Fuel Gauge

Magnetic fields of two coils moves the pointer of the fuel gauge. The coils are at right angles to each other. Battery voltage is applied to the E-coil (empty) and the circuit divides at the opposite end of the coil. One path continues to ground through the F-coil (full). Another path goes to ground through the variable resistance of the level sensor in the fuel pump and sender (Gasoline) or the fuel gauge sender (Diesel) with single fuel tank vehicles.

The fuel pump balance module averages the resistance for vehicles with two fuel tanks. When the tank is low, the resistance of the level sensor is low. A large current flow passes through the E-coil and the pointer moves toward E on the fuel gauge. When the tank is full, the level sensor resistance is high, and less current passes through the E-coil, and the pointer moves toward F on the scale. With two coils operating the pointer, the fuel gauge is not affected by changes in the system's battery voltage.

For single fuel tank vehicles, the fuel gauge includes the following functions:
^ The level sensor in the fuel gauge sender produces a resistance of about 4 ohms when the tank is empty. The resistance is about 90 ohms when the tank is full.
^ A short to ground in the sender or the wiring provides a fully empty indication.
^ An open in the circuit provides completely full indication.

For dual fuel tank vehicles, the fuel pump balance module does the following functions:
^ Measures the resistance for both of the fuel senders.
^ Averages the resistance and produces an equivalent resistance that reflects the average level of both fuel tanks.
^ Power to the module, and a change in the input resistance, produces a change in the output resistance to the fuel gauge.
^ The fuel pump balance module responds to a very low resistance (short to ground) in one or both fuel sender circuits by considering the affected tank(s) to be empty.
^ A very high resistance (open circuit) in either fuel sender circuit causes the fuel pump balance module to produce a zero output (both tanks empty) signal to the fuel gauge.
^ Equalizes the level in both of the fuel tanks. Fuel is fed to the engine from the front (primary) fuel tank.
^ Energizes the fuel pump balance relay when the level in the front tank decreases to about two gallons less than the level in the rear (secondary) fuel tank. This starts the secondary fuel pump which transfers fuel from the rear tank to the front tank.

When the level in the front tank increases to about two gallons greater than the rear tank, the secondary fuel pump stops. This then empties the rear tank first.