Removing and installing trailer detector control unit — VW
Removing and installing trailer detector control unit on VW: factory workshop procedure. Full factory text, tools and checks.
Workshop procedure for VW — the factory write-up, straight from real workshop data.
📋 Procedure
ℹ️ Torque figures below come verbatim from workshop data — always cross-check against your vehicle's factory manual before final tightening.
Removing and installing trailer detector control unit
Note
The trailer detector control unit
J345 (arrow) is located behind the rear right side panel trim.
Note
Additional information:
Self-study programme
No.
328
The Caddy 2004
Current flow diagrams, Electrical fault finding and Fitting locations
The trailer detector control unit
J345 detects from a power draw of minimum 5 W
Remark the
“trailer operation” and transmits this information to various control units via the CAN data bus.
The trailer detector control unit
J345 is supplied with information (light control) from the onboard supply control unit
J519 and convenience system central control unit
J393 via the CAN data bus.
Left turn signal light or brake light as a minimum must be switched on, in order to detect
“trailer operation”.
Fault detection and fault display:
The trailer detector control unit
J345 manufactured by Hella is not self-diagnosis capable.
Currently, only control units not capable of self-diagnosis (manufacturer Hella) are installed on Caddy vehicles.
The trailer detector control unit
J345 detects
“trailer operation” from a power draw of minimum 5 W on brake light or left turn signal switch.
Special tools and workshop equipment required
Torque wrench
V.A.G 1331
TORX bit range
V.A.G 1766
Removing:
Switch off ignition and all electrical loads, and pull out ignition key.
Remove right-hand luggage compartment side trim panel (repair group 70: Load, luggage compartment trim).
Release connector (1) and disconnect it from trailer detector control unit
J345.
Pull guide clip (2) out of bracket and place cable as far to one side as the cable length allows.
Remove securing bolts (arrows).
Remove the bracket together with trailer detector control unit
J345 from the vehicle.
Remove securing bolts (arrows).
Remove
trailer detector control unit
J345 from bracket.
Installing:
Installation is carried out in the reverse order. When installing observe the following:
Tighten bolt for securing control unit to bracket to 9 Nm.
Tighten bolt securing bracket to body to 9 Nm.
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 → · How these pages are made →Workshop data on this page last updated 20 Sep 2026.