New Blower Motor Doesn't Blow as Much Air

Summary: If you put in a new blower motor in your vehicle, make sure you wire it correctly. Reversing the red and black wires from the blower to your car’s wiring will make the fan run backwards, which makes it blow very weakly at all fan speeds.

I recently replaced the blower motor in the 2003 Mazda Protege as part of diagnosing a larger problem. When I put in the new blower motor, everything turned on and all speeds of the fan worked but very little air came out of the car’s vents. Each speed (1, 2, 3, 4) would output slightly more air and I would hear the fan spinning faster, but even the highest speed would blow very weakly.

A lot of research on internet forums gave me a lot of dead ends:

  • clogged cabin air filter (couldn’t get clogged between the old blower motor and the new one)
  • A lot of red herring suggestions of low freon, etc
  • A bad blower motor resistor (all fan speeds work, which is not true with a bad resistor)

Seeing how weakly the new blower motor was blowing, I put the old one back in. And it did the same thing! It would go through all the speeds but they all didn’t blow nearly as hard as before. So I spent a good amount of time searching for a broken or dislocated vent door – anything that would cause the air from the blower motor to lose its speed before coming out the vents. But I couldn’t find anything.

The problem was that I didn’t pay attention to the wiring when I disconnected the old blower motor. It finally occured to me that this fan might spin backwards if hooked up backwards, which is apparently possible with a brushed DC motor like the one powering the blower. The motor itself has a black and a red wire. I thought the black should connect to the blue+black wire and the red should connect to the blue+white wire. This is backwards. And if you connect it this way, the fan will actually spin backwards. This then will blow some air out but much less than it should. Like a ceiling fan reversed to blow air upwards, you still feel a bit of air blow below the fan but not near as much.

  • Blue+black == red
  • Blue+white == black

I hope this saves someone as much time as I spent figuring it out : ).