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As stated previously each tank has it's own fuel level sender as well as high pressure fuel pump. The selector switch on the dash has two sets of contacts. One set for the fuel sender and the other set to power the selected tank's pump. You can measure the fuel sender resistance at the tank selector switch using a meter (DVM) set to ohms. Typical resistance range is 22.5 ohms (empty) to 145 ohms (full). With an open circuit/very high resistance the fuel level gauge will peg past full.