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Home >> Jeep >> 2021 >> Grand Cherokee L Laredo, RWD >> Repair and Diagnosis >> Engine Performance >> System >> 3.6L - DTCS P0153 To P0304 >> DTC Troubleshooting >> P025A-Fuel Pump Module Control Circuit Open >> Diagnostic Test - Complete Fuel Pump Control Module System Testing

Diagnostic Test - Complete Fuel Pump Control Module System Testing

COMPLETE FUEL PUMP CONTROL MODULE SYSTEM DIAGNOSTICS:  Using the Mopar Scope is the preferred and best method for diagnosing the FPCM and circuits. The circuits can be monitored one at a time or all together using all four channels of the scope. This test is written with the leads back probed and connected at the FPCM connector unless otherwise specified  . Crank or start the engine while monitoring each of the following circuits:

Below are the expected results along with failure modes for each of the circuits:

  1. BATTERY SUPPLY CIRCUIT 
    1. The relay output voltage to the FPCM should remain constant and equal to battery voltage during operation.
      • If the voltage is constant battery voltage during cranking, the relay output is not the cause of the issue.
    2. If no voltage is present during cranking there is an open in the relay output circuit.
      • Check the fuse for the circuit. If the fuse is open, check the Fuel Pump Relay Output circuit for a short to ground before replacing the fuse. If the fuse is good check the relay output circuit for an open.
    3. Voltage is present but below battery voltage.
      • Check for resistance in the relay output and FPCM ground circuits.
        NOTE:

        An open ground circuit can cause the feedback circuit voltage to change from a 5v duty cycle to a constant voltage or approximately 6.5v as seen in the figure below.

        GC0200585Courtesy of CHRYSLER GROUP, LLC
        Blue Channel - Feedback circuit
        Red Channel - Fuel pump output circuit
        Green Channel - Command circuit
  2. FUEL PUMP PWM COMMAND CIRCUIT  - For a detailed description on the functionality of the command circuit, read the PCM component functional description section above.
    NOTE:

    The FPCM will still operate the fuel pump when there is an issue with the command circuit. Typically the output voltage to the fuel pump will be higher than during normal operation.

    The command circuit can be tested at the FPCM or at the PCM using the GPEC Adaptor connected in-line. Both results are described below.

    Scope lead connected at the FPCM

    1. If the PWM Command circuit is functioning normally the signal should have a 12v duty cycle between 11% and 89%.
      • If the command circuit is functioning as described above the command circuit is not the cause of the issue.
    2. If the PWM Command circuit is reading a constant 0 volts, it would indicate that the control circuit is shorted to ground.
      GC0201851Courtesy of CHRYSLER GROUP, LLC
      Blue Channel - Feedback circuit
      Red Channel - Fuel pump output circuit
      Green Channel - Command circuit
      • Test and repair the short to ground in the command circuit.
    3. If the PWM Command circuit is a reading a constant battery voltage, it would indicate an open in the command circuit or the PCM internal driver is faulty.
      GC0201453Courtesy of CHRYSLER GROUP, LLC
      Blue Channel - Feedback circuit
      Yellow Channel - Command circuit on the FPCM side of the open
      Green Channel - Command circuit on the PCM side of the open
      • Isolate and measure the resistance of the command circuit. If the circuit is open or has high resistance repair the circuit. If the circuit has less than 5.0 Ohms of resistance, the PCM driver is faulty. Replace the powertrain control module in accordance with the service information.
    4. If the PWM Command circuit is duty cycling, but the voltage is approximately 2.5v instead of the normal 12v duty cycle, it indicates that the command circuit and PCM internal driver are good. The FPCM is not producing the 12v output on the command circuit.
      GC0201453Courtesy of CHRYSLER GROUP, LLC
      Blue Channel - Feedback circuit
      Yellow Channel - Command circuit on the FPCM side of the open
      Green Channel - Command circuit on the PCM side of the open
      • Verify that the relay output battery supply to the FPCM is constant and the FPCM ground circuit is good. If both circuits test good replace the fuel pump control module in accordance with the service information.

    Scope lead connected at the PCM with the GPEC Adaptor

    1. If the PWM Command circuit is functioning normally the signal should have a 12v duty cycle between 11% and 89%.
      • If the command circuit is functioning as described above the command circuit is not the cause of the issue.
    2. If the PWM Command circuit is reading a constant 0 volts, it would indicate that the command circuit is shorted to ground.
      GC0201851Courtesy of CHRYSLER GROUP, LLC
      Blue Channel - Feedback circuit
      Red Channel - Fuel pump output circuit
      Green Channel - Command circuit
      • Test and repair the short to ground in the command circuit.
    3. If the PWM Command circuit is a reading a constant battery voltage, it indicates the command circuit and FPCM output are good. The PCM internal driver is faulty.
      GC0201453Courtesy of CHRYSLER GROUP, LLC
      Blue Channel - Feedback circuit
      Yellow Channel - Command circuit on the FPCM side of the open
      Green Channel - Command circuit on the PCM side of the open
      • Replace the powertrain control module in accordance with the service information.
    4. If the PWM Command circuit is duty cycling, but the voltage is approximately 2.5v instead of the normal 12v duty cycle, it indicates that the PCM internal driver is good. There is an open in the command circuit or the FPCM is not producing the 12v output on the command circuit.
      GC0201453Courtesy of CHRYSLER GROUP, LLC
      Blue Channel - Feedback circuit
      Yellow Channel - Command circuit on the FPCM side of the open
      Green Channel - Command circuit on the PCM side of the open
      • Isolate and measure the resistance of the command circuit. If the circuit is open or has high resistance repair the circuit. If the circuit has less than 5.0 Ohms of resistance, the FPCM output is faulty. Replace the fuel pump control module in accordance with the service information.
  3. FPCM PWM FEEDBACK CIRCUIT  - For a detailed description on the functionality of the feedback circuit, read the FPCM component functional description section above.
    NOTE:

    The PCM will command the highest command duty cycle when it cannot receive feedback from the FPCM due to an issue with the feedback circuit. Typically the output voltage to the fuel pump will be higher than during normal operation.

    The feedback circuit can be tested at the FPCM or at the PCM using the GPEC Adaptor connected in-line. Both results are described below.

    Scope lead connected at the FPCM

    1. If the FPCM PWM Feedback circuit is functioning normally the signal should have a 5v duty cycle between 3% and 97%.
      • If the feedback circuit is functioning as described above the feedback circuit is not the cause of the issue.
    2. If the PWM feedback circuit is reading a constant 0 volts, it would indicate that the control circuit is shorted to ground.
      • Test and repair the short to ground in the feedback circuit.
    3. If the PWM feedback circuit is reading a constant 5.0 volts, it would indicate that FPCM internal driver is faulty.
      GC0201229Courtesy of CHRYSLER GROUP, LLC
      Blue Channel - Feedback circuit on the FPCM side of the open
      Yellow Channel - Feedback circuit on the PCM side of the open
      Green Channel - Command circuit
      Red Channel - Fuel pump output circuit
      • Replace the fuel pump control module in accordance with the service information.
    4. If the PWM feedback circuit is reading a constant voltage approximately 6.5 volts, it would indicate that the FPCM ground circuit is open.
      GC0200585Courtesy of CHRYSLER GROUP, LLC
      Blue Channel - Feedback circuit
      Red Channel - Fuel pump output circuit
      Green Channel - Command circuit
      • Test and repair the FPCM ground circuit for an open or high resistance.
    5. If the PWM feedback circuit is duty cycling between approximately 0.3 and 0.6v instead of the normal 5v duty cycle, it indicates that the FPCM internal driver is good. Either the feedback circuit is open or the PCM is not producing the 5v output on the feedback circuit.
      GC0201407Courtesy of CHRYSLER GROUP, LLC
      Blue Channel - Feedback circuit on the FPCM side of the open
      • If the circuit is open or has high resistance repair the circuit. If the circuit has less than 5.0 Ohms of resistance, the PCM output driver is faulty. Replace the powertrain control module in accordance with the service information.
        NOTE:

        Another way to test the output and circuit is to connect a scope lead to the PCM connector using the GPEC Adaptor tool. If the 5v output is present at the PCM connector, but not at the FPCM, the PCM is good and the feedback circuit is open.

    Scope lead connected at the PCM with the GPEC Adaptor

    1. If the FPCM PWM Feedback circuit is functioning normally the signal should have a 5v duty cycle between 3% and 97%.
      • If the feedback circuit is functioning as described above the feedback circuit is not the cause of the issue.
    2. If the PWM feedback circuit is reading a constant 0 volts, it would indicate that the control circuit is shorted to ground.
      • Test and repair the short to ground in the feedback circuit.
    3. If the PWM feedback circuit is reading a constant 5.0 volts, it would indicate that the feedback circuit is open or the FPCM internal driver is faulty.
      GC0201229Courtesy of CHRYSLER GROUP, LLC
      Blue Channel - Feedback circuit on the FPCM side of the open
      Yellow Channel - Feedback circuit on the PCM side of the open
      Green Channel - Command circuit
      Red Channel - Fuel pump output circuit
      • If the circuit is open or has high resistance repair the circuit. If the circuit has less than 5.0 Ohms of resistance, the FPCM driver is faulty. Replace the fuel pump control module in accordance with the service information.
    4. If the PWM feedback circuit is reading a constant voltage approximately 6.5 volts, it would indicate that the FPCM ground circuit is open.
      GC0200585Courtesy of CHRYSLER GROUP, LLC
      Blue Channel - Feedback circuit
      Red Channel - Fuel pump output circuit
      Green Channel - Command circuit
      • Test and repair the FPCM ground circuit for an open or high resistance.
    5. If the PWM feedback circuit is duty cycling between approximately 0.3v and 0.6v instead of the normal 5v duty cycle, it indicates that the feedback circuit and FPCM internal driver is good. The PCM is not producing the 5v output on the feedback circuit.
      GC0201407Courtesy of CHRYSLER GROUP, LLC
      Blue Channel - Feedback circuit on the FPCM side of the open
      • Replace the powertrain control module in accordance with the service information.
  4. FUEL PUMP AND CONTROL CIRCUITS 
    1. There should be a relatively constant voltage on the fuel pump control circuit. When the system is operating normally without any faults the output voltage is typically around 7.0 - 9.0 volts. When there is a fault in the system the output is typically higher due to the FPCM defaulting to a higher output (between 10.5 volts and battery voltage).
      • If there is voltage on the output as described above, the fuel pump control circuit output from the FPCM is normal. Continue to step b. 
    2. Gain access to the Fuel Pump harness connector. Using the Mopar Scope, back probe the fuel pump control circuit and fuel pump return circuit. Crank or start the engine while monitor the voltage on each circuit.
      • If the Fuel Pump Control circuit voltage is 0 at the Fuel Pump harness connector, the Fuel Pump Control circuit is open or shorted to ground. Repair the circuit. NOTE: A short to ground in the Fuel Pump PWM Control circuit may open the fuse for the FPCM Battery voltage supply from the relay, which can cause a Loss of Communication fault also. 
      • If the Fuel Pump Control circuit voltage is the same as the output from the FPCM, the FPCM Return circuit is 0v, and the fuel pump is not running, the Fuel Pump is faulty.
      • If the FPCM Return circuit voltage 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. 
GC0201984Courtesy of CHRYSLER GROUP, LLC
Blue Channel - Feedback circuit
Red Channel - Fuel pump output circuit
Yellow Channel - Command circuit
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