Hi umwolvefan. Welcome to the forum. What are the symptoms or what is the problem that sent you to the pump? You're right; 9 volts is too low. Where are you grounding your voltmeter? When things don't make sense after finding low supply voltage, I have to remind myself to look at the ground circuit. Measure the voltage between the pump's ground terminal and a good clean spot on the body. If you find near 0 volts, split the supply side in half by sticking wires in the relay socket terminals so you can measure the voltage on the two contact pins while the pump is drawing current and running. If you have near full battery voltage on both pins, suspect a corroded pin in a connector going to the pump. If you find 9 volts on the relay pins, we have to work backwards toward the battery to find the 3 volt drop.
It sounds like you understand electrical theory so it shouldn't be necessary to run through all of that. Holler back with your findings.
Sunday, October 7th, 2018 AT 8:19 PM