FPCM System Description And Operation
THIS DIAGNOSTIC PROCEDURE IS APPLICABLE FOR ALL FUEL PUMP CONTROL MODULE FAULTS
KNOWN ISSUES: While the FPCM has the capability of detected failure modes, the faults are not always reported out as intended. Therefore, it is best not to rely on the DTC, or DTC Descriptions when deciding what portion of the system to diagnose. In most cases it is best to read and understand how the system operates, then diagnose the entire system to determine where the failure lies by performing the Complete Fuel Pump Control Module System Diagnostics procedure when diagnosing this system. This applies to all vehicles equipped with an FPCM. The vehicles listed below will set faults incorrectly for the model years listed and should always be diagnosed using the system approach.
- DS - model years 2016-2019
- RU - model years 2017-2018
- JL - model year 2018
- JT - model year 2020
The exceptions to the information above are the Loss of Communication, Fuel Rail Pressure Sensor and Fuel Pump Relay Control circuit faults. These faults will typically set correctly since they are detected by the PCM, not the FPCM.
GENERAL SYSTEM DESCRIPTION: When the Powertrain Control Module (PCM) requires Fuel Pump operation it energizes the Fuel Pump Relay. The Battery voltage output from the relay does not connect directly to the Fuel Pump like traditional systems. Instead, the Battery voltage is supplied to the Fuel Pump Control Module (FPCM). When the ignition is switched on, the Battery voltage is present for approximately five seconds. Battery voltage should always be present during cranking or when the engine is running. The purpose of FPCM is to operate the Fuel Pump using a Pulse Width Modulated (PWM) output to reduce electrical load on the system. Most vehicle/engine combinations operate as a fixed pressure system . Vehicles with the 6.2L engine operate as a variable pressure system . It is necessary on some vehicles with the 6.2L engine to have a second Fuel Pump and FPCM to meet engine needs. The second FPCM performs the same diagnostics as the primary module but uses different DTCs.
GENERAL SYSTEM OPERATION: To maintain the desired fuel pressure, the PCM sends a PWM duty cycle signal to the FPCM. If there are no system faults present, the Fuel Pump PWM Control output to the Fuel Pump is based on the duty cycle sent by the PCM. The graph below shows the FPCM output to the Fuel Pump in proportion to the PCM command signal.