Theory Of Operation - Dual Tank System
The Dual Fuel Tank system adds a second Fuel Tank (Auxiliary Fuel Tank) to allow the fuel capacity to be increased. Both Fuel Tanks have an independent Fuel Pump Module and Fuel Level Sensor (located inside the tanks); and fuel fill tube. The Fuel Level Sensor information is a bussed message from the Body Control Module (BCM) to the Powertrain Control Module (PCM). The Primary and Secondary Fuel Level Sensor Signals are hard-wired to the Body Control Module (BCM). Both Fuel Level Sensor signals are a constant 12.0 volts. On a normally functioning system, the raw signal voltage will read between approximately 0.5 volts (full stop) and 4.5 volts (empty stop). When there is an open in the circuitry the signal circuit voltage be 12.0 volts. The data reading value on the scan tool is clipped to read 5.0 volts when the signal circuit is open. The BCM reads the voltage and converts the analog signals to digital signals. The digital signals are broadcast over the CAN bus. The Powertrain Control Module (PCM) receives and interprets the digital signals as a voltage read. The PCM, and in most cases the BCM, perform circuit fault diagnostics on the signal input.
- The Main Fuel Tank supplies fuel to the engine for consumption. The PCM performs a Primary Fuel Level Rationality diagnostic to check for a "stuck" fuel level reading in the same way as on a single tank system. This is done to keep high or low fuel levels that are stuck from disabling OBD monitors. If a fault is set, the fuel level will default to a value that will still allow the OBD diagnostics to run. Since the amount of fuel consumed from trip to trip varies based on customer drive cycles, this diagnostic can accumulate fuel consumption over multiple drive cycles. The diagnostic will run until enough fuel has been consumed to make a decision. The PCM compares the difference between the fuel level value at the start of the diagnostic and the fuel level value after a calibrated amount of fuel consumption should have occurred. If the difference is less than a calibrated threshold, the test fails. Once a decision is made, a new diagnostic test will start.
- The Auxiliary Fuel Tank is used to fill the Main Fuel Tank when the fuel level in the Main Fuel Tank reaches a minimum threshold. It does not supply the fuel directly to the engine. The Auxiliary Fuel Pump will be de-activated when the Main Fuel Tank fuel level reaches the maximum threshold, or the Auxiliary Fuel Tank fuel level reaches a minimum threshold before or during a tank transfer event. This is to prevent an overfill of the Main Fuel Tank or the Auxiliary Fuel Pump running in a dry condition. The PCM performs a Secondary Fuel Level Rationality diagnostic during a fuel transfer event.
There are 2 relays that control the Auxiliary Fuel Pump: The Fuel Pump 2 Relay and the Fuel Pump 2 On/Off Relay. The PCM controls the low side driver of the Fuel Pump 2 Relay. The Fuel Pump 2 Relay is supplied power from the Fuel Pump 2 On/Off Relay, which is controlled by the BCM. In the event that the Fuel Pump 2 Relay is stuck on and over filling the Main Fuel Tank, the BCM can turn off the Fuel Pump 2 On/Off Relay. This will turn off the power supply to the Fuel Pump 2 Relay and the Auxiliary Fuel Pump Module.