Functional Description- Fuel Pump Control Module (FPCM) And Fuel Pump

The FPCM performs fault detection and reports faults to the PCM through the PWM Feedback circuit. The feedback circuit can be monitored using the Mopar Scope and functions as follows:

The feedback circuit is a 5.0 volt duty cycle.
The PCM supplies the 5.0 volts for the feedback circuit.
The FPCM pulses the low side of the circuit to communicate with the PCM.
The normal duty cycle operating range is between 3% and 97%.
When the system is operating normally, the FPCM will send a 50% duty cycle on the feedback circuit to the PCM.
When the FPCM detects a fault it will send a duty cycle that aligns with the failure mode detected.
If the duty cycle is 0% or 100% the PCM will set the Loss of Communication fault against the FPCM.

NOTE:

Monitoring the FPCM internal driver with an open feedback circuit using the Mopar Scope will show a pulse width signal with a very low voltage (.025v - 0.5v). This is picking up voltage from the internal driver device that is still trying to pulse the duty cycle to the PCM. This can be useful when performing diagnostics on the feedback circuitry.

Depending on the failure mode detected by the PCM and FPCM, the FPCM can react by either reducing the PWM output duty cycle, or defaulting to an 80% duty cycle for the fuel pump operation. The FPCM reactions are explained further in the PCM/FPCM Reaction column of the Enable Conditions and Failure Modes service information.

Fuel Pump: The Fuel Pump operates in the same manner as a typical Brushed DC motor. The only difference is that the speed is reduced due to the PWM output voltage from the FPCM being less than 100%. The voltage supplied is based on the command received from the PCM.

Vehicles with Dual Pump Systems: The fuel pumps operate independently, but may be serviced as an assembly on some vehicles. During normal operation both Fuel Pumps are used to provide the system pressure and volume needed during most engine operating conditions. The exception is at idle, or idle and purging where there is extremely low fuel demand. During this mode Fuel Pump 2 is typically turned off. When more fuel demand is needed (typically off idle) Fuel Pump 2 is turned on and both pumps are operated at the same duty cycle. If either pump is failing the vehicle may still operate normally during most driving conditions but would have performance issues under heavy engine load when both pumps are needed. If a failure is detected for either Fuel Pump or circuitry, the duty cycle will typically default to 80%. If Fuel Pump 1 (primary) is defaulted to 80%, Fuel Pump 2 may remain off during most operating conditions. However, if Fuel Pump 2 is defaulted to 80%, the primary pump will continue to operate at the lower duty cycle range. The reason for this is that the primary pump operates the jet pump to pull fuel from the secondary side of the Fuel Tank and also fills the Fuel Pump assembly reservoir.