CAN Bus, Function

CAN bus, function
Bus:
The term "bus" is used to describe a system for conveyance and distribution of data.
CAN:

Control Area Network (CAN) is a bus system which functions using two wires. These are referred to as "bus lines". The data is transferred serially, i.e., in succeeding order, to the control modules connected via the bus lines.

These control modules communicate by means of the CAN bus, i.e., they exchange data using the CAN bus.

The two data-bus wires are twisted around one another in order to prevent electromagnetic interference and interference- radiation emissions. The spacing of the twisted elements is important.

"Two-wire bus system". testing
If requested In the DTC table to check the bus:
^ Check how many control modules are connected to the CAN bus using the corresponding circuit diagram.
^ Check the control modules multi-pin connectors for correct seating.
^ Connect a VAG 1551/1552 or VAS5051 scan tool and press the 0 button twice for the "Automatic test sequence" function.
^ Before checking the bus lines, check to ensure that there is not a malfunction in any of the control modules connected to the bus.

Note: The term 'functional malfunction " signifies a malfunction which does not directly affect the bus system but instead in some way or other disrupts the functioning of the system. This may, for instance, take the form of a malfunctioning sensor. The result is that the sensor signal can no longer be processed for bus-system data transfer. Such a functional malfunction has a direct effect on the bus system. Communication with the other control modules which need the sensor signal affected is disrupted.

Is their a functional malfunction?
^ Repair this first
^ Note the codes and clear the DTC memories of all control modules
^ Press the 0 and 6 buttons for the "End Output' function and confirm the entry using the Q button.
^ End Output from the scan tool using the "End Output' function.
^ Repair the functional malfunctions using the DTC table.

Once you have eliminated all functional malfunctions:
^ If communication between the control modules is still disrupted, you will have to check the bus lines.

When searching for malfunctions In the bus lines, differentiate between two cases:
^ Two control modules are communicating via a 'two-line bus system"
^ Three or more control modules are communicating via a "two-line bus system".

Communication occurs between two control modules via a "Two-wire bus system".
^ Switch the ignition off.





^ Disconnect both control modules
^ Check whether there is a breakage in one of the bus lines.





^ Check whether there is a breakage in one of the bus lines.





^ Check whether one of the bus lines is short-circuiting to positive or to Ground (GND)
No malfunctions in the bus lines:
^ Try changing the control module which can be changed with the shortest time input or cost
^ If control module communication via the bus remains disrupted, change the second control module.

Malfunction messages in the CAN bus system
The communication Is performed between three or more control modules via a "two wire bus system"
^ Evaluate the codes stored in the control modules

Example 1:








The codes stored in the DTC memories enable you to establish that Control Module 1 has no connection to Control Modules 2 and 3.
^ Switch the ignition off.
^ Disconnect the control modules connected via the bus lines and check whether one of the bus lines has a breakage.
^ Electrical Wiring Diagrams, Troubleshooting & Component Locations

Example 2:








The codes stored in the DTC memory enable you to establish that Control Module 2 has no connection to Control Modules 1 and 3.
^ Switch the ignition off
^ Disconnect the control modules linked via the bus lines and check whether there is a breakage in one of the bus lines.
^ Change Control Module 2 if you are unable to find any malfunctions in the bus lines.
^ Change Control Module 1 if you are unable to find any malfunctions in the bus lines.

Example 3:








The codes stored in the DTC memory enable you to establish that transmission and reception are not possible on any of the control modules.
^ Switch the ignition off.





^ Disconnect the control modules linked by the bus lines and examine these for short-circuiting with one another.
^ Check the bus lines for a short-circuit to positive and to Ground (GND).

^ If you are unable to find any cause for the "Hardware malfunction" malfunction in the bus lines, check whether one of the control modules is responsible for this malfunction.

All the control modules which communicate via the CAN bus system must still be disconnected at this point. The ignition must be off.
^ Connect one of the control modules.
^ Connect one of the scan tools. Switch the ignition on and clear the DTC memory of the control module you have just connected. End Output from the scan tool using Function 06, "End Output'
^ Switch the ignition off and then on again.
^ Leave the ignition switched on for 10 seconds. Then check the DTC memory of the control module that has just been connected, using the scan tool.
^ If "Hardware malfunction" is read out, replace the control module you have just connected.
^ If "Hardware malfunction" is not read out, connect the next control module and repeat this procedure.