41C4398A Liftmaster RPM Sensor Assembly
The RPM interruptor cup spins
through the u-shape on the RPM sensor and counts the revolutions when
the motor starts. If the opener trolley only moves a few inches then
it's possible the RPM sensor is not working properly. There are other
possible causes for the trolley to only move a little bit but the RPM
sensor is one of the things that should be checked when diagnosing a
problem with the opener. Compatible with various Liftmaster and Raynor openers made after 1995 (see list below).