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 variable pressure systems 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.

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%).

With an open command circuit, the PCM internal driver will show a pulse width signal of approximately 2.5v if checking at the PCM connector with the Mopar Scope. 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 command circuitry. 

When the PCM loses communication with the FPCM it will default to the highest duty cycle. This will cause the FPCM to provide maximum voltage output to the fuel pump.