Scheme 1275
Theory Of Operation
Vehicles with saddle fuel tank configurations have two fuel level sending units. The primary side of the tank has the filler tube inlet near the bottom and contains the fuel pump module. During fuel tank fills, fuel must overflow the primary side to reach the secondary side. As fuel is consumed, a siphon tube is used to draw fuel from the secondary side to the primary side. Because the siphon tube flow rate exceeds the fuel consumption rate, the secondary side of the tank is empty before fuel depletes from the primary side. The rationality for each fuel level sensor is based on the float position of the opposite side of the tank. The primary sensor rationality is enabled when the secondary level is near the bottom of the tank. The secondary sensor rationality enables before the primary side begins to show fuel consumption or fuel volume is greater than a calibrated volume. The Powertrain Control Module (PCM) monitors the fuel level sensor voltage for a specific change based on a calculated value and the vehicle operating conditions.