Theory Of Operation
The Fuel Level Sensor information is a bus message sent to the Powertrain Control Module (PCM) from the Body Control Module (BCM). On the primary side of the saddle tank is the Electric Fuel Pump Module that feeds the engine and the Primary Fuel Level Sensor. On the secondary side of the saddle tank is a venturi jet pump and the Secondary Fuel Level Sensor. The jet pump is connected to the Electric Fuel Pump Module on the primary side by way of a siphon tube.
For each sensor, a high fuel level or volume will result in a low voltage reading and a low fuel level or volume will result in a high voltage reading (I.E. approximately 0.5 - 0.7 volts at the full position and 4.3 - 4.5 volts at the empty position) when operating properly. The PCM uses the average from both sides to determine the total fuel volume.
The secondary tank Fuel Level Sensor will be at an empty position before the primary tank Fuel Level Sensor will begin to decrease due to the siphon tube flow rate always exceeding the engines fuel consumption rate. Using this assumption, the PCM performs a diagnostic to check the rationality of both Fuel Level Sensors, as well as the operation of the jet pump and siphon tube as follows;
- When the total fuel volume is greater than 50% the primary tank should remain at the full position. As fuel is consumed, or sloshes over to the secondary tank while driving, the jet pump and siphon tube will replenish the primary tank to the full position and the secondary fuel level should decrease.
- When the total fuel volume is less than 50%, the secondary tank should be at empty and the primary fuel volume should start to decrease as fuel is consumed by the engine.
If either scenario fails to happen the PCM will determine that one of the Fuel Level Sensors is stuck in range, or that the jet pump or siphon tube is faulty not allowing fuel to transfer from the secondary side to the primary side of the Fuel Tank.
The rationality diagnostic for the primary side of the tank will not become enabled until the secondary side level is near empty. The rationality diagnostic for the secondary side of the tank is based on the average fuel volume in the tank. It is enabled when there is enough total fuel to fill the primary tank plus enough fuel to check the rationality of the Secondary Fuel Level Sensor when a calibrated amount of fuel is consumed. Since the amount of fuel consumed from trip to trip varies based on customer drive cycles, this monitor can accumulate fuel consumption over multiple drive cycles. The monitor will run until enough fuel has been consumed to make a decision. Once a decision is made, a new test will start.