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Thread: Outside AC Unit Fan not Running

  1. #1
    Join Date
    May 2015
    Posts
    3

    Default Outside AC Unit Fan not Running

    My outside AC fan is not running (and I can't hear the compressor). The unit is a Lennox 12ACB48-5P. I have verified the Capacitor (dual run-start) is good. I have measured voltages at the contactor with the unit set to cool and measured the following:
    - 240V coming into the contactor.
    - 0 V coming out of the contactor (obviously, this is why the fan and compressor aren't running).
    - Something much less than 24V measured across the contactor coil inputs. However, when I disconnect the leads to the coil inputs, I do read 24V across the leads. Thus, the contactor should be seeing 24V coming in (no idea if it has sufficient amperage or whether that is something I have to worry about).

    Is this likely an issue with the coils on the contactor, or is it likely the 24V in (from the house thermostat I assume) is not sufficient?

  2. #2
    Join Date
    May 2015
    Posts
    3

    Default

    I did one other check. I disconnected the leads to the contactor coil (and pulled the fuse on the AC unit to remove 240V) and then measured the resistance across the coil. It is reading 17 ohms, which I think it a little high.

  3. #3
    Join Date
    Feb 2012
    Location
    Lala land, OH
    Posts
    235

    Default

    Usually a failed coil will open. If you measure across the coil with it energized you will be measuring the voltage drop of the coil (a small number). You would need to measure a new one to see if that resistance value is close, but this is usually not a spec used to size a coil.

    With the power cut to the AC (there is exposed high voltage on the contactor) see if the plunger moves up and down freely. There is often a part at the top in the center that you can push on. Does it move when the AC should be kicking on? Also, look for badly arced terminals. You will see some arcing there with normal use, but if they get too bad it may not let enough current pass to power it up.

  4. #4
    Join Date
    May 2015
    Posts
    3

    Default

    Quote Originally Posted by Mr T View Post
    Usually a failed coil will open. If you measure across the coil with it energized you will be measuring the voltage drop of the coil (a small number). You would need to measure a new one to see if that resistance value is close, but this is usually not a spec used to size a coil.

    With the power cut to the AC (there is exposed high voltage on the contactor) see if the plunger moves up and down freely. There is often a part at the top in the center that you can push on. Does it move when the AC should be kicking on? Also, look for badly arced terminals. You will see some arcing there with normal use, but if they get too bad it may not let enough current pass to power it up.
    The "plunger" stays out and never goes in. You can push it in and the fan starts. The "issue" boils down to the contactor isn't pulling that "plunger" in. The question is why. I'm hoping it is a bad coil on the contactor.

    I bought a new contactor today, so I hopefully will find out. I measured the resistance across the coils of the new one, and it was 13.5 Ohms (higher than I expected as some things on the web had indicated it should be 9 to 12 Ohms). Maybe the 16.6 Ohms of the original isn't as high as I thought. I will install it tonight and see if it fixes the problem.

  5. #5

    Default

    Posted By: MillBaduya

    When you are testing the resistance then you must switch off your power supply. And also the contactor coil is showing the right voltage. This much of the voltage is to be there according to your fan specification. Although you bought a new contactor, install it and also share the result here with us.

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