Component Functional Description - Powertrain Control Module (Pcm)
The PCM monitors the input signal from the Fuel Rail Pressure Sensor to determine the commanded duty cycle to the FPCM. On a fixed pressure system the PCM varies the commanded duty cycle to maintain a constant pressure. On a variable pressure system the commanded duty cycle is varied to adjust fuel pressure based on engine needs. Ambient temperature and fuel temperature can also contribute to fuel pressure changes. The pressure is raised during high ambient temperatures to prevent boiling of the fuel.
On vehicles with diesel engines or high pressure gas fuel systems, the PCM monitors the Fuel Pressure Sensor on the low pressure side of the system to determine the commanded duty cycle, not the Fuel Rail Pressure Sensor on the high pressure side.
The command circuit can be monitored using the Mopar Scope and functions as follows:
- The command circuit is a 12.0 volt duty cycle.
- The FPCM supplies the 12.0 volts for the command circuit.
- The PCM pulses the low side of the circuit to communicate with the DTCM.
- The normal duty cycle operating range is between 11% and 89%.
- If the FPCM does not receive a valid command signal from the PCM it will send a fault message on the feedback circuit and operates the fuel pump at a defaulted value (typically 80%).
Monitoring the PCM internal driver with an open command circuit using the Mopar Scope will show a pulse width signal of approximately 2.5v. This is picking up voltage from the internal driver device that is still trying to pulse the duty cycle to the FPCM. This can be useful when performing diagnostics on the feedback circuitry.