What does a reading of infinity between the high and low-speed wires on a 120 VAC furnace motor indicate?

A reading of infinity between the high and low-speed wires on a 120 VAC furnace motor indicates that the winding is open. In electrical terms, an 'open' condition refers to a break in the circuit that prevents current from flowing. This can happen due to various reasons such as a damaged wire, a failed winding inside the motor, or excessive wear and tear.

When checking the continuity between the high and low-speed terminals of a motor, if you find an infinite resistance reading, it suggests that there is no path for current to flow between these terminals. This would typically mean that one of the windings in the motor is not functional. In operational terms, this means that the motor will likely not run at either speed, affecting the overall performance of the furnace.

In contrast, other conditions like a properly functioning motor would show some measurable resistance between the windings, whereas a shorted winding would present as a low resistance (possibly close to zero). The necessity for motor replacement usually arises when it has a definitive open winding, as it is unlikely to function correctly without a full circuit.