Pump cycles on and off

Sometimes a motor will just cycle on and off at regular intervals.  This occurs when the motor is running just a bit too hot.  When it gets hot enough, the thermal sensor will cut it off until it cools down, then it comes back on.  There are several possible causes for this:

The motor may be receiving improper voltage

If the voltage to the pump motor is 10% above or below the voltage stated on the motor specifications, then it may overheat. 

Check the voltage at the terminals at the back of the motor. If it is too low, either the wiring leading to the motor is insufficient or the power to the house is just low.  Sometimes the power company fails to properly regulate the power and high or low voltage can ruin a motor over a period of time.  We have seen situations where the installer wired up the pump with wire that was too small for the power requirements.
As a result, voltage is lost due to the resistance of the wire.

Check the wiring diagram on the motor. Most pool motors can be connected either 115 or 220 and sometimes installers get it wrong.  If a motor is hooked up improperly, it may run for a while, but the likelihood for long term damage is pretty high.

Check the wiring connections to make sure they are all tight. Sometimes connections will loosen up over time and will result in much less contact between wires.

The pump vents may be blocked

Pool motors are air-cooled and if the air flow is blocked, this may contribute to overheating.  Be sure to keep leaves, dirt and anthills from collecting around the back of the motor.

The pump may have internal blockage

The pump motor may be running hot because of internal blockage.  Sometimes sand or fibers gets into the impeller and can make the motor work extra hard to get the job done.

The pump may have an internal short in the windings

If there is an internal short in the windings, the motor may overheat.  In this case, the motor will need to be replaced.  The windings have a clear insulation on them which can get burnt off and cause the motor to overheat.  This is generally the result of old age.