Towing / Trailer System: Description and Operation

Functional Description For Trailer Module AHM_01

Functional description for trailer module AHM_01
The trailer module is an auxiliary control unit for the footwell module.
The trailer module is fitted if the vehicle is equipped with a trailer coupling. The trailer coupling is a fully automatic trailer coupling.

Component description
Alongside a microprocessor, the trailer module consists essentially of overload-resistant sense FET power circuit breakers for the trailer power circuits. The sense outputs of the power circuit breakers enable monitoring of the power circuits for excess current or wire breaks. Also implemented is an input for direct activation of the brake light by the brake light power circuit of the towing vehicle (hard-wired). The emergency function guarantees the function of the brake light on failure of the CAN bus. The trailer module controls and monitors the current circuits of the trailer. The trailer module also separates the current circuits of the trailer from the vehicle network of the towing vehicle. In the event of system errors, the trailer module creates the Check Control messages. The trailer module detects overvoltage and undervoltage in the vehicle network. Outside the valid range of 9 to 15 Volts, functions that are not relevant to safety are switched off. The control unit for the fully automatic trailer coupling is required for operation. The control unit is connected to the vehicle network by one 16-pin and one 4-pin connector. It has no direct connection to the bus system, but rather a LIN bus connection to the trailer module. The fully automatic trailer coupling is operated using a button in the luggage compartment. The control unit is not programmable.

System functions
- Trailer lighting
The trailer module activates the individual lamp electrical circuits of the trailer:
- Brake lights
- Side lights / parking light
- Turn indicators, hazard warning lights, crash alarm hazard warning lights, DWA alarm flashing
- Rear fog light
- Reversing light
- Trailer detection
The trailer module detects a trailer as connected when at least two loads (cold measurement) are detected at the lighting outputs. The trailer module then reports the status "Trailer connected". This status report is implemented in the towing vehicle in a number of control units. This disables the rear Park Distance Control, for example.
In vehicles built up to 2006, the reversing light and rear fog light of the towing vehicle were disabled during trailer towing.
In newer vehicles, the reversing light is no longer disabled. If the rear fog light on the trailer fails, there is a switch to the rear fog light of the towing vehicle.
- Fully automatic trailer coupling
The fully automatic trailer coupling is a component of the optional extra S03AC: trailer coupling. A more precise description can be found in the test module for the fully automatic trailer coupling.

Information on coding / programming
After installation of a trailer module, the control unit must be recoded. In doing so, observe the various national versions.

No liability can be accepted for printing or other errors. Subject to changes of a technical nature