LEMON Manuals: Even more car manuals for everyone
Home >> Fiat >> 2018 >> 500 Lounge, Automatic Trans >> Repair and Diagnosis >> External Pages >> Different variant/trim >> Section 58 (Communication - Non-DTC Based Diagnostics - 500 - Electric) >> Diagnosis And Testing >> Can C Bus No Response >> Theory Of Operation

Theory Of Operation

WARNING: This page is about a different variant/trim than selected.

The Body Control Module (BCM) provides the central connection point for vehicle communication. The BCM is the only module connected to both the CAN B and CAN C buses. The scan tool uses only the CAN C Bus to communicate with the BCM. The scan tool communicates directly with each of the CAN B modules. The BCM serves as the CAN B star connection point and provides a wire harness in-line connection for the scan tool CAN B diagnostics.

The primary communication network between electronic control modules is the Controller Area Network (CAN) data bus system. The Controller Area Network (CAN) data bus allows all electronic modules connected to the bus to share information with each other. Regardless of whether a message originates from a module on the higher speed CAN C (500K) Bus or on the lower speed bus, CAN B (50K) Bus, the message structure and layout is similar, which allows the Body Control Module/Central GateWay (BCM or BCMCGW) to process and transfer messages between the CAN buses. The BCM stores Diagnostic Trouble Codes (DTCs) for certain bus network faults on the CAN C (500K) Bus and CAN B (50K) Bus.

The CAN C bus modules are connected in series, and the CAN B bus modules are connected in parallel. Both use a two-wire bus using a twisted pair, where the wires are wrapped around each other to provide shielding from unwanted electromagnetic induction, thus preventing interference with the relatively low voltage signals being carried through them. While the CAN bus is operating (active), one of the bus wires will carry a higher voltage and is referred to as the CAN High or CAN bus (+) wire, while the other bus wire will carry a lower voltage and is referred to as the CAN Low or CAN bus (-) wire.

A short to ground or voltage on either of the CAN C bus circuits will prevent the CAN C modules from communicating. For the CAN B bus, a short to ground or voltage on only one bus circuit will not prevent the modules from communicating.

Possible Causes
CAN C BUS (+) CIRCUIT SHORTED TO GROUND
CAN C BUS (+) CIRCUIT SHORTED TO VOLTAGE
CAN C BUS (-) CIRCUIT SHORTED TO GROUND
CAN C BUS (-) CIRCUIT SHORTED TO VOLTAGE
CAN C BUS (+) CIRCUIT SHORTED TO CAN C BUS (-) CIRCUIT
ANY MODULE CONNECTED TO THE CAN C BUS
    NOTE:

    The CAN C bus modules are connected in series. The Data Link Connector (DLC) is connected to the bus through the Body Control Module (BCM). The termination resistors for the CAN C bus are located in the BCM and Powertrain Control Module (PCM). Normal bus resistance is approximately 60 Ohms when both the BCM and PCM are connected. The bus resistance is approximately 120 Ohms when only the BCM or PCM is connected.

  1. CHECK THE CAN C BUS CIRCUITS FOR A SHORT TO VOLTAGE 
    1. Disconnect the scan tool.
    2. Turn the ignition on.
    3. Measure the voltage between ground and the CAN C Bus (+) circuit at the DLC.
    4. Measure the voltage between ground and the CAN C Bus (-) circuit at the DLC.

      Is either voltage above 10.0 volts?

      Yes 

      • Go To  2

      No 

      • Go To  3
  2. CHECK THE CAN C BUS CIRCUITS FOR A SHORT TO VOLTAGE IN THE HARNESS FROM THE DLC TO BCM 
    1. Turn the ignition off.
    2. Disconnect the BCM C3 harness connector.
    3. Turn the ignition on.
    4. Measure the voltage between ground and the CAN C Bus (+) circuit at the DLC.
    5. Measure the voltage between ground and the CAN C Bus (-) circuit at the DLC.

      Is any voltage present?

      Yes 

      No 

      • Diagnose the short to voltage in the CAN C Bus circuit. The concern is located in one of the CAN C Bus circuits or modules between the BCM and the PCM. Starting with the PCM and working towards the BCM, use the module connectors to isolate portions of the bus to assist in determining the location of the concern. Repair as necessary. Only replace a module after all harness concerns are diagnosed.
      • Perform the BODY VERIFICATION TEST. Refer to BODY CONTROL MODULE (BCM) - BODY VERIFICATION TEST - 500 - ELECTRIC .
  3. CHECK THE CAN C BUS CIRCUITS FOR A SHORT TO GROUND 
    1. Turn the ignition off.
    2. Disconnect and isolate the negative battery cable.
    3. Measure the resistance between ground and the CAN C Bus (+) circuit at the DLC.
    4. Measure the resistance between ground and the CAN C Bus (-) circuit at the DLC.

      Are the resistances above 5.0 Ohms?

      Yes 

      • Go To  5

      No 

      • Go To  4
  4. CHECK THE CAN C BUS CIRCUITS FOR A SHORT TO GROUND IN THE HARNESS FROM THE DLC TO BCM 
    1. Disconnect the BCM C3 harness connector.
    2. Measure the resistance between ground and the CAN C Bus (+) circuit at the DLC.
    3. Measure the resistance between ground and the CAN C Bus (-) circuit at the DLC.

      Are the resistances above 10k Ohms?

      Yes 

      • Diagnose the short to ground in the CAN C Bus circuit. The concern is located in one of the CAN C Bus circuits or modules between the BCM and the PCM. Starting with the PCM and working towards the BCM, use the module connectors to isolate portions of the bus to assist in determining the location of the concern. Repair as necessary. Only replace a module after all harness concerns are diagnosed.
      • Perform the BODY VERIFICATION TEST. Refer to BODY CONTROL MODULE (BCM) - BODY VERIFICATION TEST - 500 - ELECTRIC .

      No 

  5. CHECK THE CAN C BUS CIRCUITS FOR A SHORT TOGETHER 
    GC0143419Courtesy of CHRYSLER GROUP, LLC
    1. Measure the resistance between the CAN C Bus (+) circuit and the CAN C Bus (-) circuit at the DLC.

      Is the resistance below 5.0 Ohms?

      Yes 

      • Go To  6

      No 

      • Go To  7
  6. CHECK THE CAN C BUS CIRCUITS FOR A SHORT TOGETHER IN THE HARNESS FROM THE DLC TO BCM 
    GC0143419Courtesy of CHRYSLER GROUP, LLC
    1. Disconnect the BCM C3 harness connector.
    2. Measure the resistance between the CAN C Bus (+) circuit and the CAN C Bus (-) circuit at the DLC.

      Is the resistance above 10k Ohms?

      Yes 

      • Diagnose the short between the CAN C Bus (+) circuit and the CAN C Bus (-). The concern is located in one of the CAN C Bus circuits or modules between the BCM and the PCM. Starting with the PCM and working towards the BCM, use the module connectors to isolate portions of the bus to assist in determining the location of the concern. Repair as necessary. Only replace a module after all harness concerns are diagnosed.
      • Perform the BODY VERIFICATION TEST. Refer to BODY CONTROL MODULE (BCM) - BODY VERIFICATION TEST - 500 - ELECTRIC .

      No 

  7. CHECK THE CAN C BUS CIRCUITS FOR AN OPEN IN THE HARNESS FROM THE DLC TO BCM 
    1. Disconnect the BCM C3 harness connector.
    2. Measure the resistance of the CAN C Bus (+) circuit between the DLC and the BCM C3 harness connector.
    3. Measure the resistance of the CAN C Bus (-) circuit between the DLC and the BCM C3 harness connector.

      Are the resistances below 5.0 Ohms?

      Yes 

      • Go To  8

      No 

  8. CHECK FOR CORRECT BUS TERMINATION RESISTANCE 
    GC0143419Courtesy of CHRYSLER GROUP, LLC
    1. Reconnect the BCM C3 harness connector.
    2. Measure the resistance between the CAN C Bus (+) circuit and the CAN C Bus (-) at the DLC.

      Is the resistance below 67 Ohms?

      Yes 

      • Test complete, the condition or conditions that originally set this DTC are not present at this time. Using the wiring diagrams as a guide, check all related splices and connectors for signs of water intrusion, corrosion, pushed out or bent terminals, and correct pin tension.
      • Perform the BODY VERIFICATION TEST. Refer to BODY CONTROL MODULE (BCM) - BODY VERIFICATION TEST - 500 - ELECTRIC .

      No 

      • For a resistance measurement of approximately 120 Ohms, Go To  9
      • For all others, repair the open or high resistance in the CAN C Bus (+) circuit or the CAN C Bus (-) circuit. The concern is located in one of the CAN C Bus circuits or modules between the BCM and the PCM. Starting with the PCM and working towards the BCM, use the module connectors to isolate portions of the bus to assist in determining the location of the concern. Repair as necessary. Only replace a module after all harness concerns are diagnosed.
      • Perform the BODY VERIFICATION TEST. Refer to BODY CONTROL MODULE (BCM) - BODY VERIFICATION TEST - 500 - ELECTRIC .
  9. DETERMINE WHICH TERMINATION MODULE IS NOT CONNECTED TO THE CAN C BUS 
    GC0143419Courtesy of CHRYSLER GROUP, LLC
    1. Disconnect the PCM harness connectors.
    2. Measure the resistance between the CAN C Bus (+) circuit and the CAN C Bus (-) at the DLC.

      Is the resistance approximately 120 Ohms?

      Yes 

      • The BCM is connected to the CAN C bus. The open is between the BCM and PCM. Use the module connectors to isolate portions of the bus to assist in determining the location of the concern. Repair as necessary. Only replace a module after all harness concerns are diagnosed.
      • Perform the BODY VERIFICATION TEST. Refer to BODY CONTROL MODULE (BCM) - BODY VERIFICATION TEST - 500 - ELECTRIC .

      No 

scientia non olet · About LEMON Manuals