Carrier Central AC

Old 08-06-15, 09:10 AM
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Carrier Central AC

The compressor is working but the fan only runs on low speed. About a month ago, nothing was working. I changed the non programmable thermostat & all was well for about a month. I changed the thermostat again. It didn't help. I reluctantly made it to the attic balancing on joists & I could hear the fan running on a low speed. There was a cardboard box with an old motor. The label said direct drive 3 speed reversible motor. I presume that the motor was once replaced.

There are only 3 wires on the stat, red, black & white. The black was on G, red on RH with a jumper to RW, I think & white on Y. The red is the feed, the Y is the fan & the G is the compressor AFAIK.

If it's a 3 speed motor with only one fan wire on the stat, shouldn't there be a relay somewhere? The fan motor is enclosed where the duct meets the unit in the attic.
Old 08-06-15, 09:55 AM
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Actually, the Y terminal controls the A/C outside unit, and the G terminal controls the inside fan (blower). The number of motor speeds has nothing to do with the number of thermostat wires. The motor speed is a function of what speed tap (wire) is being used. Often there is a fan relay on the furnace control board that controls operation of the blower fan. When the G terminal on the control board gets 24VAC from the thermostat, it energizes a relay which then sends power to the blower motor.

If you need the blower motor to run at a faster speed, you need to check the wiring diagram on the motor to see which wire(s) to use for the desired speed. In all likelihood, you would remove one of the motor wires where it connects to the control board and connect one of the other motor wires instead. However, you'll need to refer to the wiring diagram for the proper connections.

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