Checking exchange of data between connected control units — AUDI

AQF AQH ARU AKC AUW AVN AVP
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 signal wires in sequence, i.e. in a specific order to the connected control units (e.g. engine speed and accelerator pedal position).

Testing the bus system

The fault table includes instructions to check the data exchange between engine control unit, gearbox control unit and ABS control unit and/or dash panel insert.

Check that multiple connectors for control units are properly seated.

Connect the vehicle diagnostic, testing and information system VAS 5051 or fault reader V.A.G 1551 and select engine electronics control unit 1 with "Address word" 01 => Page
01-15
.
For this purpose, the engine must be running at idling speed.

Rapid data transfer
HELP
Select function XX
Indicated on display:

Press keys 0 and 8 for the function "Read measured value block" and confirm entry with Q key.

Read measured value block
Enter display group number XXX
Indicated on display:

Press keys 1, 2 and 5 for "display group number 125" and confirm entry with Q key.

Read measured value block 125
⇒
1
2
3
4
Indicated on display:

Display zones
1 · 2 · 3 · 4
Display group 125: CAN bus messages
Display · Gearbox 1 · ABS 1 · Dash 1
Display · CAN bus / Gearbox control unit · CAN bus / ABS control unit · CAN bus / Dash insert
Range · 1 = OK / 0 = not OK · 1 = OK / 0 = not OK · 1 = OK / 0 = not OK
Specified value · Gearbox 1 · ABS 1 · Dash 1
Note
Note · If specified value is not obtained for one or more control units perform automatic test sequence

Automatic test sequence

Press key 0 twice for the address word "Automatic test sequence" and confirm entry with the Q key. The V.A.G 1551 transmits all known address words in sequence.

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

Any system faults stored will be displayed in sequence and printed out. The V.A.G 1551 will then transmit the next address word.

V.A.G - SELF-DIAGNOSIS
HELP
1 - Rapid data transfer*
2 - Flash code output*
The automatic test sequence has ended when the following appears on the display:

If a fault relating to "Data bus"... or "...no message" is displayed:

Check to make sure that the vehicle is fitted with the correct engine control unit, gearbox control unit, ABS control unit and dash panel insert (check part numbers and code).

If the correct control units are installed:

Check that multiple connectors for control units are properly seated.

When multiple connectors are tight

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-wire bus system".

Evaluate the faults stored in the control units.

Note:

This helps to locate 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.

Control unit · Faults stored in fault memory
1 · - No message from control unit 2 / - No message from control unit 3
2 · - No message from control unit 1
3 · - No message from control unit 1

Switch ignition off.
Disconnect the control units connected to each other via the bus wires and investigate whether there is a short 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, replace 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.

Control unit · Faults stored in fault memory
1 · - No message from control unit 2
2 · - No message from control unit 1 / - No message from control unit 3
3 · - No message from control unit 2

Switch ignition off.
Disconnect the control units connected to each other via the bus wires and investigate whether there is a short 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, replace control unit 2.

Example 3:

With the faults stored in the fault memories it can be seen that there is no sending or receiving operation possible in any of the control units.

Control unit · Faults stored in fault memory
1 · - Data bus drive defective
2 · - Data bus drive defective
3 · - Data bus drive defective

Switch ignition off.
Disconnect the control units that are linked by the bus wires and check for a short circuit between the bus wires

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

Check the bus wires for short to positive and short to earth.

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

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

Connect one of the control units.
Connect fault reader V.A.G 1551. Switch the ignition on and erase the fault memory of the control unit that has just been connected Terminate fault readout by selecting 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.