How does the M-body climate control work exactly?

Interior and Electrical

  1. 8v-of-fury

    8v-of-fury Well-Known Member

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    I was doing some light reading of the manual that came with the car (grandfather kept a fairly good record ;) ) from the dealer. It states that with the temperature slider set to a certain temperature, the internal temperature of the cabin will be kept at that temperature automatically by the control of the coolant hose/valve in the engine bay?

    Does it use a probe to signal the vacuum source?
     
  2. kkritsilas

    kkritsilas Well-Known Member

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    I would guess that there is a temperature sensor somewhere in the cabin. When the temperature sensor goes above the set temperature, it sends a electrical signal to engage the compressor. I think the air blend door is open all the way when the system is put into semi-automatic mode.

    System is semi-automatic in that it can only cool the car, it cannot heat it, in automatic mode. To get heat you would have to turn the system to heater mode. Modern automatic temperature controlled systems can either heat or cool the cabin based on the interior temperature. That is why I don't think the blend door is moved by the temperature control; if it was it would have been able to heat or cool the cabin.
     
    Last edited: Oct 9, 2016
  3. 89.Fifth

    89.Fifth Well-Known Member

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    Look under the glove box and you'll see a small round vent shaped sort of like a pizza pie. That's the temp sensor. There's probably a small fan than pulls air across a ballast resistor with a heatsink, as the air temp changes, so does the resistance, and that voltage changes movements in the blend door. Just a guess. The FSM may have more details.
     
  4. BudW

    BudW Moderator Staff Member

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    The ATC (Automatic Temperature Control) uses both the A/C and heater. The A/C runs full time and adjusts the amount of air flow through the heater core (which is also running full time), via the blend air door/door actuator, to maintain the desired temperature.

    It is quite simple, when it works. Quite the pain in the rear, when it doesn’t work.

    There is an aspirator (in car temp sensor), ambient temp sensor (screws into intake side of evaporator case) and a sliding resistor (attached to A/C control head. Those three items are wired in series which is used to move the servo motor located on topside of evaporator.
    ATC diagram.JPG
    That is it.


    Cars with normal A/C, the temperature is controlled a cable which in turns controls the amount of air flow through the heater core.

    If you can get the (correct) cable, the lever that attaches to top of evaporator case and lever for on top of the A/C control head (or just find a non/ATC control head). Not hard to do other than dash top, gauge bezel, right side lower dash and A/C control head needs to be removed for access.

    BudW
     
  5. 8v-of-fury

    8v-of-fury Well-Known Member

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    Well this is the thing I am not sure it works, as I havent needed any temps between full heat and full AC. lol

    It has a vacuum controlled heater valve in the engine bay, so the core in this system see's full air flow and coolant is not circulated when in the "cold" positions of the temperature gauge I am assuming.

    With say the knob for bilevel push/pulled out so that the AC is not running it should still change the amount of hot/cold from the vents if you were to set it at a certain temperature on the slider yeah? Obviously, not as well at the colder end without the AC on.
     
  6. BudW

    BudW Moderator Staff Member

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    The Hot Water valve under hood is either open or closed (no mid position).
    It is turned off (no coolant flow) when HVAC system is off or in Max A/C.
    All other times it is full open. This applies if it has regardless if regular A/C or ATC.

    What controls the temperature coming out of the vent is the blend air door (temp door), that has a lever (or ATC servo) on top of the evaporator case. It adjusts how much air flow goes through the heater core. The more air flow = hotter vent temperature.


    In all positions, except for off and heater, you can pull the mode switch button (chrome or black) out and doing so will turn off the A/C compressor.

    On a mild day, I want air flow but not necessarily really cold air – so in that case, you can leave heater off, and manually push/pull out the mode button which will in turn off the A/C compressor then back on when you push button back in. Sometimes it is more pleasant this way than turning A/C off or other methods.

    My ’86 has a small finger grip on top of the chrome mode switch buttons.
    I have seen switches that do not have a small finger grip on them.

    BudW
     
  7. 8v-of-fury

    8v-of-fury Well-Known Member

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    Yes, mine also has the pull tabs on the NORM and BI-LEVEL. HOWEVER! You can also pull back on the defrost one (although there is no pull back tab) and it will disengage the A/C clutch as well. This is beneficial for many cold mornings where the AC running isn't helping more than to burn more gas lol. This past week has seen below freezinf temperatures each morning-car covered in frost windows need scraping.. But The insides weren't fogged, so I did not need the A/C running on the defrost mode.

    So yes, going by how you describe the system working -- mine is fully functioning correctly! :) If you slide the temperature control lever either way slightly you can feel it adjust the temps of the vent. Gonna have to get my temp gun out and see how accurate it is.. ;)
     
  8. BudW

    BudW Moderator Staff Member

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    If you have ATC and there is no change, on a properly warmed up engine, when you move the lever, then “you have a problem”, an ATC problem to be more exact.

    The most likely suspect is the servo. The in car temp sensor, the ambient temp sensor and A/C control head resistor (and wiring harness) can also fail, but not as often as the servo.

    Matter of fact, I don’t recall ever seeing the A/C control head resistor ever going bad before.

    The second most often failed part of ATC, that I have found, is a crack in the right lower dash part going to the in car temp sensor hole. Yes, not what you are referring to, but still a common failure (the cracked plastic).

    BudW
     
  9. Darth-Car

    Darth-Car Well-Known Member

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    My control stays set on 68 all year, and I am happy. The only thing that changes is the vent selector.