Diagnostic Test - Communication And FPCM Internal Faults

FPCM INTERNAL PERFORMANCE (P1205, P120A): 

1. The FPCM is reporting that it has lost the capability of performing fault detection on the system.
   1. Replace the FPCM in accordance with the service information.

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. Choose one of the two methods below to test the system.

TESTING THE BATTERY SUPPLY AND FEEDBACK USING A MOPAR SCOPE 

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. Back probe and connect the scope leads at the FPCM connector as follows:
   1. Channel A to the fuel pump relay output to FPCM circuit (power supply to module).
   2. Channel B to the feedback signal circuit.
3. Monitor the signals with the engine cranking or running. When the feedback circuit is connected and functioning properly, the scope pattern generated will be a 5v square wave signal. The 5.0 volts is generated in the PCM, with the FPCM controlling the pulse width on the low side. The power supply should remain constant and equal to battery voltage during operation of the fuel pump.
   1. If the pattern on the feedback signal is a constant 5v signal it would indicate that the PCM and feedback circuit are good. The issue is internal to the FPCM control. Replace the Fuel Pump Control Module in accordance with the service information.
   2. If the pattern on the feedback signal is showing a slight pulse width signal switching between 1mv and 0.5v, it would indicate that the internal FPCM control is active and either the feedback circuit is open or a faulty PCM. Isolate and check the feedback circuit for an open. If the feedback circuit checks good, replace the Powertrain Control Module in accordance with the service information.
   3. If the pattern is a constant 0v, it would indicate a short to ground in the feedback circuit. Test and repair the circuit for a short to ground.
   4. If the FPCM ground path is open the feedback circuit will have a constant voltage of approximately 6.6v present. Continue testing in step 4  to load test the FPCM.
   5. If the power supply drops during operation it would indicate resistance in the battery supply from the fuel pump relay output. Repair the resistance in the power supply circuit. If no voltage is present, repair the circuit for an open.
4. Disconnect the FPCM harness connector and load test the FPCM Ground circuit. 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 .

   Courtesy of CHRYSLER GROUP, LLC

   1. A way to verify that there is no resistance in the circuit being tested is to 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, replace the Fuel Pump Control Module in accordance with the service information.

BASIC LOAD TESTING OF FPCM POWER AND GROUND CIRCUITS 

1. Disconnect the FPCM harness connector and load test the FPCM Ground circuit. 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 .

   Courtesy of CHRYSLER GROUP, LLC

   1. A way to verify that there is no resistance in the circuit being tested is to 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 2. 
   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.

2. Load test the battery supply to the FPCM harness connector.
   1. If the battery supply circuit fails load testing, repair the fuel pump relay output circuit for an open or high resistance. If the fuse is open, test the circuit for a short to ground
   2. If the battery supply circuit checks good, continue testing in step 3. 
3. Reconnect the FPCM harness connector and check the FPCM PWM Feedback circuit. The preferred method is to back probe and check the feedback duty cycle at the FPCM harness connector with a Mopar Scope. Monitor the signal with the engine cranking or running. When the feedback circuit is connected and functioning properly, the scope pattern generated will be a 5v square wave signal. The 5.0 volts is generated in the PCM, with the FPCM controlling the pulse width on the low side.
   1. If the pattern is a constant 5v signal it would indicate that the PCM and feedback circuit are good. The issue is internal to the FPCM control. Replace the FPCM in accordance with the service information.
   2. If the pattern is showing a pulse width signal switching between 1mv and 0.5v it would indicate that the internal FPCM control is active and there is an open feedback circuit or faulty PCM. Isolate and check the feedback circuit for an open. If the feedback circuit checks good, replace the Powertrain Control Module in accordance with the service information.
   3. If the pattern is constant 0v it would indicate a short to ground in the feedback circuit. Test and repair the circuit for a short to ground.