Theory Of Operation
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 Main Fuel Tank supplies fuel to 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 has dropped to a low threshold. It does not supply the fuel directly to the engine. The PCM will not activate the Auxiliary Fuel Pump if the Main Fuel Tank fuel level is above a calibrated maximum threshold. The Auxiliary Fuel Pump will be de-activated when the Main Fuel Tank fuel level reaches this maximum threshold during a tank transfer event. This is to prevent an overfill of the Main Fuel Tank. The Auxiliary Fuel Pump will be de-activated if the Auxiliary Fuel Tank fuel level reaches this minimum threshold before or during a tank transfer event. This will prevent having the Auxiliary Fuel Pump running in a dry condition.
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.
The PCM also determines if the signal circuit has an open or shorted condition.
- The primary sensor is powered by a voltage close to vehicle battery (minus a diode voltage drop). If disconnected, the signal circuit voltage will be equal to source voltage, which is approximately 12.0 volts. On a normally functioning system with no circuit faults, the voltage reading on the signal circuit at the Fuel Level Sensor should be between approximately 1.0 volt (full tank) and 10.5 volts (empty tank). The voltage is used up through the resistor on the Fuel Level Sensor, therefore the voltage reading on the return circuit should be zero volts with the connector plugged in. The voltage reading for the scan tool is scaled to read between approximately 0.6 volts (full tank) and 4.5 volts (empty tank) on a normally functioning system.
- If the signal voltage measures source voltage with the connector plugged in, it would indicate either a short to voltage on the signal circuit or an open Fuel Level Sensor or Return circuit.
- The secondary sensor operates like a typical 5.0 volt 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.