And be sure you connect the neg. lead to the pump ground and not to a common or battery ground, this way you'll be measuring the voltage at the pump.
A “common” in a DC circuit?.
Every ground on the boat must be tied back (have continuity) to a common source (battery post) to prevent ground loops. Grounding point for voltage tests should not matter
As such, the entire ground path is easily verified by checking continuity between component ground and the battery post.
Once continuity is confirmed, it’s easy enough to flip the meter over to ohms and measure the resistance back to ground.
Ideally, you want the lowest possible resistance value, but anything under 5 ohms is considered a “good” ground. When testing communications networks such as NMEA 2000, resistance values over 1 ohm can be problematic
