Data Transfer Between Engine Control Module And Can-Bus Capable Control Modules
Data is transferred between control modules through the CAN-bus system. CAN-bus refers to a network connecting various control modules. Data is transferred between modules serially (one module after the other), through CAN-bus network. If DTC testing suggests checking data exchange between ECM and CAN capable control modules perform following procedure:
- Connect VAS 5051 or equivalent scan tool to DLC. DLC is located behind cover under left side of steering column. Start engine. Using scan tool, select vehicle system 01 - ENGINE ELECTRONICS. From selection 1, select diagnostic function 08 - READ MEASURING VALUE BLOCK. From button field 2, enter 125 for DISPLAY GROUP 125. Press "Q" button to confirm input, and go to next step.
- CAN capable control modules are displayed in display fields 1-4 of display group 125. No indication: indicates control module is not CAN capable. Indication 1: CAN capable control module is connected to databus. Indication 0: CAN capable control module is not connected to databus. Press forward arrow button to switch to display group 126. Check display group 126 for CAN capable modules. From selection 1, select diagnostic function 06 - END OUTPUT. Go to next step.
- Using scan tool, from selection 1, select diagnostic function 00 - CHECK DTC MEMORY - COMPLETE SYSTEM. DTC memory of all OBD capable systems in the vehicle will be checked. When each control module responds with it's identification, scan tool display indicates number of stored DTCs, or NO MALFUNCTIONS RECOGNIZED. DTC stored in the system are indicated in sequence and printed out. VAG scan tool sends next address word. Go to next step.
- If malfunction related to powertrain databus or CAN-bus is indicated. Check if ECM or other CAN capable control modules installed are appropriate for vehicle (check part number and coding). Check for poor or loose terminals in control module harness connectors. If problem is found, repair as necessary. If no problem is found, go to next step.
- Using stored DTCs and data link connectors wiring diagram, analyze which control modules are not communicating. See DATA LINK CONNECTORS article in ACCESSORIES & EQUIPMENT. Use example in illustrations to determine non-communicating modules. See Fig 1 - Fig 3 . Turn ignition off. Disconnect CAN-bus connected control module harness connectors. In examples 1 and 2 check for open circuits in bus wires of connected control modules. In example 3 check for short to voltage in bus wire circuits of connected control modules. If problem is found, repair as necessary. If no problem is found, replace suspect control module. Go to next step.
- If cause for Powertrain CAN-bus malfunction DTC can not be determined, check if DTC was caused by one of the control modules. Turn ignition off. Disconnect all control modules on CAN-bus circuit.
- Connect one of the control modules.
- Connect VAG 5051 scan tool. Turn ignition on. Using scan tool erase DTC memory of last connected control module. End scan tool output using function 06 - END OUTPUT.
- Turn ignition on and off.
- Leave ignition on for 10 seconds or more. Using scan tool read DTC memory of last connected control module.
- If scan tool displays, POWERTRAIN CAN-BUS MALFUNCTION, replace last connected control module.
- If scan tool does not display, POWERTRAIN CAN-BUS MALFUNCTION, connect next control module and repeat procedure.
- Continue repeating procedure until malfunctioning module is determined. Using scan tool, clear DTCs. Test drive vehicle. Using scan tool, retrieve DTCs. If DTC does not reset testing is complete.
NOTE:
Measuring value blocks 125 and 126 indicate control modules connected to powertrain data-BUS.