Data Transfer Between ECM and CAN-Bus Capable Control Modules
Data Transfer Between Engine Control Module (ECM) And CAN-bus Capable Control Modules, Checking
Special Tools and Equipment
- VAG1526A
- VAG1527B
- VAG1594A
- VAG1598/31
- VAS5051 with VAG5051/1
NOTE:
- Data exchange between individual control modules occurs via a bus system.
- The term "CAN-bus" refers to a system that transports and distributes data.
- The wire connections between the control modules, via which data is transferred, are referred to as data wires.
- Data is transferred to the connected control modules serially (one after the other) via these data wires (i.e. engine speed, accelerator position).
Checking bus system
The DTC table suggested checking the data exchange between the Engine Control Module (ECM) and CAN capable control modules.
- Connect VAS5051 tester and select vehicle system "01 - Engine electronics". Ignition must remain switched on for this. Connecting VAS 5051 Scan Tool
Display on VAS5051:
- Select diagnostic function "08 - Read Measuring Value Block" in selection -1-.
Display on VAS5051:
1 - Enter display group Max. input value = 255
NOTE: Measuring value blocks 125 and 126 indicate the participants in the powertrain data-BUS.
- Select function "125" in button field -2- for "display group number 125" and press Q button to confirm input.
Display on VAS5051:
- Check indications in display fields -1- through -4-.
CAN capable control modules connected to the Engine Control Module (ECM) are indicated:
- No indication: Control module not CAN-capable
- Indication 1: CAN capable control module is connected to data bus
- Indication 0: CAN capable control module is not connected to databus
- Press - button to change into display group 126.
- Check under display group 126 in the same manner.
- Press "08 - erase DTC memory" to end function <- button.
Display on VAS5051:
- Select diagnostic function "06 - end output" in selection -1-.
Display on VAS5051:
- In selection -1-, press diagnostic function "00 - Check DTC memory - complete system".
- DTC memory of all OBD capable systems in the vehicle will be checked.
When a control module responds with its identification, the display indicates the number of stored errors or indicates "no malfunctions recognized".
DTCs stored in the system are indicated in sequence and printed out. Then the VAG1551 scan tool sends the next address word.
If a malfunction related to the "powertrain databus" or the "CAN-bus" is indicated:
- Check whether the Engine Control Module (ECM) and other CAN capable control modules installed are appropriate for this vehicle (part no. and coding).
If the correct control modules are installed:
- Check to be sure the multi-pin connectors of the control modules are securely connected.
If multi-pin connectors are properly secured:
- Check CAN bus system
Checking a "two-line bus system"
Three or more control modules are communicating across a "two-line bus system".
- Analyze the DTCs stored in the memories of the control modules.
NOTE: This analysis will help you locate the cause of the line malfunction.
Example 1:
From the DTCs stored in the memories, you have determined that control module 1 is not communicating with control modules 2 and 3.
- Switch ignition off.
- Disconnect the control modules connected across the bus wires and check the bus wires for an open circuit.
- Replace control module 1 if no malfunctions can be found in the bus wires.
Example 2:
From the DTCs stored in the memories, you have determined that control module 2 is not communicating with control modules 1 and 3.
- Switch ignition off.
- Disconnect the control modules connected across the bus wires and check the bus wires for an open circuit.
- Replace control module 2 if no malfunctions can be found in the bus wires.
Example 3:
Using the DTCs stored in the memories, you have determined that none of the control modules are sending or receiving signals.
- Switch ignition off.
- Disconnect the control modules connected across the bus wires and check bus wires for a short circuit to B- and Ground (GND).
If you cannot determine a cause for the DTC "Powertrain CAN-bus malfunction, check whether the DTC is caused by one of the control modules.
All control modules that use the CAN-bus are still disconnected. Ignition is switched off.
- Connect one of the control modules.
- Connect VAG1551 scan tool. Switch ignition on, and erase the DTC memory of the control module you just connected. End scan tool output using the "End Output" function 06.
- Switch ignition off and on again.
- Leave ignition on for 10 seconds. Then, read the DTC memory of the control module you just connected.
- If the DTC "Powertrain CAN-bus malfunction" is displayed, replace the control module you just connected.
- If the DTC "Powertrain CAN-bus malfunction" is not displayed, connect the next control module and repeat the procedure.