But then I hook up signal (red wire) and it drops to 10.
Perfectly understandable when you realize that you're measuring the resistance across the sender AND the gauge coil in parallel.
The only way that you get a true reading of 0-30 0hms from the sender is with it isolated from everything but the meter.
If your tank is 3/4 full and the gauge reads 3/4, then what are you worrying about?
