| Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.
With the thermostat calling for cooling, check to see if the ''Y'' terminal at the furnace is hot. The fan on position bypasses some of the internal wiring of the furnace. You need to trace the circuit from the ''Y'' terminal at the furnace to the blower. There may be a limit switch or timer, plus a relay. If you are lucky, there will be a schematic inside the furnace. If you need to, post back with what you find. | 
Linear Mode
