87 ls swap a/c system wiring

Discussion in 'Tech Discussion' started by RummedMonkey, Aug 11, 2018.

  1. RummedMonkey

    RummedMonkey Junior Member

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    colby
    Truck Year:
    1987
    Truck Model:
    c10
    Engine Size:
    5.3
    Hey guys, first time poster. I have a 1987 c10 with the ls swap. I am stuck on the wiring. The light green wire going to the low pressure switch is hooked up and receives a/c request. The dark green wire is not hooked up and I'm not sure where it needs to go. The high pressure switch on the back of the compressor- the ground is hooked up. The power wire is not connected. If I tie the high pressure 12v wire to the low pressure 12v wire I can get the compressor to come on but does not cycle in stages. Im sure that is not the correct way of connecting the wires so I disconnected the wire going to the compressor. Any help will be appreciated. So I am needing to know where the dark green wire on the low pressure switch @ dryer goes and also the 12v high pressure on switch wire on compressor goes.
    Am I missing just a relay? Do I need to wire a/c request to relay-then term30+85 relay then 86term to low pressure switch- wire high pressure 12v to term 87 relay? If so does the ecm detect wot shut off?
     
    Last edited: Aug 11, 2018
  2. Charlie

    Charlie Full Access Member

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    350/TH350/AC/4 BBL Quadrajet
    :welcome:
     
  3. 73c20jim

    73c20jim Supporting Member Supporting Member

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    Truck Year:
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    Truck Model:
    C20 2WD Custom Deluxe
    Engine Size:
    350 CID 195/260 HP Goodwrench crate
    Check out the wiring diagrams in the reference section.
     
  4. yevgenievich

    yevgenievich Full Access Member

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    Need a bit more info. What compressor, is it only coil connector on it or actual high pressure switch and if you want to let the ls computer controlled. If you skip computer control and using stock system without high pressure switch then the request from the cab does go to low pressure switch, then from it straight to the coil on the compressor. Other side of the coil goes to ground. If there is a high pressure switch then run power through it. First low pressure, then high pressure, then coil.
    For ls computer control you would run from low pressure switch to ecm request ac pin. Then ac engage pin from ecm to ground side of a relay that would then feed the coil on the compressor. Ls style pressure sensor should be wired in for it to know pressure. Without pressure sensor connected it will think no Freon and act as low pressure interlock.
     
  5. Camar068

    Camar068 Supporting Member Supporting Member

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    Truck Year:
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    Engine Size:
    10 yrs Air Force
    I thought there was only one signal from the vehicle that went to ECM to tell it that AC was on....and to increase the idle a bit. I'll try to dig it up tomorrow as I'll need to know the same info when I get my AC hooked up.
     
  6. chengny

    chengny Full Access Member

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    1. They are connected together at the firewall - near the junction block. The connector style makes them unmistakable (see below)

    2. There is no relay involved in the compressor control circuit

    3. No

    4. The ECM will receive an input signal that the compressor is engaged using just the factory wiring harness.


    From low side switch:

    IMG_20121106_094752.jpg

    To compressor clutch:

    IMG_20121106_094722.jpg

    Another image of the control switch side of the connector


    BC3E523A-46D1-4924-8CA3-AD51F3E83199.jpeg


    A schematic view of the circuit - while your connector may not look like this, the wiring goes to (and comes from) the same places:

    Pages from ST_350_89_1989_GM_RV-P_Light_Trucks.JPG
     
  7. yevgenievich

    yevgenievich Full Access Member

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    On tbi trucks it is just ac is on signal, on some ls computers it does the control of the ac and fans according to the pressure sensor that allows to indicate low/high and in between pressures
     

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