Brief component description

Alongside a microcontroller, the trailer module (AHM) essentially consists of overload-resistant power switches (sensory field- effect transistor power switches) for the trailer power circuits.

The sense outputs of the power circuit breakers enable monitoring of the power circuits for overload current or wire breaks. The power switches are activated via CAN messages.

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 following control units are involved in the functions of the trailer module (AHM):

• AHM: Trailer module 

  The trailer module (AHM) controls and monitors the load circuits of the trailer. The trailer module (AHM) also separates the load circuits of the trailer from the vehicle electrical system of the towing vehicle. In the event of system faults, the trailer module (AHM) creates the Check Control messages.

• Control unit for the electrical folding trailer tow hitch (E9x) 

  The control unit is connected to the vehicle electrical system via a 16-pin connector. The control unit has no direct connection to the bus system. The electrical folding trailer tow hitch release is operated using a button in the luggage compartment. The control unit is installed in the luggage compartment in the equipment holder. The control unit is neither programmable nor self-diagnosable.

• Control unit for the fully automatic trailer tow hitch (E6x, E70) 

  The control unit is connected to the vehicle electrical system via a 16-pin and a 4-pin connector. The control unit has no direct connection to the bus system. The fully automatic trailer tow hitch is operated using a button in the luggage compartment. The control unit is installed in the luggage compartment in the equipment holder or in the spare wheel well. The control unit is neither programmable nor self-diagnosable.

• CAS: Car Access System 

  The CAS control unit supplies the signal for terminal 15. This signal is connected at the control unit of the trailer tow hitch release.

• JBE: Junction Box Electronics 

  The junction box supplies the trailer module (AHM) with voltage (terminal 30 switched). Moreover, the junction box also supplies the control unit of the trailer tow hitch release with voltage.

  The trailer module (AHM) detects overvoltage and undervoltage in the vehicle electrical system. Outside the valid range of 9 to 15 Volt, functions that are not relevant to safety are switched off. For example, the trailer coupling release cannot be activated with undervoltage or overvoltage.

  Fig 1: RDC System Functions Diagram

  Courtesy of BMW OF NORTH AMERICA, INC.

The graphic shows the E90