Module, Automatic Headlamp Leveling (AHLM): Operation

The microcontroller in the Automatic Headlamp Leveling Module (AHLM) contains the logic circuits and controls all of the features of the AHLM system. The AHLM uses On-Board Diagnostics (OBD) and communicates with other electronic modules in the vehicle as well as with a diagnostic scan tool using the Controller Area Network (CAN) data bus. The AHLM is also a Local Interface Network (LIN) master node and uses this method of communication to communicate with the two stepper motors within each right and left front lamp unit.

The AHLM microcontroller continuously monitors electronic message inputs from the Body Control Module (BCM) (also known as the Common Body Controller/CBC), the Integrated Center Stack (ICS), the Steering Column Control Module (SCCM) microcontroller and the Controller Antilock Brake (CAB) (also known as the Antilock Brake Module/ABM, the Antilock Brake System/ABS module or the Electronic Stability Control/ESC module). The AHLM also energizes or de-energizes the front and rear axle sensors as appropriate to monitor the vehicle height. When the axle sensors are energized, the AHLM monitors and evaluates the Pulse Width Modulated (PWM) return signals from those sensors.

The AHLM uses these inputs to determine steering wheel angle, vehicle speed, accelerator pedal position, vehicle yaw rate and ABS status. It then uses internal programming algorithms to compute the proper electronic message control outputs to provide over the LIN bus to the LIN slave headlamp leveling and headlamp swiveling stepper motors to adjust the headlamp beams vertically or horizontally as required. The AHLM also continuously monitors return inputs from the leveling and swiveling motors to confirm stepper motor responsiveness as well as the proper positioning of the headlamp beams.

The AHLM receives battery voltage on a fused ignition switch output (RUN) circuit, and is grounded at all times through a hardwired remote ground point. These connections allow the AHLM module to operate only when the ignition switch is in the ON position. The AHLM also monitors all of the AHLM system circuits and components, then sets active and stored Diagnostic Trouble Codes (DTC) for any monitored system faults it detects.

The hardwired circuits of the AHLM may be diagnosed using conventional diagnostic tools and procedures. Refer to the appropriate wiring information. However, conventional diagnostic methods will not prove conclusive in the diagnosis of the AHLM or the electronic controls or communication between modules and other devices that provide some features of the AHLM. The most reliable, efficient, and accurate means to diagnose the AHLM or the electronic controls and communication related to AHLM operation requires the use of a diagnostic scan tool. Refer to the appropriate diagnostic information.