[tonyde]

I'm having trouble diagnosing a york furnace downdraft gas unit with 3-speed blower to run a cool down cycle. After the heat cycle the blower shuts off unless the fan is in the on position. I believe the card is bad, as it is set for 120 seconds. My question though is what should the other leads on the blower motor read when the blower is running in the heat cycle, With the wires to the other speeds hanging free, I get a reading of 97 vts and 154 vts on the free leads and ground. I believe something is taking out my card circuit for blow down. Any help would be appreciated.

[NorthernHeat]

All taps on a motor are hot (have voltage) if any one winding is energized, never apply voltage to more than 1 tap at a time unless you want to buy a new motor. If this is a modern PC board, the motor will not damage the board if installed correctly. The timing of the fan off is controlled by timing circuitry which is controlled by the 24 V. low voltage transformer.

[NorthernHeat]

Assuming this is a 120 V motor, the reason you might be scratching your head, about the 154 V. tap being so high. Is because of back voltage. Voltage that is induced back into another winding, that is why capacitors are usually rated at 370 V or above.

[tonyde]

Thanks for the quick reply. Do you know if this back voltage could have caused the furnace card failure? Is this back fead normal? Thnaks again for your help. I'm out for a few days so I won't be able to respond until then