AC Dies when 4WD Activated?

Discussion in '99+ 7.3L PowerStroke Engine & Drivetrain' started by 1956ford, Aug 14, 2021.

  1. 1956ford

    1956ford Pre-Retired

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    Here in NW Arkansas, I don't need 4WD often. Last week, I was dragging a trailer up an incline, rear started to spin, so I engaged 4WD to assist. Cold air stopped coming out of the vents, 95 degrees outside. (But the blower could still be heard.)

    When I got home, I checked every fuse inside and under hood. All good.

    Then - in 2WD - AC started working again.

    I have never used the 4WD in summer, so I have never seen it happen.

    Question for the Knowledgeables here: Do the older pickups stop the air con when 4WD is electrically engaged? Perhaps to allow all power to the drivetrain? (I did not lock the front hubs.)

    If not, any ideas about this would be extremely welcome.

    Thank you.
     
  2. Eddie Longest

    Eddie Longest SDD Junior Member

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    This info might help. I did not stay at a Holiday in Express but I have been trying to repair vacuum lines that some 4 wheeling mice ate. You are dealing with a vacuum issue. The vacuum pump is located on the pass. side inner fender. It supplies vacuum to engage the HVAC dampers and Hub engagement. The pump vacuum line goes fwd. to a relay looking device and big black container. Then one line goes to the HVAC in the dash and one line goes to a hub and splits to the other side hub. In the fender wells at the frame there are connectors that connect the smaller vacuum lines to bigger rubber lines leading to the axle to engage the hubs.
    I would say you have a leak in the vacuum lines going to the hubs. My rubber lines had pulled off the connectors in the wheel well. Another guess your axel hub seals could be bad. Or you could have a rodent problem that have eaten the hard plastic lines.
    My issue was a vacuum splitter harness at the big black box. Cost about $35 for the vacuum harness from Ford. Auto Zone carries a repair kit for the hard plastic vacuum lines that can be used to repair your lines.
    Bottom line if you have a vacuum leak on the hub locking side your HVAC dampers won't work.
    Good Luck
    Eddie
     
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  3. 1956ford

    1956ford Pre-Retired

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    Thank you Eddie. Pretty interesting. I will absolutely have some looks at those.
    Steve
     
  4. Eddie Longest

    Eddie Longest SDD Junior Member

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  5. bcvickers

    bcvickers SDD Junior Member

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    I agree that it's a vacuum issue. I would bet your AC was coming out the defrost vents when this happened, as that is the default blend door direction.
     

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