Originally Posted by 70ChevelleSS
Just a note, if there is a pump controller involved they supply the ground to turn on the pump. So you may have 12v at the pump but no ground if the controller is shot.
There will be two wires leading to the pump. Generally the 12V. supply wire will be red while the ground wire will be black. These wires should be disconnected and checked with a volt meter hooked between the supply wires (not the pump) for full battery voltage with the pump switch on.
If you get a voltage reading under battery voltage you have to correct it before you replace the pump.
If you don't have power when testing find a good ground on the chassis and test for voltage between the pump's power wire and the good ground.
If you have a voltage reading you have a ground supply problem.
At that point I would go to the switch and test it. This may require some long jumper wires if you can't easily access the wiring on the switch.
You will have to have a 12V. supply at the switch location available to test the switch for a ground input and output. With the pump switch turned ON you should have a ground on both switch terminals and the voltmeter will read battery voltage. With the switch off there should be a voltage only on one switch terminal.
Most of this testing could be done with a test light. The drawback here is the test light may illuminate but the voltage might be low due to a poor ground or voltage connection and you wouldn't know it. I prefer a volt meter for electrical testing.
Low voltage can or will damage an electric motor. You need to know you are getting full battery power supplied to the pump. If you replaced a burnt out pump and supplied the new pump with low voltage the new pump will fail in a short time period.
If you are unsure of the testing put some pictures of the pump and switch on here and we will try to walk you through the procedure.