posted 07-19-2009 11:48 AM ET (US)
Check the instructions which came with the pump. In some of these bilge pumps there is a built-in controller circuit which tries to automate the operation of the pump. I will explain the general technique they use:When power is applied to the pump motor, the motor runs briefly. The controller senses the current being drawn by the motor. The controller makes an inference about the load on the motor as follows:
--if the current is low, the pump is running dry, that is, there is no water to be pumped;
--if the current is high, the pump is moving water
If the controller decides the pump is running dry, it shuts off the motor and goes into a period of dormancy. The dormant period is of a duration on the order of ten minutes. At the end of the dormant period, the controller again runs the motor briefly to see if there is any water to pump. If no water, it goes dormant and repeats the cycle.
If there is a load on the motor, the motor runs.
The precise behavior of your pump is unknown to me, but it could very well have such a circuit. When the power to the unit goes OFF, the unit could still remain in some sort of dormant state due to a stored charge on a timing capacitor.
If you make the assumption that the pump you bought contains such a self-controlling apparatus, the behavior of the pump in your narrative is consistent with that sort of design.