AC Blower Motor runs only at low speed.

[RickHomeOwner]

Our furnace and air conditioner has a four-speed blower with four leads: Red (low speed), yellow (medium-low speed), brown (medium-high speed), and black (high speed). Only the low and high speeds are used. When the air conditioner is running, the blower should run at high speed; however, it runs at low speed. I checked, and I have current in the black wire, and no current in the red wire. Is it possible for a multi-speed motor to have a failure mode that causes it to run a low speed when power is supplied to the high-speed input?

Before a buy a new blower motor, I want to make sure I shouldn’t do more trouble shooting.

[Grady]

A weak capacitor will cause the same problem. Have you tried it in the heat mode? Check voltage going to the motor in cooling then heat. If you have a way to check amperage, that too would be a good test to do. Are you sure brown is med-high? Usually brown is for the capacitor & blue is med-high.