until it simply wore itself out.
    If I need to add these switches, now's the time to do it I guess .
    I'm looking on line and see the R4 compressor accepts a high pressure switch right into the backside. Easy nuff.
    As for a.low pressure switch, I guess I can add one into anyplace along the low pressure line?
     
  7. onetrickpony
    Joined: Sep 21, 2010
    Posts: 536

    onetrickpony
    Member
    from Texas

    Most GM systems with an R4 used a low pressure switch to protect the compressor if it ran low on freon. They also used a clutch cycling switch if the system had a fixed orifice instead of an expansion valve. Some guys use a trinary switch to activate an electric fan when the compressor comes on. It all depends on your exact setup.
     
  8. SPEC
    Joined: Feb 1, 2021
    Posts: 396

    SPEC
    Member

    The trinary switch turns on the electric fan when the head pressure reaches 243 psi.
     
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  9. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    Ok...I truly appreciate the help, but you’re talking over my head. Let’s go back to “I am NOT an AC proficient kind of guy”...
    I have purchased a Vintage Air under dash unit which uses an expansion valve on the evap.
    My under hood setup is a serpentine driven R4 compressor.
    I have no binary, trinary, quadrupinary , ..or any other switches.
    What do I need to make the under dash unit play well with an R4 compressor?
     
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  10. LOL!
    First off , you need a low pressure and a high pressure tap. Schrader valve if you will. I am using a Sanden and both are at the compressor. They CAN be any place as long as the evaporator, the under dash unit, is in between. A binary [ means it is doing two jobs ] switch somewhere. Mine is at the dryer. Shuts the compressor off, by interrupting the current to the clutch, if unit is turned off, if Freon pressure is too low or two high. Vintage can/should sell one to you.
    Good luck

    Ben
     
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  11. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    Found some tech articles on the Vintage Air site which explain some compressor ops, safety switches, and general operation. I'll call their tech support line tomorrow and see what they have to offer for advice.
     
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  12. You will need at the least a binary switch on your drier. If you want the system to turn on an electric fan, get a trinary switch. Look at the diagram in the Vintage Air box. Just follow what it says. The binary switch allows the clutch on the compressor to engage when the system has enough pressure in it to activate the switch. No pressure, no transfer of electricity to energize the compressor and make the system cool.
     
  13. tim troutman
    Joined: Aug 6, 2012
    Posts: 375

    tim troutman
    ALLIANCE MEMBER

    good advice from all. I have been to the fairgrounds at the nats or good guys have heard belts squealing and hoses blow due to no high pressure switch. it is what make your compressor cycle on and off on your daily driver you need one . it needs to be wired to a relay not straight to the compressor
     
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  14. jimmy six
    Joined: Mar 21, 2006
    Posts: 10,277

    jimmy six
    ALLIANCE MEMBER

    The switch on my dryer starts the compressor.
     
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  15. Almostdone
    Joined: Dec 19, 2019
    Posts: 635

    Almostdone
    ALLIANCE MEMBER

    Easy peasy - call the tech line at Vintage Air. They’ve been good folks to me.

    John
     
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  16. SS327
    Joined: Sep 11, 2017
    Posts: 448

    SS327

    On a GM the low pressure port on the drier is a service port. That is where you add your Freon. If you tried at the high pressure side the can will blow up in your hand. The pressure switches on the Vintage Air system to cycle the compressor may be near the evaporator core. If the fittings are the correct size just put a cap on the Schrader valve port.
     
    AGELE55 likes this.
  17. Ok, let’s hold on a minute, the fitting on the drier should NOT be used for a high pressure cut out switch. Why? The drier is located after the condenser and the pressure after the condenser is lower than before the condenser. The pressure in the A/C system is highest between the compressor and the condenser, this is where the high pressure cut out switch should be located. The switch port on the drier can be used to turn on an electric fan, or in my world activate the air solenoid turning on the fan, or simply as a service port. Incidentally, EPA laws require a high pressure cut out switch in A/C systems, weather or not you run one is up to you.
     
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