Body Control Module (BCM)

U0002-00-CAN C BUS OFF PERFORMANCE

For a complete wiring diagram, refer to the Wiring Information.

Theory of Operation

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 Interior High Speed (IHS) (125K) 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. An additional gateway module, the Telematics Gateway (TGW) Module is used to transfer messages between the CAN IHS (125K) Bus and the CAN Audio and Telematics (AT) Bus. The BCM stores Diagnostic Trouble Codes (DTCs) for certain bus network faults on the CAN C (500K) Bus and CAN IHS (125K) Bus. The TGW stores DTCs for certain bus network faults on the CAN AT Bus.

All modules transmit and receive messages over one of these buses. Data exchange between the modules is achieved by serial transmission of encoded data messages (a form of transmission in which data bits are sent sequentially, one at a time, over a single line). Each module can both send and receive serial data simultaneously. Each data bit of a CAN Bus message is carried over the bus as a voltage differential between the two bus circuits which, when strung together, form a message. Each module uses arbitration to sort the message priority if two competing messages are attempting to be broadcast at the same time. Corruption of a single bit within a message will corrupt the entire message. Each message contains a Cyclic Redundancy Check (CRC) which specifies the message size exactly. If the message detected conflicts with the CRC the ECU receiving it will determine the message to be an error and consider that communication has not been possible. Diagnosis of this condition using a lab scope may reveal activity that appears to be Bus data messages even if no actual communication is possible. Communication problems that affect the whole bus, as a result of opens and terminal push outs are more likely to occur on data busses that operate at a high speed than a data bus that operates at a lower speed.

When an open circuit or terminal push out occurs one or more modules can become isolated from the remainder of the bus. The isolated module will attempt to communicate, but will not be able to receive messages or determine arbitration from other modules. Each time the isolated module attempts to communicate it alters the bus voltage on the intact bus circuit. Without functioning arbitration the isolated module alters the bus voltage while other bus messages are being sent thereby corrupting the messages on the remainder of the bus.

The CAN bus modules are connected in parallel to the 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.

For additional information on the communication network and star connector locations, Communication - Description.






- When Monitored:
With the ignition on.

- Set Condition:
The Body Control Module (BCM) detects a short condition on one of the CAN C Bus circuits.





1. TEST FOR INTERMITTENT CONDITION

NOTE: Disconnecting the Radio Frequency (RF) Hub Module results in a loss of ignition status messages and may cause scan tool communication concerns.

1. Turn the ignition on.
2. With the scan tool, record and erase DTCs.
3. Cycle the ignition from on to off three times.
4. Turn the ignition on.
5. With the scan tool, read DTCs.

Does the scan tool display U0002-00-CAN C BUS OFF PERFORMANCE?

Yes

- Go To 2

No

- 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. Body Verification Test.

2. CHECK THE CAN BUS (-) CIRCUIT FOR A SHORT TO VOLTAGE




1. Disconnect the scan tool.
2. Measure the voltage between ground and the CAN C Bus (-) circuit.

Is the voltage above 10.0 volts?

Yes

- Diagnose the short to voltage in the CAN C Bus (-) circuit. Use the Star 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.
- 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 voltage between ground and the CAN C Bus (+) circuit.

Is the voltage approximately 0 volt?

Yes

- Diagnose the short to ground in the CAN C Bus (+) circuit. Use the Star 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.
- Perform the BODY VERIFICATION TEST. Body Verification Test.

No

- Go To 4

4. CHECK FOR CORRECT BUS TERMINATION RESISTANCE




1. Turn the ignition off.
2. Disconnect and isolate the negative battery cable.
3. Measure the resistance between the CAN C Bus (+) circuit and the CAN C Bus (-) circuit.

Is the resistance between 53.5 and 67 Ohms?

Yes

- Test complete, the condition or conditions that originally caused this concern are not present at this time. Using the wiring diagrams as a guide, check the Star Connectors and all related splices for signs of water intrusion, corrosion, pushed out or bent terminals, and correct pin tension.
- Perform the BODY VERIFICATION TEST. Body Verification Test.

No

- For a resistance measurement above 67 Ohms, an open or high resistance is present in the CAN C Bus (+) circuit or the CAN C Bus (-) circuit between the Star Connectors or one of the termination resistors internal to the Star Connector is damaged. Diagnose and repair as necessary.
- For a resistance measurement below 53.5 Ohms, a short between the CAN C Bus (+) circuit and the CAN C Bus (-) is present. Diagnose the short between the CAN C Bus (+) circuit and the CAN C Bus (-) circuit. Use the Star 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 resistance to return to normal.
- Perform the BODY VERIFICATION TEST. Body Verification Test.