Atwood 8500-IV furnace problem

I said a bad motor would not cause the short cycling. Not having your exact furnace model number, I was working from the wrong wiring diagram in the manual I downloaded so that's why I thought there were only two wires to the thermostat and of the five wires you found only two were for the furnace. My method of troubleshooting would have negated the need to replace the thermostat.

In my second post detailed the operation of the time delay relay that controls the fan. Have you been able to locate the relay and do any tests on it?

"RELAY" description from the furnace manual wrote:
This component is commonly referred to as a time delay relay. The same relay is used on the 7900, 8500 and 8900 series furnaces.

FUNCTION - The relay has one primary function.
-to purge the plenum of heat and the chamber of any unburnt gases after each heating cycle.

OPERATION - The motor voltage path of relay is normally open. There should always be voltage from the circuit breaker to the relay terminal of the circuit breaker. There should always be continuity between the thermostat terminal connection and ground terminal connection or the relay.

Only when the thermostat contacts are closed is voltage supplied to the thermostat terminal of the relay. This voltage heats a coil in the relay body. In approximately 20 seconds this heated coil causes a bimetal disc to close. Voltage now passes through the relay and on to the motor, which in turn should allow the furnace to ignite and start a heating cycle.

When a heating cycle is complete, the contacts of the thermostat open and voltage ceases to the heater coil of the relay. In approximately 45 - 90 seconds, the heater coil cools down, the bi-metal disc opens and voltage ceases to the motor as well.