Auto temp 1 A/C heather switch -> does it also switch the AC on when on DEF?

Electrical & Ignition

  1. gransedan

    gransedan Member

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    Hi all,

    I think my A/C heather switch is bad because it blew a fuse.
    I had a switch laying around and tried that. The fuse does not blow.
    But I have a question if this is correct?
    The AC compressor switches on when in AUTO ans HI AUTO -> that must be ok.
    But ALSO when I push DEF and HI DEF?
    Does anybody know if this is correct?
    '69 imperial.

    THE AC system is not filled at the moment.


    Thanks, Pieter
     
  2. Ripinator

    Ripinator Senior Member

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    Yes. I think that is normal operation. The purpose is to keep the inside surface of the windshield from fogging up and to also quickly clear any fogging that may be present.
     
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  3. Davea Lux

    Davea Lux Senior Member FCBO Gold Member

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    A/C compressor will run in the defrost mode, they use the A/C evaporator as a dehumidifier to help prevent the inside of the windshield from frosting or fogging in cold weather. If your system is low on refrigerant, the compressor should not run. If it is running, you have a bad low pressure switch and the compressor should be unplugged immediately as it will destroy itself without refrigerant.

    Dave
     
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  4. gransedan

    gransedan Member

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    Thanks, very helpful!

    Where can i find the low pressure switch, so i can check it?
     
  5. Davea Lux

    Davea Lux Senior Member FCBO Gold Member

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    The switch is usually located on the filter/dryer canister, (Most C Bodies, the canister is mounted on the core support or close to it) It will have two wires coming off of it. You can use a volt meter to test the switch. On a system that is discharged, the switch should be open, ie no circuit. The switch will also be open if the system has high operation pressures as this is a hi/low type switch. With normal operating pressure, the switch is closed to complete the run circuit for the compressor. If the switch tests ok and the compressor still runs, you have a bad A/C control circuit, most likely shorted wiring. (Edited Post)

    Dave
     
    Last edited: Jul 13, 2018
  6. gransedan

    gransedan Member

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    Thanks Dave,

    The old dryer/filter didn't have the preassure switch... My new one has (aftermarket).
    Do you think that in the old setup there was not preasure switch? Or dealt with it in a different way.

    Where would the AC relay be? It deals with the compressor switch right?
    I have a Service manual but did not see a AC relay (maybe it's not called like that)?

    Greetings, Pieter
     
  7. commando1

    commando1 Sergeant at Arms FCBO Gold Member

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    I can't recall ever seeing a bad lo pressure switch not defaulting to "Off" (Normally open).
     
  8. Davea Lux

    Davea Lux Senior Member FCBO Gold Member

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    Some of the ATC systems have the Hi/Low pressure switch installed near the expansion valve on the high pressure line. All mopar systems of this era have this switch although the location may vary. Trace your high pressure line from the evaporator back to the condenser, the switch will be someplace along that line.

    Your system probably has a run circuit that is wired thru what the service manual calls a "compressor switch" which works when the ambient temperature switch is active (system on and temperature above 32 degrees F). This unit supplies power to the ambient temperature delay switch located on the air box in the engine compartment and will start the compressor if the Hi/Low switch is closed. It looks like there is no relay per se to the A/C compressor. As long as the compressor is not running with the A/C or defrost control off, the control circuit is probably not shorted. The service manual shows the Hi/Low pressure switch on the filter dryer, check your wiring to be sure that someone did not bypass Hi/Low switch.

    Dave

    I edited this post after doing further research on the control circuit.
     
    Last edited: Jul 13, 2018
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  9. Davea Lux

    Davea Lux Senior Member FCBO Gold Member

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    Most of the time they don't. I have run across some on poorly maintained systems that were corroded and stuck because of the corrosion. Usually resulted in a burned out compressor because the system had leaked out all of the R-12 and the compressor kept running.

    Dave
     
  10. gransedan

    gransedan Member

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    Thank you very much Dave!

    Very much appriciated that you took the time to write it all down and explain all this!

    Greetz from Holland