A stalled motor will draw more current and therefore drop the voltage.
What voltage is present on the feed wires with the pump disconnected?
A stalled motor will draw more current because there is no back emf caused by the spinning motor (generator effect) to counteract it.
Still not convinced? - try this.
With your cars engine at idle operate electric windows. When the motor stalls with window fully up you will hear slight extra load on engine from alternator.