Contactor stuck on always has voltage

Discussion in 'HVAC Heating & Cooling' started by Guyerme, Aug 3, 2017.

  1. Guyerme

    Guyerme New Member

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    Aug 3, 2017
    Location:
    Illinois
    we have ordered a new contactor but I'm not sure that's the problem any more! Our ac is always on unless we shut it off at the breaker. The contactor is constantly engaged but if we disconnect a wire it pops out. So it's only stuck on because it always has voltage, the actual contact points aren't welded or pitted or anything....what would cause this? A bad relay switch on the furnace board? A short?
     
  2. bgard

    bgard Member

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    Jun 11, 2017
    Location:
    NW Indiana
    most likely the thermostat itself, remove the thermostat from it's sub base and see if it shuts off the. is it just the out door unit the keeps running or the fan inside also? either way removing the thermostat would be the first place to check.
     
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  4. Reach4

    Reach4 Well-Known Member

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    Or probe the wires/terminals to or at the thermostat.
     
  5. DonL

    DonL Jack of all trades Master of one

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    A model number would help.

    I agree T-stat unless it has a exotic controller.
     
  6. Guyerme

    Guyerme New Member

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    Aug 3, 2017
    Location:
    Illinois
    It's a Honeywell lyric T5, we just replaced it last month. I pulled it off the base where the wires attach and the ac stayed on. When it started doing this a few weeks ago first nothing would turn on and the furnace gave me a 24 error so I replaced the fuse on the furnace board and the system kicked back on but then everything stayed on all the time!

    I think it's just the outdoor unit. Before we realized what it was doing the pipes kept freezing up because it was constantly running although the air wasn't circulating.
     
  7. Stuff

    Stuff Well-Known Member

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    Pennsylvania
    This sounds a lot like wires that are shorted/pinched somewhere. Could be behind the thermostat base or even in the furnace.
     
  8. Tony Hwang

    Tony Hwang New Member

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    Retired EE (digital electronics)
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    Calgary, AB
    +1, control voltage is always present which is originating from thermostat Y terminal. Analog or digital thermostat? Trouble-shoot with multi meter.
     
  9. WorthFlorida

    WorthFlorida The wife is still training me.

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    Retired
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    If it was the thermostat the fans both inside and outside would be running. The outside contractor also turns on the condenser fan but some units l have a small circuit board if there is a two speed fan.

    The control board inside the air handler probably went bad. You did blow a fuse. You need to trace back the wire from the compressor relay back to the air handler. Look for any burn marks on the control board. FYI 24v ac operates the contactor.
     
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  10. Reach4

    Reach4 Well-Known Member

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    Good point.
     
  11. DonL

    DonL Jack of all trades Master of one

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    Rocket Scientist
    Location:
    Houston, TX
    Was the fuse that blew the one for the T-Stat transformer ?

    If that voltage is missing the outside unit may run, But the inside blower may not.

    Good Luck. Never use a bigger fuse.
     
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    New England
    With the thermostat off (i.e., not calling for cooling), double-check the voltage at the contactor's coil. If it has 24vac across it, double check it at the thermostat base. It should be open. without the thermostat in the circuit, if there's 24vac on the yellow to common, there's a short somewhere.

    Essentially, the thermostat is just a smart switch...when cooling is called for, it connects the R to Y to supply voltage to the ac contactor (the return line is wired from the ac unit to the source of the 24vac). That turns the ac unit on. It may or may not immediately turn on the circulation fan.

    IOW, you're looking for where the coil is getting 24vac when the thermostat is not calling for cooling. It could be a defective thermostat, a connection between the R and Y somewhere else, or a defective contactor, but that doesn't seem to be the case. But, if you miswired the contactor, it's possible it could be getting the coil energized, but it would be a weird set of circumstances.
     
  13. Tony Hwang

    Tony Hwang New Member

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    Retired EE (digital electronics)
    Location:
    Calgary, AB
    Youtbe will give some visual help.
     
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