Theory Of Operation

Fuel Tank pressure can build because the vehicle may operate for some time in electric mode without Fuel Tank vapors being purged to the engine and burned. The PHEV Fuel Door locks to prevent the vehicle from being fueled while the Fuel Tank is pressurized. The components used to control the opening of the Fuel Door are the Fuel Tank Isolation Valve (FTIV)  , Fuel Door Release Switch  and the Fuel Door Unlock Solenoid  , which also includes an internal Fuel Door Position Switch  .

• The Fuel Door Release Switch  is hard-wired to the BCM and is the input that begins the refueling process. When the BCM receives a pressed signal from the switch it sends a wake up command to the Powertrain Control Module (PCM) along with a corresponding fuel refill bus message.
  • The Fuel Door Release Switch signal is a 12.0 volt supply signal. The switch has two internal resistors and operates similar to a multiplex switch. When operating normally the input signal voltage is approximately 3.1 - 3.2 volts when the switch is in a not pressed  state. The voltage will change to approximately 1.6 - 1.7 volts when the switch is in a pressed  state. The PCM determines any voltage reading above 3.0 volts (but less than the open circuit threshold) to be not pressed  and any voltage reading under 2.0 volts (but not less than the circuit low threshold), for 0.40 seconds, to be pressed  . A voltage reading between 2.0 volts and 3.0 volts is considered to be an irrational signal.
• The PCM will check the Fuel Tank pressure and open the Fuel Tank Isolation Valve (FTIV)  to depressurize the Fuel Tank. Once pressure is relieved below a calibrated threshold, the Fuel Door is unlocked using the Fuel Door Unlock Solenoid  and refueling can begin.
  • If there are no Fuel Door Unlock Solenoid faults present, the BCM supplies 12.0 volts to the Fuel Door Unlock Solenoid  and the PCM controls the operation of the solenoid through a Low Side Driver  .
  • The PCM also monitors the Fuel Door position to determine if it is opened or closed. This is done by monitoring the status signal input from the Fuel Door Position Switch  which is integrated with the Fuel Door Unlock Solenoid.

Typical Low Side Driver Operation and Fault Detection:  This type of driver circuit is generally used for relay control, solenoid control or a similar type of driver device. The PCM provides a ground to operate the device when switched on. The ground could be constant or Pulse Width Modulated (PWM). The PCM also provides fault detection for the device, wiring and internal driver. Fault detection can be done by monitoring voltage on the circuit, current draw, or a combination of both. For diagnostic purposes the PCM uses an internal pull down diagnostic resistor connected in series and a voltage reference (V-Ref) comparator for fault detection:

• Circuit Open and Circuit Low Detection:  The PCM monitors for an open circuit  and short to ground  when the driver is switched off. When switched off, the available voltage passes through the device and the internal pull down resistor  connected in series. The voltage at the comparator circuit  should be close to Battery voltage since the majority of the voltage drop occurs through the diagnostic resistor. If the available voltage is less than the V-Ref, a fault is set. In this scenario the V-Ref would be slightly below Battery voltage. An alternative method of fault detection for an open or short to ground  that is used is to monitor current draw when the internal driver is switched on. If the module does not detect any current draw it determines that the component or circuitry is open. Excessive current draw detected would indicate a short to ground.
• Circuit High Detection:  The PCM monitors for a short to voltage  when the driver is switched on. When the driver is switched on providing a path to ground through the transistor, the available voltage should be pulled low, near zero volts since the comparator circuit  is monitoring the ground side of the device. If the voltage is greater than V-Ref, a fault is detected. In this scenario V-Ref would be slightly above zero volts.

  Courtesy of CHRYSLER GROUP, LLC