I'm facing an issue with my Craftsman 1/2 HP Garage Door Opener Model 139. I found it dead one morning, no power, no led light was lit in the head unit, and wall controller light was flickering.

On troubleshooting, I found that as soon as I disconnect the RPM sensor 4 prong wire, I can hear one of the 3 relay kicks in from the power circuit board and unit comes backs to life. Everything then works normally, of course except without the RPM sensor, the chain only drives 6 inches back and forth and the led indicator flashes 5 times which is expected. I inspected the RPM sensor for physical damage or burn out, but everything looks normal pictures attached.


  1. Can an RPM sensor cause a short circuit making relay to cut power off? Or the fault is somewhere else and is masked when disconnecting the RPM sensor as the circuit is open.
  2. Is there any way to test the RPM sensor for faults using multimeter off the circuit, or any other recommended way in the circuit?
  3. Or if there are any other specific troubleshooting steps I should follow to diagnose and fix this issue


Logic Control Board & Power Control Board:
Logic Control Board & Power Control Board

RPM Sensor Back:
RPM Sensor Back

RPM Sensor Front:
RPM Sensor Front

1 Answer 1


Not sure why you are calling this an RPM sensor.

That looks like an optical sensor intended to detect whether something is blocking the beam, presumably acting as a limit switch to stop the motor when the door has moved far enough. As such, it it is always reporting "stop now", it might not be surprising the door doesn't move.

The question would be whether the sensor is bad, the calibration is bad, or something else is going on.

If you've convinced yourself that this is probably the cause of the problem, contact the manufacturer about getting a replacement, or find someone who can look at what components were used and test/rebuild it.

  • Thanks for your response. But it is an rpm sensor. amazon.com/Liftmaster-41C4398a-RPM-Sensor-Assembly/dp/… Commented Jan 9 at 21:25
  • Measuring Revolutions Per Minute is one specific application of this kind of sensor. Limit switch is another.
    – keshlam
    Commented Jan 9 at 21:38
  • Fair point, do you have any suggestions on the setup (on the workbench) I can do to actually test if it's working correctly? From the circuit, I could deduce that it takes 5V DC input. Commented Jan 10 at 1:48

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