When I turn on the air conditioner, the compressor runs ok but the fan motor is not running at any speed (Lo, Med or Hi). I checked the fan motor and the winding is not open and the thermal fuse is also OK. I tested these latter with an ohmmeter. Any idea what's wrong ?
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All you need is 2 fan speeds, 1 for cool and 1 for heat. Leave the low speed disconnected. Normally we have high speed for cooling, and med. or low for heat. So either speed connected to the heating circuit is fine.
Sounds like a relay problem but if your fan is running in heating and cooling and operating ok , leave it how you got it. It is possibly the low speed winding is shorted but will be fine with med and high. Sounds like a psc motor, doesn't use 3 speeds
It could be many things, the most common are Low R22 refrigerant, bad fan motor, bad compressor. obstructed condenser coil, bad starting relay, bad capacitor. among other things.
To check fan motor, set unit to HI Fan Only setting and let it run for a while, check that fan did not loose speed and it is not getting too hot.
If fan is OK, then turn unit to HI Cool 60-65 degrees and see what happens. Hear if compressor kicked in or if just made a humming noise and stopped. if this happens, replace capacitor.
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I would check to make sure that the control is sending the "cool" signal to the control unit. LO on the room temp display usually means that the fan speed is set to low. Is LO and Auto showing on the display at the same time. Try resetting the thermostat first. Hold down the Mode and Zone buttons with the tstat off. While holding them down, turn the tstat back on and hold the buttons down until FF appears on the screen. Release the buttons and try the AC. If this does not work, on the control board is a freeze control switch plug (blue usually). Try unplugging this unit and insert a small jumper wire. If the Compressor comes on and runs then you have a bad freeze control switch. If not, then you should replace the control board.