Not sure if this is the right place to ask this, but perhaps someone can help...

I have a soap dispenser that stopped working. Taking it apart, I see that it uses one of these 6v RF-370 motors screwed onto a white diaphragm pump device:

When I remove the white part (only leaving the silver RF-370 motor), the motor works as expected. The motor shaft spins everytime I activate the sensor. Tried it probably a hundred times and all seems well. I reconnect the white part and test and it works as expected (air and/or liquid enters the inlet and blows out the outlet). It will work a few times (maybe 5 or 10 times) and then it quits; The motor refuses to run....When I loosen the screws on the white part, the motor starts back working. Will keep working until I tighten the screws again at which point it will run 5 or 10 more times then stops. Inside of the white part looks completely clean; No evidence of anything that would cause problems. The white part must be connected air tight for the pumping to work, yet the motor stops running after a few tries when I do this. Added a drop or two of oil to the motor, but that did not change anything. Which the motor sounds like it's running smoothly (low noise, average sound, no squeaking or dragging sounds)...Outside of the motor is cool to the touch, so I'm assuming there's no overheating issues inside?

Any ideas of what could possibly be wrong? I realize the motor can be replaced at low cost. My attempt to diagnose/repair it is purely for the educational value. Any help is much appreciated!

 

Some reasons:

1) the motor has worn/ been damaged and produces less than normal output power.
2) the power supply (drive circuit) is supplying reduced voltage- less than normal.
3) the mechanical load has increased due to wear/loss of lube, etc.

My money is on the combo of 1-3 the mechanical parts have increased friction AND the commutator/bearings are worn.

 

Measure the voltage going to the motor as it is running smoothly.
Monitor the voltage as it begins to labor and slow down.
If the voltage decreases substantially then there is something wrong with the motor electronics.