Data Exchange Between Engine, ABS & TCM Control Modules Check
Data Exchange Between Engine/ABS/Transmission Control Module, Checking
NOTE:
- The data exchange between the various control modules is carried out via a bus system.
- The term "Bus" refers to a system for transmitting and distributing data.
- The wires which connect the control modules and convey the data between them are called data wires.
- Data is transmitted via data wires in serial, i.e. in a specific order to the connected control modules.
Checking bus system
The principle of the test is independent of the number of control modules/systems connected to the CAN-bus.
The DTC table suggested checking the data exchange between the Engine Control Module (ECM) and CAN capable control modules.
- Start engine (if engine will not start, switch ignition on).
- Connect VAS5051 tester or VAG1551 scan tool and select the control module for engine electronics using "address word" 01. Refer to "Scan Tool Testing/Scan Tool Connecting and Initial Checks" Testing and Inspection
When indicated on display:
Rapid data transfer HELP
Select function XX
- Press buttons -0- and -8- to select "Read Measuring Value Block" and press -Q- button to confirm input.
When indicated on display:
Read measuring value block Q
Enter display group number XXX
- Press buttons -1-, -2- and -5- to select "display group number 125" and press -Q- button to confirm input.
When indicated on display:
Read measuring value block 125 ->
1 2 3 4
- Check indications in display fields 1 through 4.
CAN capable control modules connected to the Engine Control Module (ECM) are indicated:
- No indication (display field empty): Control module not CAN-capable
- Indication 1: CAN capable control module is connected to databus
- Indication 0: CAN capable control module is not connected to databus (e.g. in case of malfunction)
Example:
Read measuring value block 125 ->
ABS 1 Combi 1
NOTE: Display group 125 indicates all systems that communicate with the Engine Control Module (ECM) via the CAN-bus. (In this example, the ABS control module and the instrument cluster). If these systems are not installed, the display fields in display group 125 that are intended for them will be empty.
Display field 1 will indicate "Transmission 1" and display field 4 will indicate "A/C 1" if these systems are communicated with the Engine Control Module (ECM) via the CAN-bus.
Read measuring value block 126 ->
1 2 3 4
- Check under display group 126 in the same manner.
NOTE: Display group 126 indicates all other systems that communicate with the Engine Control Module (ECM) via the CAN-bus. If these systems are not installed, the display fields that are intended for them will be empty.
The following systems will be indicated in display group 126, assuming they are communicating with the Engine Control Module (ECM) via the CAN-bus:
Display field 1: automatic distance control; indication: "Distance 1".
Display field 2: Steering angle sensor;
indication: "Steering angle 1"
Display field 3: Airbag; indication: "Airbag 1".
Display field 4: System voltage; indication: " cnt. ELEC 1".
- Press -> button.
When indicated on display:
Rapid data transfer HELP
Enter address word XX
- Press button 0- twice to select "Automatic test sequence", address word 00. Press -Q- button to confirm input.
- 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.
The automatic test sequence is over when the following indication appears on the display:
VAG - On Board Diagnostic (OBD) HELP
1- Rapid data transfer*
2- Blink code output*
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.
The following trouble shooting procedure is recommended if three or more control modules communicate via a "two-wire bus system".
- Read out malfunctions stored in the control modules.
This will help to identify a malfunction in the wiring.
Example 1
From the malfunctions present in the DTC memories, you can see that control module 1 has no connection to control modules 2 or 3.
Control module Stored malfunctions in DTC memory:
1 - No message from control module 2
- No message from control module 3
2 - No message from control module 1
3 - No message from control module 1
- Switch ignition off.
- Disconnect control modules that are linked by bus wires and check whether there is an open circuit in one of the bus wires
Locations
- If no malfunction can be identified in the bus wires replace control module 1.
Example 2
From the malfunctions present in the DTC memories, you can see that control module 2 has no connection to control modules 1 or 3.
Control module Stored malfunctions in DTC memory:
1 - No message from control module 2
2 - No message from control module 1
- No message from control module 3
3 - No message from control module 2
- Switch ignition off.
- Disconnect control modules that are linked by bus wires and check whether there is an open circuit in one of the bus wires
- If no malfunction can be identified in the bus wires replace control module 2.
Example 3
From the malfunctions present in the DTC memories, you can see that none of the control modules are able to transmit or receive signals.
Control module Stored malfunctions in DTC memory
1 - Data Bus Powertrain Malfunction
2 - Data Bus Powertrain Malfunction
3 - Data Bus Powertrain Malfunction
- Switch ignition off.
- Disconnect the control modules which are linked by the bus wires and check for a short circuit between the bus wires
- Check bus wires for short to B+ and short to Ground (GND).
If the cause of the malfunction "Data Bus Powertrain Malfunction" cannot be found in the data bus wires, check whether one of the control modules is causing the malfunction.
At this stage all the control modules which communicate via the CAN data bus are disconnected. The 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 which has just been connected. End output from scan tool with function 06 "End Output."
- Switch ignition off and then on again.
- Leave ignition switched on for 10 seconds. Then check DTC memory of the control module that has just been connected, using the scan tool.
- If the malfunction "Data Bus Powertrain Malfunction" is now indicated, replace the control module which has just been connected.
- If malfunction "Data Bus Powertrain Malfunction" is not indicated, connect next control module, and repeat above procedure.