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Symptoms: AC kicks on, but blows warm air

Found issues: AC kicks on and AC condenser appears to turn on (makes noise), but the fan won't spin. A few seconds later, the condenser fan motor overheats (almost too hot to touch), then the reset switch is flipped. If I kickstart the fan, it'll run, but the motor will eventually overheat under 15 minutes and also flips the reset switch.

Other details: AC condenser fan spins by hand (long screw driver) fairly freely.

From what it looks like, it's either the condenser motor and/or the capacitor. My question is how do I properly diagnose which it is?

  • Do you have a multimeter? Does the meter have a capacitance setting? Are you comfortable poking around in the equipment? – Tester101 May 12 '16 at 4:37
  • I do have a multimeter, but I'll have to check tomorrow evening if it has capacitance. And yes, I'm definitely comfortable with electrical, motors, and caps. HVAC is a bit of a new area for me though. – Travis May 12 '16 at 4:48
  • What's the make and model of the unit? – Tester101 May 12 '16 at 9:54
  • You can use a meter on ohms to get an idea if the cap is good or bad if you do not have the capacitance feature. With the cap out of circuit put the leads on the cap and watch for a increase in ohms as the cap charges. Swap leads and repeat. if you have a constant value and there is no bleed resistor in the cap it is bad. – Ed Beal May 12 '16 at 12:59
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Before you begin, make sure to pull out/turn off the serviceman switch, and/or turn off the breaker, to remove power to the unit. Once you open the unit, you'll want to discharge the capacitor(s). A charged capacitor can store enough energy to kill you, so you want to make sure you discharge them.

Test the motor

To check the motor, you're going to want to check the resistance of the coils in the motor. To do this, you'll need a multimeter set to measure resistance in Ohms. You'll also have to identify the wires, which will depend on the unit you're working on. Use the schematic printed inside the unit, or in the manufacturer's documentation, to determine which wires are which. You'll want to determine which wires connect to the Common, Run, and Start motor terminals.

Once you know which wires go where, you'll measure the resistance as follows:

  • Common -> Start
  • Common -> Run
  • Start -> Run

If you sum the value measured between Common and Run, with the value measured between Common and Start. You should end up with a value right around what you measured between Start and Run. If the values are way off, the motor is bad. If any of the readings are infinity (open), it means the coil is broken and the motor is bad.

NOTES:

  • Depending on the motor, there may be multiple speeds. If so, you'll have to measure them as well, to determine if those coils are good.

Test the capacitor

When checking the capacitor, the first thing to do is a visual inspection. If the capacitor is bulging, or has released the magic goo. Then the capacitor is bad, and needs to be replaced.

Make note of all the wires connected to the capacitor, then disconnect them. Then set your meter to measure capacitance in microfarads (uF). Touch one meter lead to the terminal marked C (common), and the other to the terminal marked FAN. Hold the leads on the terminals until the meter settles, then take note of the reading. With one lead still on C, put the other lead on HERM. Again, wait for the meter to settle, and note the reading.

Compare the readings you took, to the values printed on the side of the capacitor. The measured values should be within the tolerances printed on the capacitor, otherwise the capacitor needs to be replaced.

  • Also the centrifugal switch that disconnect the capacitor once it's at speed may be broken (if it is just a capacitor-start motor and not a capacitor-run motor). If it failed in the always closed position then the starter wire and capacitor would remain on for too long and destroy the capacitor or trigger the thermal safety. – ratchet freak May 12 '16 at 11:34
  • I went ahead and ordered a new motor and capacitor. The capacitor was barely registering any capacitance (~100 nF) on both sides. The motor checked out as you described checking the resistance, however there was a good amount of discoloration on the housing from over heating and decided it wasn't worth salvaging. I ordered the new motor and cap for $140 and should be here next week. Thanks! – Travis May 15 '16 at 16:56

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