Theory Of Operation

WARNING: This page is about a different car, the 2019 RAM 3500 HD and 2019 RAM 2500 HD. However, it is still accessible from the selected car via links, so may be relevant.

The dual fuel tank system adds the ability to use a second fuel tank (Auxiliary Fuel Tank). This allows the fuel capacity to be increased. The Auxiliary Fuel Tank is used to fill the Main Fuel Tank when the fuel level has dropped to below a threshold. It does not supply the fuel directly to the engine. The Auxiliary Fuel Tank has an independent fuel level sensor; fuel pump module (located inside of the tank); and fuel fill tube.

There are 2 relays that control the Auxiliary Fuel Pump: The Fuel Pump 2 Relay and the Fuel Pump 2 On/Off Relay. The Powertrain Control Module (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 Body Control Module (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 will not activate the Auxiliary Fuel Pump if the Main Fuel Tank fuel level is above a calibrated threshold. The Auxiliary Fuel Pump will be de-activated when the Main Fuel Tank fuel level reaches this threshold during a tank transfer event. This is to prevent an overfill of the Main Fuel Tank. The PCM also will not activate the Auxiliary Fuel Pump Module if the Auxiliary Fuel Tank fuel level is below a calibrated threshold. The Auxiliary Fuel Pump will be de-activated if the Auxiliary Fuel Tank fuel level reaches this threshold during a tank transfer event. This will prevent having the Auxiliary Fuel Pump running in a dry condition. A message will be sent from the PCM to the BCM to command the Fuel Pump 2 On/Off Relay on or off. The BCM reports fuel level to the PCM over the CAN bus.