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Diagnostic Test - Fuel Pump Control Module System

For most vehicles the fuel pressure desired and actual values, as well as the Fuel Pump output duty cycle data can be viewed on the scan tool. These readings can also be very useful during the diagnosis of the system.

TECH TIP:  A vehicle will typically have one of two issues;

  1. The vehicle has no fuel pressure/pump is not operating.
    • The first thing to check is for a Loss of Communication fault. The reason is that the same Battery supply and ground circuit that are used to power up the module are also used for the Fuel Pump motor to operate. A loss of Battery supply, or a poor chassis ground for the module will cause both issues.
    • The next thing to look for is for an output from the FPCM to the pump. This can be done with the scan tool or by checking the duty cycle output from the FPCM. If there is any output from the FPCM to the Fuel Pump, the pump should be operating and creating fuel pressure. If the pump is not running then the Fuel Pump and circuits between the FPCM and Fuel Pump should be focused on during diagnostics. If the Loss of Communication fault is not present the Battery supply and chassis ground are most likely good.
  2. The vehicle runs normally, has a DTC or DTCs present, and the output duty cycle to the Fuel Pump is defaulted. The output is commonly defaulted to 80% and the actual pressure is greater than the desired pressure.
    • The first thing to check is for a Loss of Communication fault. If the vehicle is running with a Loss of Communication fault, and an 80% output duty cycle, it would typically point to an issue with the FPCM PWM Feedback circuit. A loss of Battery supply or a poor chassis ground would usually cause no Fuel Pump operation and a no start condition.
    • If there is no Loss of Communication fault then this condition is most likely present due to an issue with the command signal, or an over current draw/over temp condition with the Fuel Pump.

Keep this information as a source for where to focus diagnosing while navigating through the diagnostic procedures below.

COMPLETE FUEL PUMP CONTROL MODULE SYSTEM DIAGNOSTICS:  Using a lab scope is the preferred and best method for diagnosing the FPCM PWM Command circuit, FPCM PWM Feedback circuit, and Fuel Pump circuits. An alternative testing method would be to checking the Duty Cycle of each circuit with a DVOM. The first step is being performed to determine which path to take in diagnosing the system failure. Crank or start the engine while monitoring the duty cycle when each circuit is being checked.

  1. Using the wiring information as a guide, check the fuse that supplies power to FPCM through the Fuel Pump Relay. Verify the fuse is not open and has a good connection with the terminals in the PDC.
    1. If the fuse is good, continue testing in step 2.
    2. If the fuse is open, check the Fuel Pump Relay Output circuit and the Fuel Pump PWM Control circuit for a short to ground before replacing the fuse.
      NOTE:

      The Fuel Pump PWM Control circuit voltage is supplied on the Fuel Pump Relay Output circuit through the FPCM.

  2. Using a lab scope, back probe the FPCM PWM Command circuit, the Fuel Pump PWM Control circuit and the FPCM PWM Feedback circuit at the FPCM harness connector.
    NOTE:

    During normal operation the duty cycle on the Fuel Pump PWM Control circuit would match approximately to the FPCM PWM Command circuit and the FPCM PWM Feedback circuit duty cycle should be 50%.

    1. If the FPCM PWM Feedback circuit duty cycle is between 3% and 97%, but not 50%, the FPCM is reporting a fault to the PCM. This is normal when a system fault is present. Continue diagnosing the rest of the system and circuits for the failure mode. The FPCM PWM Feedback circuit should not require diagnosis unless the duty cycle is 100% or 0% (LOSS OF COMMUNICATION DIAGNOSTICS).
    2. If the FPCM PWM Command circuit duty cycle is 0% or 100%, continue testing in step 2.
    3. If the FPCM PWM Command circuit duty cycle is between 11% and 89%, and the Fuel Pump PWM Control circuit is reading 0%, it could indicate that the Fuel Pump PWM Control circuit is shorted to ground, or the FPCM is faulty.
    4. If the Fuel Pump PWM Control circuit is reading any duty cycle, the Fuel Pump should be operating and producing fuel pressure.
      • If the Fuel Pump is operating but no fuel pressure is present, the most likely cause is a leak or blockage in the line or a faulty Fuel Pump.
      • If the Fuel Pump is not operating, continue testing in step 4.
  3. Check the FPCM and PCM harness connectors for proper locking, pushed out, spread, corroded or dirty terminals.
    1. If the connectors and terminals appear good, continue testing in step 3.
  4. Isolate and check the FPCM PWM Command circuit for an open or short to ground.
    1. If an open or short are found, repair the circuit.
    2. If the circuits checks good, replace the FPCM and retest. If the condition continues, replace the PCM.
  5. Gain access to the Fuel Pump harness connector. Using the lab scope, back probe the Fuel Pump PWM Control circuit and Fuel Pump Return circuit. Crank or start the engine while monitor the duty cycle on each circuit being checked.
    1. If the Fuel Pump PWM Control circuit duty cycle is 0% at the Fuel Pump harness connector, the Fuel Pump PWM Control circuit is open or shorted to ground. Repair the circuit. NOTE: A short to ground in the Fuel Pump PWM Control circuit can also likely open the fuse for the FPCM Battery voltage supply, which can cause a Loss of Communication fault also. 
    2. If the Fuel Pump PWM Control circuit duty cycle is the same as the output from the FPCM, and the FPCM Return circuit is 0%, The Fuel Pump is faulty.
    3. If the FPCM Return circuit duty cycle matches the Fuel Pump PWM Control circuit duty cycle at the Fuel Pump harness connector, the FPCM Return circuit is open to ground. NOTE: The return circuit grounds through the FPCM chassis ground circuit. 

LOSS OF COMMUNICATION DIAGNOSTICS (U0109/U016C):  The PCM can lose communication with the FPCM if the FPCM does not power up or the FPCM PWM Feedback circuit is open or shorted. The FPCM is powered when the PCM energizes the Fuel Pump Relay. A poor chassis ground can also cause the FPCM not to power up.

NOTE:

If the P0627-Fuel Pump Relay Control Circuit fault is present, diagnose that DTC before continuing.

  1. Using the wiring information as a guide, check the fuse that supplies power to FPCM through the Fuel Pump Relay. Verify the fuse is not open and has a good connection with the terminals in the PDC.
    1. If the fuse is good, continue testing in step 2.
    2. If the fuse is open, check the Fuel Pump Relay Output circuit and the Fuel Pump PWM Control circuit for a short to ground.
      NOTE:

      The Fuel Pump PWM Control circuit voltage comes from the Fuel Pump Relay Output circuit through the FPCM.

  2. Disconnect the FPCM harness connector and load test the FPCM Ground circuit.
    GC0174957Courtesy of CHRYSLER GROUP, LLC
    1. A load test is used to determine if a circuit is capable of carrying the amperage needed to perform properly. The 3156 bulb in the load tool illustrated, is a simple but effective method of testing circuit functionality. A 3156 Bulb has approximately 6.0 Ohms of resistance when the bulb is powered and draws approximately 2.0 amps of current. Read the CIRCUIT LOAD TESTING PROCEDURE for information on building a simple load test tool and for additional load testing information and alternative methods of load testing or voltage drop testing a circuit. Refer to CIRCUIT LOAD TESTING PROCEDURES .
    2. To verify that there is no resistance in the circuit being tested, perform a simple voltage drop test across the 3156 bulb of the load test tool. The voltage drop across the bulb should be equal to the voltage reading across the Battery terminals if there is no resistance in the circuit being tested.
      • If the ground circuit fails load testing, repair the FPCM chassis ground.
      • If the ground circuit tests good, continue testing in step 3.
  3. Check the Battery supply on the Fuel Pump Relay Output circuit to the FPCM. The Battery supply can be checked one of the two ways below.
    NOTE:

    Keep in mind when performing this step that when the ignition is switched on, and the engine is not cranking, the Fuel Pump Relay is energized for approximately 5.0 seconds and then powered down. It will be powered continuously when the engine is cranking or running.

    1. Load test the Battery supply with the FPCM harness connector disconnected.
    2. Reconnect the FPCM harness connector. Back probe and measure the voltage when the ignition is turned on, and during cranking. The Battery voltage supply to the module is also used to power the Fuel Pump. If there is resistance in the Fuel Pump Relay Output circuit it may read Battery voltage with no load but drop when the Fuel Pump is energized.
      • If the Battery supply fails load testing, repair the Fuel Pump Relay Output circuit for an open.
      • If the Battery supply and ground circuits checked good, continue testing in step 4.
  4. Check the FPCM PWM Feedback circuit for an open and shorted to ground.
    1. The preferred method is to measure the duty cycle with a lab scope. If the duty cycle is reading 100% or 0% it indicates a possible open circuit or short to ground.
    2. The FPCM PWM Feedback circuit can be isolated and checked for high resistance or continuity to ground with a DVOM.
      • If the FPCM PWM Feedback checks good, replace the FPCM and retest. If the condition continues, replace the PCM.
        NOTE:

        Before replacing any modules it is a good practice to check all in-line harness connectors for pushed out, spread, partially broken or corroded terminals that can cause an intermittent condition.

FUEL SYSTEM PERFORMANCE DIAGNOSTIC (P062A): 

NOTE:

The Fuel Rail Pressure Sensor is used to detect this failure (Low Fuel Pressure Sensor on the fuel high pressure system engines). A drifted Fuel Rail Pressure Sensor reading due to a faulty sensor or high circuit resistance can cause this DTC to set. The Fuel Rail Pressure Sensor rationality is checked by the PCM at each engine start up event. If it fails it will set a Fuel Rail Pressure Sensor Performance fault. If the Fuel Rail Pressure Sensor Performance fault is present, perform that test procedure before continuing.

This fault usually sets along with other faults since the FPCM will either default to 80% duty cycle, or reduce the duty cycle PWM Output to the Fuel Pump when another fault is present. Either condition can cause delta between the actual fuel pressure and the desired fuel pressure to be greater than the calibrated thresholds. If this fault is present with any other FPCM related faults, perform the other appropriate diagnostic procedure to find the failure. If only this fault is set, perform this diagnostic procedure.

  1. If available on the scan tool, check the Fuel Pump duty cycle, the actual and desired fuel pressure data readings.
    1. If the duty cycle is 80%, and the actual pressure is approximately 64 - 65 psi, continue testing in step 2.
    2. If the actual pressure is lower than the desired pressure, there is likely a fuel pressure issue.
      • Verify that the vehicle is not out of fuel.
      • Check the fuel lines for leaks or restrictions.
      • Before condemning the Fuel Pump, the Fuel Pump PWM Control and return circuits should be checked for high resistance causing low Fuel Pump output. If both circuits check good, the Fuel Pump is most likely faulty.
  2. It is possible to have resistance in Fuel Rail Pressure Sensor signal or sensor ground circuit causing the signal to read off. Check the actual fuel system pressure with a pressure gauge and compare it to the Fuel Rail Pressure Sensor reading on the scan tool. Refer to the CHECKING THE FUEL DELIVERY SYSTEM (HARD START) test procedure for checking fuel pressure. Refer to DIAGNOSIS AND TESTING .
    1. If the actual fuel pressure and the Fuel Rail Pressure Sensor reading is the same, replace the Fuel Pump. The Fuel Pump Assembly can be failing and still produce high fuel pressure in the system when the FPCM is defaulted to 80% duty cycle.
      • If the actual fuel pressure and the Fuel Rail Pressure Sensor reading is different, continue testing in step 3.
  3. Isolate and check the Fuel Rail Pressure Sensor circuit for high resistance.
    1. If any of the circuits measure above 5.0 Ohms of resistance, repair the high resistance.
    2. If the circuits check good, replace the Fuel Rail Pressure Sensor.

SYSTEM VOLTAGE HIGH OR LOW DIAGNOSTICS (P025C/P025D/P027C/P027D):  The system voltage low fault indicates that the voltage supply to the module is below a calibrated threshold (approximately 6.0 volts). This typically is due to high resistance in the Battery voltage supply or ground circuits for the FPCM.

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