U0121
U0121-LOST COMMUNICATION WITH ABS
Theory of Operation
The primary communication network between electronic control modules on this vehicle 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 CAN Interior High Speed (IHS) (125K) Bus the message structure and layout is similar, which allows the Totally Integrated Power Module/Central GateWay (TIPM or TIPMCGW) to process and transfer messages between the CAN buses. The TIPM also stores Diagnostic Trouble Codes (DTCs) for certain bus network faults.
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. Since each module contains it own termination a module that is isolated may receive its own messages correctly 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.
- When Monitored:
With the ignition on.
Battery voltage between 10 and 16 volts.
Ignition Off Draw (IOD) fuse installed.
Totally Integrated Power Module (TIPM) is configured correctly.
- Set Condition:
Bus messages not received from the Anti-Lock Brake Module (ABS) for approximately two to five seconds.
1. VERIFY THE DIAGNOSTIC TROUBLE CODE (DTC) IS ACTIVE
NOTE: Make sure the IOD fuse is installed and battery voltage is between 10 and 16 volts before proceeding.
1. With the scan tool, read active DTCs.
Is this DTC active?
Yes
- Go To 2
No
- Stored Lost Communication DTCs Perform the STORED LOST COMMUNICATION DTCS diagnostic procedure.
2. CHECK FOR ANY OF THE FOLLOWING ACTIVE DTCS
1. With the scan tool, read all active DTCs from all modules.
NOTE: Check for TIPM configuration, CAN C hardware electrical, VIN Missing/Mismatch, battery or ignition related DTCs.
Does the scan tool display any active DTCs to the conditions listed above?
Yes
- Perform the appropriate diagnostic procedure.
No
- Go To 3
3. VERIFY THE ABS IS ACTIVE ON THE BUS
1. Turn the ignition on.
2. With the scan tool, select Network Diagnostics.
3. Verify the ABS is active on the bus.
Is the ABS active on the bus?
Yes
- Go To 4
No
- No Response From ABS (Antilock Brake Module) Perform the appropriate NO RESPONSE FROM ABS diagnostic procedure.
4. CHECK FOR ADDITIONAL COMMUNICATION RELATED DTCS
1. With the scan tool, select Network Diagnostics.
Is there more than one module with active DTCs "Logged Against" the ABS?
Yes
- Replace/update the Anti-Lock Brake Module.
- Perform the ABS VERIFICATION TEST. ABS Verification Test.
No
- Go To 5
5. CLEAR DTC IN MODULE SETTING FAULT
1. With the scan tool, select the module setting the DTC against the ABS.
2. Clear active DTCs.
Is this DTC still active?
Yes
- Replace/update the module that set this DTC.
- Perform the appropriate verification test for the module being replaced. If there is no verification test for the associated module perform the BODY VERIFICATION TEST. Body Verification Test.
No
- Go To 6
6. CYCLE IGNITION
1. Cycle the ignition from RUN to OFF three times ending with the key in the RUN position.
Does this DTC become active?
Yes
- Go To 7
No
- The condition is 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 ABS VERIFICATION TEST. ABS Verification Test.
7. CHECK FOR ADDITIONAL LOST COMMUNICATION FAULTS
1. With the scan tool, select Network Diagnostics.
Does the TIPM and other CAN C BUS modules show Lost Communication with the remaining CAN C BUS modules?
Yes
- Go To 8
No
- Replace/update the module that set this DTC.
- Perform the appropriate verification test for the module being replaced. If there is no verification test for the associated module perform the BODY VERIFICATION TEST. Body Verification Test.
8. (D52) OR (D65) CAN C BUS (+) CIRCUIT SHORTED TO VOLTAGE
1. Measure the voltage between the (D52) CAN C Bus (+) circuit and ground.
2. Measure the voltage between the (D65) CAN C Bus (+) circuit and ground.
Is the voltage above 5 volts?
Yes
- Repair the short to voltage in the (D52) or (D65) CAN C Bus (+) circuit.
- Perform the ABS VERIFICATION TEST. ABS Verification Test
No
- Go To 9
9. (D51) OR (D64) CAN C BUS (-) CIRCUIT SHORTED TO GROUND
1. Turn the ignition off.
2. Measure the resistance between ground and the (D51) CAN C Bus (-) circuit.
3. Measure the resistance between ground and the (D64) CAN C Bus (-) circuit.
Is the resistance below 10K Ohms?
Yes
- Repair the short to ground in the (D51) or (D64) CAN C Bus (-) circuit.
- Perform the ABS VERIFICATION TEST. ABS Verification Test.
No
- Replace/update the module that set this DTC.
- Perform the appropriate verification test for the module being replaced. If there is no verification test for the associated module perform the BODY VERIFICATION TEST. Body Verification Test.