Body Control Module (BCM)
U0001-88-CAN C BUS OFF (NO CAN C COMMUNICATION)
For a complete wiring diagram, refer to the Wiring Information.
Theory of Operation
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.
- When Monitored:
With the ignition on.
- Set Condition:
The Body Control Module (BCM) detects a short to ground or voltage on the CAN C Bus circuit(s).
Diagnostic Test
1. CHECK THE BCM FOR CORRECT OPERATION
NOTE: The CAN C Bus modules are connected in series in the following order: DLC >> BCM >> EPS >> YRS >> ESM >> TCM >> ABS >> PCM. Disconnecting a module will remove the module and all the modules downstream of that module from the CAN C bus, preventing communication with the scan tool.
1. Turn the ignition off.
2. Disconnect the BCM A harness connector.
3. Turn the ignition on.
4. With the scan tool, read Body Control Module (BCM) DTCs.
Does the scan tool display U0001-88 CAN C BUS OFF as stored in the BCM?
Yes
- Go To 2
No
- Verify the correct operation of the scan tool being used.
- Using the wiring diagrams as a guide, check all the related connections between the scan tool, Data Link Connector (DLC), and Body Control Module (BCM). Repair as necessary.
- If the above conditions are correct, replace the Body Control Module (BCM). Body Control Module - Removal.
- Perform the BODY VERIFICATION TEST. Body Verification Test.
2. CHECK THE CAN C BUS (+) CIRCUIT FOR A SHORT TO GROUND
1. Turn the ignition off.
2. Measure the resistance between ground and the CAN C Bus (+) circuit at the BCM C1 harness connector.
Is the resistance below 10K Ohms?
Yes
- Repair the short to ground in the CAN C Bus (+) circuit. Use the module connectors to isolate portions of the bus to assist in determining the location of the concern. Repair the circuit concern or replace the module that when disconnected allowed the bus voltage to return to normal. Only replace a module after all harness concerns between the modules are diagnosed.
- Perform the BODY VERIFICATION TEST. Body Verification Test.
No
- Go To 3
3. CHECK THE CAN C BUS (-) CIRCUIT FOR A SHORT TO GROUND
1. Measure the resistance between ground and the CAN C Bus (-) circuit at the BCM C1 harness connector.
Is the resistance below 10K Ohms?
Yes
- Repair the short to ground in the CAN C Bus (-) circuit. Use the module connectors to isolate portions of the bus to assist in determining the location of the concern. Repair the circuit concern or replace the module that when disconnected allowed the bus voltage to return to normal. Only replace a module after all harness concerns between the modules are diagnosed.
- Perform the BODY VERIFICATION TEST. Body Verification Test.
No
- Go To 4
4. CHECK THE CAN C BUS (+) CIRCUIT FOR A SHORT TO VOLTAGE
1. Turn the ignition on.
2. Measure the voltage of the CAN C Bus (+) circuit at the BCM C1 harness connector.
Is voltage present?
Yes
- Repair the short to voltage on the CAN C Bus (+) circuit. Use the module connectors to isolate portions of the bus to assist in determining the location of the concern. Repair the circuit concern or replace the module that when disconnected allowed the bus voltage to return to zero. Only replace a module after all harness concerns between the modules are diagnosed.
- Perform the BODY VERIFICATION TEST. Body Verification Test.
No
- Go To 5
5. CHECK THE CAN C BUS (-) CIRCUIT FOR A SHORT TO VOLTAGE
1. Measure the voltage of the CAN C Bus (-) circuit at the BCM C1 harness connector.
Is voltage present?
Yes
- Repair the short to voltage on the CAN C Bus (-) circuit. Use the module connectors to isolate portions of the bus to assist in determining the location of the concern. Repair the circuit concern or replace the module that when disconnected allowed the bus voltage to return to zero. Only replace a module after all harness concerns between the modules are diagnosed.
- Perform the BODY VERIFICATION TEST. Body Verification Test.
No
- Test complete.