What I do is test them before installing in the tank. I hook it up to the connector at the tank and ground it. I have someone watch the gauge as I slowly run it through it's range. Start at the FULL position, stop at 3/4, then 1/2, then 1/4 and finally at EMPTY. Then I check by gauge position and note where the float arm is when the gauge reads FULL, 3/4,1/2,1/4 and finally EMPTY. It is amazing to see how bad some sending units are. It's putsy work, but beats installing it and finding out it's junk.
