Rick's Free Auto Repair Advice

Blower motor slows down when accelerating

Blower motor slows down when accelerating

If your blower motor slows down when accelerating, it may have nothing to do with the motor itself. Many vehicle manufacturers use vacuum motors to operate heating/AC doors. Most vehicles have at least two of these vacuum motors. One motor is used to move a door that switches airflow “modes,” like defrost, heat, vent, etc. The other motor controls a door that regulates air temperature by “blending” outside or re-circulated air with freshly heated or cooled air. Other vehicles have a third vacuum motor to control a door that allows fresh air to enter the car or restricts the airflow to re-circulate. In yet another variation, manufacturers require two vacuum motors and two door to regulate the “mode” settings.

When you accelerate rapidly, the throttle plate opens wide and intake manifold vacuum decreases. That means the vacuum motors lose vacuum and the doors they control move to their “default” position. But engineers know that vacuum always decreases on acceleration, so they design the vacuum motors to work despite the vacuum drop. However, if the vacuum motor develops a small leak, or there is a leak in the vacuum hose or the dashboard switch, the drop in vacuum may be too much to overcome. That’s when you get a change in airflow. When the door doesn’t operate properly, it may sound like a Blower motor slows down when accelerating

Sometimes, when a vacuum motor defaults due to loss of vacuum, and the system uses two vacuum motors to switch modes, it may seem as if the fan is losing speed. What is really happening is that one door is closing off the airflow and the fan only sounds as if it’s slowing down.

The fix involves tracking down the bad vacuum motor or the leaking vacuum line.


© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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