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Why am I getting only 21 volts to blower motor in air handler ?

Hi. My blower motor stopped coming on a while ago. I am in the process of testing all sorts of things. The latest test is for voltage going to the motor when it should be on.

What I got was 21 volts, no matter if it is in 'fan on' or 'fan off'.

It is a Trane air handler from 1986. Has been working fine until recently.

Outdoor unit still works fine, by the way.

I'm wondering why the voltage isn't ever zero or 120. Why would it be at this strange value of 21 ?



  • Jamie Hall
    Jamie Hall Member Posts: 23,650
    Between what and what are you measuring that voltage? And what meter are you using?

    It sounds to me like a "phantom" voltage. Those do occur -- and can have the full range of what one thinks of as normal voltage -- on wires (including what should be grounds) where the connection at both ends is poor -- or non-existent.

    For example, the hot power wire to a fixture which is controlled by a distant disconnect can (and, in fact, often will) have a phantom voltage on it when the connection at the disconnect is open. A return neutral with a poor connection at the panel can. And so on.

    So... what to do? The thing to recognize is that while there is voltage, and you can measure it with your high impedance multimeter (which most of them are), there is actually no power there to speak of. The best way to find out if that is the case is to construct yourself a test light. Simply a light bulb -- perhaps a 4 watt night light type bulb) of the appropriate power voltage, with a fuse (! please!) and clip leads. Now -- clip the leads to the circuit in question. Does it light? One of the leads is connected to something hot, the other to a good neutral or ground. Connect it to a neutral and the other side to ground? Should be no light. Neutral to a known good hot? No light? Problem with the hot. And so on.

    I know. It sounds pretty back woods -- and in a way, it is. But it works...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    Your 21 volts is likely due to induced voltage that the high impedance digital meter is able to read. It really means 0.

    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    EBEBRATT-Ed Member Posts: 15,839
    Agree with @Solid_Fuel_Man most likely "ghost voltage"