when mine did it, it was a break in the wire right at the battery end, there are 2 smallish red wires that connect to the pos terminal of the battery that then go down a larger red sheath, even though the outer insulation was ok, the copper had work hardened inside and snapped off, i found it by waggling all the wires and hearing the pump momenterilly kicking in.... just cut back a few inches to find it and then re soldered some new tails on, caused by the fact that the wires had been moved so many times while disconnecting the battery and removing it, bend int heback out of the way.