Audi Workshop Service and Repair Manuals

Notes:

• ◆ Data is exchanged between individual control units by means of a bus system.
• ◆ "CAN bus" is used to describe a system that transports and distributes data.
• ◆ The wires between the control units that are used to transfer the data are known as signal wires.
• ◆ Data is transmitted via data wires in sequence, i.e. in a specific order to the connected control units (e.g. engine rpm and accelerator pedal position).

Checking the bus system

The fault table instructed you to check the data exchange between the engine control unit and CAN-capable control units.

• ‒ Connect vehicle diagnostic, testing and information system VAS 5051 and select vehicle system "01 -Engine electronics" in the selection. For this purpose, the ignition must be switched on.

→ Display on VAS 5051:

• ‒ Under -1- select the diagnostic function "08 - Read measured value block".

→ Display on VAS 5051:

•     1 -    Enter display group
  max. input value = 255

Note:

The measured value blocks 125 and 126 indicate CAN drive users.

• ‒ Enter "125" for "display group number 125" in zone -2- and confirm the entry by pressing the Q key.

→ Display on VAS 5051:

• ‒ Check displays in display zones -1- to -4-.

• CAN-capable control units are displayed with the engine control unit:
• ‒ No display: Engine control unit not CAN-capable
• ‒ Display 1: CAN-capable control unit is a data bus user
• ‒ Display 0: CAN-capable control unit is not user of the Databus

• ‒ Select the font=symbol charset=fontspecific code=115 TeX='\sigma ' descr='[sigma]' -key in order to change to display group 126.
• ‒ Check in the same way with display group number 126.

• ‒ Exit the function "08 - Read measured value block" by pressing the ◂key.

→ Display on VAS 5051:

• ‒ Under -1- select the diagnostic function "06 - End output".

→ Display on VAS 5051:

• ‒ Under -1- select the diagnostic function "00 - interrogate fault memory - whole system".
• ‒ The fault memory is then interrogated for all systems in the vehicle which are capable of self-diagnosis

If a control unit answers with its identification, the number of faults stored or "No fault detected" appears on the display.

Any system faults that are stored are displayed successively. The VAS 5051 will then transmit the next address word.

If a fault is displayed relating to "Data bus drive" or "...CAN bus":

• ‒ Check that the engine control unit and other CAN-capable control units installed are suitable for this vehicle (part no. and code).

If the correct control units are installed:

• ‒ Check that multiple connectors for control units are properly seated.

If the multi-pin connectors are firmly seated:

• ‒ Check the CAN bus system.

Checking a "two-wire bus system"

The communication between three or more control units is carried out over a "two-line bus system".

• ‒ Evaluate the faults stored in the control units.

Note:

This helps to localise wiring faults.

Example 1:

The faults stored in the fault memories indicate that the control unit 1 does not communicate with control units 2 and 3.

• ‒ Switch ignition off.
• ‒ Disconnect the control units linked to one another via the bus wires and investigate whether there is an open circuit between the bus wires.

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

• ‒ If no fault can be detected in the bus wires exchange control unit 1.

Example 2:

The faults stored in the fault memories indicate that the control unit 2 does not communicate with control units 1 and 3.

• ‒ Switch ignition off.
• ‒ Disconnect the control units linked to one another via the bus wires and investigate whether there is an open circuit between the bus wires.

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

• ‒ If no fault can be detected in the bus lines exchange control unit 2.

Example 3:

The faults stored in the fault memories indicate that sending or receiving is not possible in any of the control units.

• ‒ Switch ignition off.

• ‒ → Disconnect the control units that are linked to one another by the bus wires and check the bus wires for short circuits to positive and earth.

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

If cause of fault "Data bus drive defective" cannot be found in bus lines check whether one of the control units is responsible for the fault.

Test requirements:

• ● Vehicle diagnosis, measurement and information system VAS 5051 connected and vehicle self-diagnosis selected => Page 01-3.

All the control units that communicate via the CAN data bus are still disconnected. Ignition is switched off.

• ‒ Connect one of the control units.
• ‒ Switch the ignition on.

→ Display on VAS 5051:

• ‒ Select the applicable vehicle system under -1-.
• ‒ Interrogate and erase fault memory in the control unit which has just been connected.
• ‒ Press the ◂ key.
• ‒ Under -1- select the diagnostic function "06 - End output".

• ‒ Switch the ignition off and then on again.
• ‒ Leave the ignition switched on for 10 seconds. Then interrogate the fault memory of the control unit that has just been connected, using the fault reader.
• ‒ If the fault "Data bus drive defective" is now indicated, replace the control unit which has just been connected.
• ‒ If fault "Signal wire defect" is not read out, connect the next control unit and repeat the procedure.