3 pin blower motor

Does anyone know how to bench test a 3 pin blower motor? I'm working on a 97 Cadillac Seville sls and I'm trying to determine if it's the ac programmer that's the problem or the blower motor itself. Pin A is constant 12v and pin C is constant ground. Pin B is the square wave from the programmer. How do I spin this motor with just jumper cables? How do i bench test this Cadillac blower motor?

Hey MercenarySed! I moved this to the Repair section, FYI.

Looking at the wiring diagram and pictures of the blower motor, it appears that the motor controller is integral to the motor itself. :blink: That doesn't leave you many options for jumpering or bench testing the motor. Besides, if RockAuto.com is telling the truth, then you can't buy the motor or controller separately.

Wiring diagram, for anyone following along:



With the blower motor built the way it is, I'd say you're pretty close to done. You've tested for power and ground, so the only question left is the controller signal wire from the HVAC control head.

I don't know to a certainty what you're looking for on that wire, but I can tell you from experience that it's usually a pull down circuit. The blower motor controller will provide a voltage, and the control head will ground it to command higher blower speeds. Watching that circuit with a DMM or scope is your best bet. If you see voltage with no ground as you dial up the blower speed, I'd suspect a control head problem. If you see voltage that gets pulled down with higher commanded fan speeds, I'd suspect a failed blower motor.

Thanks for the quick response. In this case the motor's speed is controlled by a separate module (ac programmer) that sends a square wave to the middle pin of the blower motor to control the speed. On my scope the square wave never shows up so im sure the ac programmer is faulty but I want to "be the computer" and make the blower spin so i can make sure the blower isn't a broken as well... Is there a way to spin the blower without the programmer input? I tried giving ground to the middle pin to give it full power (like the square wave does when not reducing speed) and nothing happens. Is the blower motor bad too?

On your scope trace, was the signal fixed high, or fixed low? Do you recall what the voltage was, exactly? I think knowing that will help point us in the right direction.

With the harness unplugged it was 0v but with the harness plugged in it was fixed at 12v. No square wave present. I checked the data bus lines from the climate controller that sends the request to the ac programmer n it has signals (although i have nothing to compare them to) so I'm assuming the programmer is receiving the requests to control the speed but the signal wire is fixed at 12v. All tests done with engine running and ac on. Voltage never changed no matter what speed i set