Audi Workshop Service and Repair Manuals

If fault table indicates that bus should be checked:

• ‒ Use appropriate current flow diagram to establish how many control units are linked to CAN bus.
• ‒ Check for proper connection of multi-pin connectors of control units.
• ‒ Connect up one of the fault readers V.A.G 1551/1552 or VAS 5051 and press key 0 twice for "Automatic test sequence" function.

• Before examining bus wires, make sure there are no malfunctions in any of the control units connected to the bus.

The term "malfunction" refers to a fault which does not directly affect the bus system but does somehow impair the operating sequence of a system. This may for example be a defective sensor. As a result, the sensor signal can no longer be conditioned for bus system data transfer. Such a malfunction has an indirect effect on the bus system. Problems will be encountered with communication with the other control units requiring the sensor signal concerned.

Is there a malfunction?

• Start by rectifying this.

• ‒ Note down faults and erase fault memories of all control units .
• ‒ Press keys 0 and 6 for "End output" function and confirm entry with Q key.
• ‒ Conclude fault reader output with "End output" function => Page 01-16.
• ‒ Repair malfunctions using fault table in appropriate Workshop Manuals.

After rectifying all malfunctions:

• ‒ Examine bus wires if there are still problems with control unit communication.

A distinction must be made between two situations when performing bus wire fault-finding:

• ◆ Two control units communicating by way of a "two-wire bus system"
• ◆ Three or more control units communicating by way of a "two-wire bus system"

Communication between two control units by way of a "two-wire bus system"

• ‒ Switch off ignition.
• ‒ → Unplug the two control units.
• ‒ Check for open circuit in one of the bus wires.

=> Current Flow Diagrams, Electrical Fault-finding and Fitting Locations binder

• ‒ → Check for mutual short circuit in bus wires.

=> Current Flow Diagrams, Electrical Fault-finding and Fitting Locations binder

• ‒ → Check for short to positive/earth in one of the bus wires.

If no faults are found in bus wires:

• ‒ Try replacing the control unit which is quickest and least expensive to exchange.
• ‒ Replace second control unit if there are still problems with control unit communication via bus.