Checking data exchange between engine/ABS/gearbox control unit — AUDI
Checking data exchange between engine/ABS/gearbox control unit on Audi: factory workshop procedure for engines APS, APZ, ARJ.
Workshop procedure for AUDI — the factory write-up, straight from real workshop data.
📐 31 workshop diagrams for APS
Real factory drawings for this job — timing marks, bolt locations and routing. Locked in the free view.
📋 Procedure
APZ ARJ
Special tools, testers and workshop equipment required
V.A.G 1526 A
V.A.G 1594 A
V.A.G 1598/31
VAS 5051 with VAS 5051/1
or
V.A.G 1551 with V.A.G 1551/3 A
Notes:
Data is exchanged between individual control units by means of a bus system.
"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.
These signal wires are used for serial transmission of data, i.e. data is sent to each of the connected control units in turn.
Testing the bus system
The fault table includes instructions to test the data exchange between engine control unit, gearbox control unit and ABS control unit.
Interrogate the fault memory of the gearbox control unit.
=> Automatic gearbox, Self-diagnosis; Repair group 01; Performing self-diagnosis
Automatic gearbox, Self-diagnosis
Performing self-diagnosis
Interrogate the fault memory of the ABS control unit.
=> Running gear FWD and 4WD; Repair Group 01; Self-diagnosis; Electrical tests
Running gear FWD and 4WD
Self-diagnosis
Electrical tests
If a fault is displayed relating to "Databus..." or "...communication":
Check whether appropriate engine, gearbox and ABS control units have been fitted in the vehicle. If they have:
Switch off ignition.
Connect the V.A.G 1598/31 test box to wiring harness leading to engine control unit; engine control unit should not be connected => Page
24-20
.
Use current flow diagram to check for open circuit/short in data lines from engine control unit pin 6 and 58 (contacts 79 and 77 on engine control unit are used for data lines to infinitely variable speed gearbox control unit; refer also to "Important Note") to gearbox control unit (if fitted) or to ABS control unit.
=> "Current Flow Diagrams, Electrical Fault Finding and Installation locations" binder
Important note:
Contacts 79 and 77 on the engine control unit are used for the data wires to the control unit for the infinitely variable speed gearbox (if fitted). For all other "bus users" measurements are to be taken at contacts 60 and 58 on engine control unit.
If the wiring is OK, try installing a new engine/gearbox/ABS control unit in turn.
TorqFix answers live for your specific car — tell it the engine code and it serves the matching sequence, torque figures and wiring for that exact variant.
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Who makes these pages, and how
TorqFix was built by a mechanic — after a 2,700 km drive with five roadside repairs that no diagnostic tool could explain.
The figures on this page come from manufacturer workshop documentation and are shown as they are. The page is assembled automatically from that data — it is not written by an AI. The AI works in the chat, where it answers for your specific car. It writes nothing here.
Variants of the same model differ. Cross-check torque against the factory manual for your engine before final tightening.
Seen a figure that does not match your manual? Write to us — we correct it and note when. support@torqfix.com
The story behind TorqFix →Workshop data on this page last updated 20 Sep 2026.