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HVAC speed control

Discussion in 'Tech Discussion' started by Marcos, Jan 16, 2019.

  1. Marcos

    Marcos Full Access Member

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    Truck Year:
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    Truck Model:
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    Engine Size:
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    hello I have a 1980 GMC Jimmy that I purchased this summer. Never really used the heater until recently but noticed it only works on high speed. I replaced both the resistor and relay but still only works on high. This Jimmy was originally equipped with AC however some of the AC components have been removed and is not functional. Also I noticed some of the wiring for the AC was cut at the firewall. Do any of these wires affect the blower speeds?

    thank you in advance
    Marcos

    [​IMG]
     
  2. bucket

    bucket Super Moderator Staff Member

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    The cut harness on the left side of the picture should be the culprit. I'm not around my trucks at the moment.
     
  3. 75gmck25

    75gmck25 Full Access Member

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    I'd have to verify wiring colors, but I believe those cut wires run to the resistor pack on the plenum, which is what gives you the lower speeds.

    The large red wire running across the top probably runs to the blower relay, which is used to turn on the high speed when you turn the switch to high or run A/C in the max position. That's probably why high speed still works.

    Bruce
     
  4. Marcos

    Marcos Full Access Member

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    thank you, would I be able to splice those 2 wires and plug them directly to the resistor? if so does anyone have a picture so I can see where they should be plugged in?
     
  5. 75gmck25

    75gmck25 Full Access Member

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    I've attached a copy of the wiring diagram for my '75. I think it will be very similar for your '80 truck.

    Bruce

    HVAC_wiring.jpg
     
  6. bucket

    bucket Super Moderator Staff Member

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    Here's how my '78 HVAC harness is routed.

    IMG_20190201_151407933.jpg
    IMG_20190201_151419728.jpg
    IMG_20190201_151437325.jpg
     
  7. Marcos

    Marcos Full Access Member

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    thank you Bruce, ill see if I can splice those 2 wires I have and plug them to the resistor. Ill keep you posted on the outcome
     
  8. Marcos

    Marcos Full Access Member

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    looks like the wire colors on my vehicle, of course the splice that's there with a different color doesn't help. Any idea if the black/white wire or the green wire should be plugged. Wondering if one wire should go to resistor and the other to relay?[​IMG]

    IMG_0090 (002).jpg into the resistor and if so where should the other wire connect? this is how mine is currently plugged in IMG_0088 (002).jpg
     
  9. bucket

    bucket Super Moderator Staff Member

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    The green wires should be AC related. The cut wires over towards the engine should be unrelated. That's the location for the electronic timing control in the later years (but didn't think '80 would have that yet?). Honestly, it really would be best to get a good used HVAC harness from a parts truck and install that.
     
  10. Marcos

    Marcos Full Access Member

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    hello all im having a hard time locating a harness at any of my local junkyards. Not sure if I can wire those 2 cut wires directly to the resistor or relay or if they need to go plugged into any other hvac parts under the hood. If anyone can help me figure out where these 2 wires go I would greatly appreciate your help. My plan is to eventually get all the HVAC that was removed and get everything working as it was originally intended. For now id like to just get the lower speeds functioning.

    thank you in advance
     
  11. Marcos

    Marcos Full Access Member

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    ok I finally found a harness at the junkyard but im a little confused. The donor vehicle was a different year but the setup looks identical so I snatched it. Im not sure if the different years used the colored wires in a different area. on my model the light blue plugs into the top connector of the resistor and the pinkish brown plugs into the middle whereas the donor harness plugs in opposite? The Dark brown on my original harness is cut and the donor plugs into the slot that is empty on my resistor. was thinking of wiring it directly to that open slot and see if I get any lower speeds. The green wire appears to go to the ac canister which im assuming is the condenser? which I don't have on my vehicle. wondering if I can bypass the green wire completely. Any ideas if the Dark Brown wire not plugged to resistor might possibly be my culprit?
     
  12. chengny

    chengny Full Access Member

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    You're good to go. The same resistor stack was used with the C60 system from 73 -91 (P/N 336403).

    Pages from Group 9 - Body  Mounts & AC.JPG

    Pages from 9 - Body mounting and AC comp - 26-Illustrations-Group-9[1].JPG



    However, you are correct about the wiring colors - there was a change over the years. But it only effected the 12 volt power supply (the one at the top of the "pyramid"). It was YEL in earlier models, but at some point it was changed to BRN/WHT. The color of the other three leads remained the same: LT BLU in the middle, DK BLU on the bottom left and TAN on the bottom right. Exactly as shown in the images that Andy provided.

    Here is your original schematic:

    Pages from 1980 Wiring Dia's Only.JPG
    An example using the 1986 system diagram:

    AC.JPG

    Just a little advice: the wiring in your truck is a mess. I would disregard everything associated with it (i.e. wire colors, terminal connections and especially - the splicing methods used). Butt splices are bad medicine!

    BTW - there are no GRN leads involved in the blower control circuit. The LT GRN is 12 volts to the low side control switch (located on the filter/drier). There should also be a DK GRN that exits the control switch and then supplies the compressor clutch with power when pressure in the low side rises above 46 psi. When it drops below 25 psi the power is cut off.

    Control of the refrigeration cycle (on and off operation of the compressor) is done with a switch that senses low side pressure as an indicator of evaporator pressure. During air temperatures of 60-80°F, the equalized pressures within the charged air conditioning system will close contacts of the pressure switch.

    When an air conditioning mode is selected, voltage is supplied to the compressor clutch coil. As the compressor reduces the evaporator pressure to approximately 25 psi , the pressure switch will open, de-energizing the compressor clutch. As system equalizes and the pressure reaches approximately 46 psi , the pressure switch contacts close, re-energizing the clutch coil.
     
    Last edited: Feb 18, 2019
    TerColHan likes this.
  13. chengny

    chengny Full Access Member

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  14. Marcos

    Marcos Full Access Member

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    Thank u Chengny appreciate your response my apologies on the schematics as they’re kind of confusing for a guy like me but now that u marked them in color I understand now. On another note I realize my wiring is a mess thus the reason for me trying to fix it. Looks like the harness I picked up from the donor should work perfect! The wiring currently in the truck is plugged into the wrong terminals.

    Now on to the installation, you mentioned the “bad medicine” butt splices, I have no choice but to splice the donor harness at the firewall where the current splice is. What do u recommend other than butt splice?
     
  15. Snoots

    Snoots Supporting Member Supporting Member

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    Slide a piece of heat shrink over the wire, solder the wires together, smear a dab of RTV on the joint then, slide the heat shrink over the joint and shrink it.
     

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