How can I confirm the RPM Sensor needs to be replaced in my garage door opener?

expert EXPERT January 10, 2008

There are no specific voltages or tests developed for the RPM Switch. Typically it is diagnosed through a process of elimination, you just eliminating other potential problem areas until it is the only option left. On some models there are LED error codes that might flash to help with the diagnosis also.

There are four things that typically would create the same effect as a failed RPM Switch even if the switch were actually okay:

1. The Interrupter Cap/Cup is not on far enough. There is no specified position, but the cap needs to be on far enough for the notched ends to pass by the sensor so that the sensor is blocked, then open, blocked, then open, and so on. Normally that just means pushing the cap on until it stops sliding. The sensor needs to see that change or it assumes the door cannot move for some reason and reverses the motor to prevent any potential damage or safety issues.
2. The wiring to the sensor is connected correctly. The plug is not notched or asymmetric so it can be pushed on incorrectly with one or more of the prongs not actually in the plug. Again, the sensor cannot respond correctly and the motor is reversed to prevent damage or safety issues.
3. The Limit Switch is timed, or synchronized, incorrectly. For example, if the trolley is in the up position and the Limit Switch is installed set in the down position (as it would have been if removed when the door was closed) the mechanisms cannot travel far enough to reach the limit set on the Limit Switch and the safety systems will interpret this as something stopping the door movement before it is closed and reverse back open as a safety precaution.
4. Something in the Worm Gear Assembly or Motor is binding. If the gears were assembled incorrectly or had a bad tooth or there were a bad spot in the motor, then the drag created there could be interpreted by the system as something forcing the door to stop moving. Again, as a safety precaution, the system would reverse back open.

If you cover all the other options then the sensor may have failed. It could also be a problem with the Logic/Control board of the opener, but it is more likely to be the RPM Sensor and, given the cost of a Logic Board, your best option would be to try replacing the RPM Sensor first.