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Thread: AC Control Question - 84 Coupe DeVille

  1. #1
    zimmstoy is offline Cadillac Owners Member
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    AC Control Question - 84 Coupe DeVille

    Hey everyone,

    Dad picked up an 84 Coupe DeVille last week. The car has the 4100 in it and we are planning out an LS swap to breathe some new life
    into this Cadillac. I have most of what we are going to do figured out. My question here pertains to the A/C. I have heard rumor that
    the A/C is controlled by the Engine Computer which doesn't make sense to me. Can anyone clarify how the A/C is controlled? Just want to make sure I don't eliminate the ability to use A/C with the engine swap.

    Thanks so much,
    Scott


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  3. #2
    carnut is offline Cadillac Owners Master
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    Re: AC Control Question - 84 Coupe DeVille

    This car has one computer, so yes it controls the A/C as well as the engine. Get a factory service manual. You'll need it anyway.

  4. #3
    jayoldschool's Avatar
    jayoldschool is offline GM RWD V8 addict
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    Re: AC Control Question - 84 Coupe DeVille

    You don't need the computer to control the AC, however.

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    zimmstoy is offline Cadillac Owners Member
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    Re: AC Control Question - 84 Coupe DeVille

    Talk to me Jay - am I looking at trouble going down this road or can I work around it?

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    jayoldschool's Avatar
    jayoldschool is offline GM RWD V8 addict
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    Re: AC Control Question - 84 Coupe DeVille

    Hang on, I'll ask a buddy on another forum. He just completed a 350 olds swap for the computer controlled 307 in his mid 80s 442. I recall it was easy to make work.

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    zimmstoy is offline Cadillac Owners Member
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    Re: AC Control Question - 84 Coupe DeVille

    cool. thanks Jay. I appreciate it man!

  8. #7
    carnut is offline Cadillac Owners Master
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    Re: AC Control Question - 84 Coupe DeVille

    The Olds has a manually operated A/C, heater system. The 84 computer completes the grounding of various circuits to, among other things, delay fan operation, engage the compressor and read system pressures. With the engine swap maybe the 84 ECM can remain for the climate system and the new ECM for the swapped engine added? I will admit a lack of knowledge regarding this type of swap, hopefully someone like jayoldschool can be more help.

  9. #8
    The Ape Man's Avatar
    The Ape Man is offline Cadillac Owners Connoisseur
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    Re: AC Control Question - 84 Coupe DeVille

    I used to leave the HT-4100 ECM in place and just remove the check engine lamp.

    HVAC always worked OK for me.

  10. #9
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    jayoldschool is offline GM RWD V8 addict
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    Re: AC Control Question - 84 Coupe DeVille

    I was thinking the exact same thing, just run the Caddy ECM for the HVAC, and a separate PCM for the new engine.

    Anyways, here is what my buddy got back to me with:

    -----FOR THE A/C SYSTEM OPERATION:

    There is a green wire from the A/C switch to the computer, but this simply tells the computer that the A/C is turned on and to increase the idle speed. The computer uses the tach feed from the distributor to sense engine RPM. If you disconnect it, the computer assumes the engine is idling too slowly and shuts off the A/C to prevent the car from stalling.

    There is also a brown wire from the A/C compressor relay that is controlled by the computer. The computer disconnects the ground on this relay to turn off the compressor if it senses low freon pressure. This brown wire is in position 19 on computer connector C1. There is also a black wire in the same connector in position 15. This is the wire that tells the computer there is low freon pressure.

    By connecting the black wire (which is switched ground) to the brown wire, the compressor will work and the relay will automatically cut out when the freon pressure drops too low.

  11. #10
    zimmstoy is offline Cadillac Owners Member
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    Re: AC Control Question - 84 Coupe DeVille

    This is great. Thank you.

  12. #11
    The Ape Man's Avatar
    The Ape Man is offline Cadillac Owners Connoisseur
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    Re: AC Control Question - 84 Coupe DeVille

    Hi Jay,

    Something doesn't sound right to me. I'm fairly sure the RWD ECMs didn't care about refrigerant pressures in those years.

    I also remember just leaving the ECM in and disabling the check engine lamp. The HVAC worked like that.

    One was a 1983 CDV converted to a 500 and the other was an '85 Fleetwood RWD converted to a 425.

    I also converted an HT-4100 stretch limo to a 368 but kept the ECM.

    Getting more difficult to do this from memory............


    Quote Originally Posted by jayoldschool View Post
    I was thinking the exact same thing, just run the Caddy ECM for the HVAC, and a separate PCM for the new engine.

    Anyways, here is what my buddy got back to me with:

    -----FOR THE A/C SYSTEM OPERATION:

    There is a green wire from the A/C switch to the computer, but this simply tells the computer that the A/C is turned on and to increase the idle speed. The computer uses the tach feed from the distributor to sense engine RPM. If you disconnect it, the computer assumes the engine is idling too slowly and shuts off the A/C to prevent the car from stalling.

    There is also a brown wire from the A/C compressor relay that is controlled by the computer. The computer disconnects the ground on this relay to turn off the compressor if it senses low freon pressure. This brown wire is in position 19 on computer connector C1. There is also a black wire in the same connector in position 15. This is the wire that tells the computer there is low freon pressure.

    By connecting the black wire (which is switched ground) to the brown wire, the compressor will work and the relay will automatically cut out when the freon pressure drops too low.

  13. #12
    jayoldschool's Avatar
    jayoldschool is offline GM RWD V8 addict
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    Re: AC Control Question - 84 Coupe DeVille

    I have no idea if the above will work on the C/D body. It did work perfectly on his Cutty, however.


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