Strange A/C problem

Discussion in 'The Big Chill' started by DeeVeeEight, Apr 20, 2014.

  1. DeeVeeEight

    DeeVeeEight Well-Known Member

    My A/C works well at all fan speeds except for the highest fan speed. When I turn the fan switch all the way up the compressor disengages. The fan runs as it should at all speeds. What is causing the compressor to drop out at max speed?
    I have a used A/C control that I substituted and the results are the same.
    I pulled the fan speed resistor and it is as stout and primitive a piece as you will ever see, it is fine.
    Even though the fan relay was OK I replaced that too.

    Your thoughts?
  2. Briz

    Briz Platinum Level Contributor

    I had a sort of opposite problem where the compressor would come on when I didnt want it to. Turned out to be something in the defrost control switch. Most likely not your issue here. GL.
  3. mrolds69

    mrolds69 "The Cure"

    Maybe it's the blower relay on the firewall.
  4. SteeveeDee

    SteeveeDee Orange Acres

    Check the thermal limiter fuse (if your car has it). Trace the compressor power wires back to the fender and look for a three-pronged gizmatchie and check it for an open. It usually blows when you are low on refrigerant. I don't remember which year this switch started to be used, but somewhere around '69, I think. It was used until CCOT systems came into use with the 4-cylinder radial instead of the axial 6-cylinder. 30-year old memory there, too, so maybe they used that fuse on the CCOT, too. I'd have to go dredge up my AC service manuals to be sure.
  5. philbquick

    philbquick Founders Club Member

    High speed is the only fan speed that doesn't go through the resistor pack. If the fan and the compressor get their voltage from the same fuse circuit, it could be a dirty connection that drops the voltage below the threshold that will pull the clutch in when the fan is on high since the fan draws the most current on high and would cause the greatest voltage drop across a dirty connection. See if the clutch voltage goes down when the fan is switched to high,or does it go to zero? If it goes down, start cleaning connections, start with the fuse and fuse holder,then clean all of the connections in that circuit. If it goes to zero I would look at the switches in the circuit. Get some contact cleaner and spray it in the switches and work them back and forth. I think there's another switch in the control other than the fan switch and maybe a relay, clean the contacts.
  6. DeeVeeEight

    DeeVeeEight Well-Known Member

    Awesome information here. philbquick - the detailed information about the high fan speed not going through the resistor is very promising and seems to make the most sense at this point. I will check for a bleed down voltage versus a complete voltage drop at the compressor as you suggested and proceed from there.

    Is there a separate wire for the high fan speed if it is not going through the resistor? If there is one I would be inclined to trace it and see what is up with it. I'll have to see if I can find a wiring diagram for it.

    My thanks to all who replied here!
  7. 70SherwoodGS

    70SherwoodGS Well-Known Member

    That was my first thought. It sounds like the same issue I had. They a/c on my dad's Chevelle and on the Cutlass we are finishing up, (72 & 70) worked great except on High. We replaced the "High Blow Relay" on both and it cured the issue on both cars.

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