On a central unit with gas the fan motor has to run on slow speed for heat. For cooling the fan must blow on high speed. A relay is used to do that job. The relay has five wires. 2 wire operate a low voltage solenoid. The relay is always wired so that when the relay is not energized it runs the slow speed for heat. For cooling the relay gets energized and the fan goes to high speed. I would look for the relay to be bad. On older furnaces the relay mounts near the blower. Usually has a white common wire and a black wire for high a red wire for slow, with a couple of smaller wires operating the solenoid from the thermostat. newer furnaces have it mounted on the board.
The picture you posted ain't working.MORE HERE
« Last Edit: September 13, 2012, 03:26:19 PM by JWWebster »
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