Everyone always assumes it the power going out to the device that is failing, but it could just as easy be the power returning from the device, the negative return.
You need to check this first. Put your jumper cables onto your battery and take the other ends to your pump. Run a short wire from the positive jumper cable to the positive input to your pump. Did that help? Yes? Your failure is in the positive feed. No? Disconnect that short wire from the positive feed and put it on the negative battery cable and connect that to the negative return line on the pump. Did that help? Yes... your problem is in the return wiring and you have perfectly good positive wiring to your pump. No.... Then connect both positive and negative wires to the jumper cables at the same time. That should make it work if the pump is working. If not.. the problem is in the pump motor itself.
