Bench testing two cheap eBay fuel sending units

They will act differently on the bench than in the car. The issue is the shape of the fuel tank. The fuel tank has a big chunk of it missing near the top for the spare tire. Since the top part of the tank doesn’t hold near as much gas as the bottom part, you don’t need to use much gas to get the float to drop a lot when it is full. You can’t really duplicate this just moving the float on the bench.

The thermal gage design takes awhile (several seconds) to respond to changes in the float position. This effectively “averages” the voltage coming into the gauge over the period of time it is on. This means that any temporary change, like the gas sloshing around for a few seconds won’t show up in the gauge reading very much. The gauge takes a finite period of time to heat up or cool down.

The threads Dana67 posted have a lot of good info. There can be all kinds of problems with your car, the gauge, the wiring, the IVR, the sender, grounding. Maybe one of them, maybe all of them combined.

I made a resistor box with switches to simulate the correct resistances for empty, half & full and connected it back at the fuel tank. That way I could verify that the wiring, gauge & IVR together were working properly. I got the right readings with the simulator box, so I knew it was my sender causing the errors. I used a device called a Meter Match to fix it. It is not perfect, but works pretty good.