AC freezing up

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by PSM379, Jul 23, 2021.

  1. PSM379

    PSM379 Medium Load Member

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    Good afternoon. I have a older truck, I was really excited my AC was blowing 40 degrees on a 100 degree day. Until it starts freezing up. I have a new expansion and drier w/ trinary switch I am going to install this weekend. My compressor never kicks out I’m noticing, I was speaking to another man with the same exact system and truck as me, he said there’s a “freeze switch” in the evaporator, upon further research it seems as if there’s a thermostat in the evaporator that senses when it’s getting too cold and cuts the compressor out. Isn’t this the job of the trinary switch? Just trying to gather as much info as I can, before I chase ghosts and replace stuff not needed. Has anyone ever dealt with a situation like this? It’s a red dot system in a 1985 western star. Thanks in advance for any replies.
     
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  3. sailfish27

    sailfish27 Bobtail Member

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    No idea how to fix but if you turn AC off and just run fan on high you will cold air until it defrosts. Once that happens you turn AC back on. This works for home/office as well.
     
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  4. PSM379

    PSM379 Medium Load Member

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    Thanks that’s what I’ve been doing
     
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  5. MLC Adventures

    MLC Adventures Medium Load Member

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    No trinary is a pressure switch, the thermostatic switch in the evaporator core kicks A/C clutch out when temps get to a set point.

    The older ones are adjustable, and I try to set cutout at 30, and cut in at 33.

    But my initial assumption would be that you're low on freon. At least I'd start there.
     
  6. PSM379

    PSM379 Medium Load Member

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    thank you, Freon is good. I will post a picture of the thermostatic switch, not sure if I need to pull the evaporator, or how to even remove it.
     
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  7. MLC Adventures

    MLC Adventures Medium Load Member

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    Just pull the two wires off the switch, and the sensor lead goes into the core. Easy to change, no need to remove evaporator or heater core.
     
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  8. PSM379

    PSM379 Medium Load Member

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    i really appreciate your reply, how do I get it off this Mount? Also how do I know where I’m sticking the lead? Just push it in the same location it comes out of? Picture to follow
     
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  9. PSM379

    PSM379 Medium Load Member

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    Here is pictures
     

    Attached Files:

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  10. Don379

    Don379 Medium Load Member

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    Your truck is old and the evaporator is probably caked with years of dust and debris dust and mud pies under your dash all over the fins. Gonna have to pull it and clean it. I would just replace it. Easy job I just did in my truck a week ago. Maybe not easy for you
     
  11. MLC Adventures

    MLC Adventures Medium Load Member

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    Normally they're held on with two screws. That one may slip onto a plastic attachment point. Yes, just put the lead back where it was, just needs to be able to sense core temp.

    And Don above is correct. If you don't have enough flow across the core, they freeze up. Most older 379 HVAC filters have been removed and left out.
     
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