Automatic Headlamp Levelling, Xenon Headlamps: Notes
Cars with xenon lamps are equipped with automatic headlamp levelling. The control system ensures that the headlamps are always at an optimum angle during acceleration, braking and at varying loads. When the front of the car lifts during fast acceleration, the headlamps will be angled down and during heavy braking when the front of the car dips, they will be angled up. The correct angle is maintained even when the car is heavily loaded.
The levelling system has two load angle sensors located on the front and rear axles respectively. These Hall type sensors detect whether the front of the car is rising or dipping. The two headlamp/AHL control modules, one for each headlamp, receive information concerning the movement of the car from the load angle sensors and operate the motors to adjust the angle of the headlamps.
When the xenon lamps are turned on, the headlamps will be tilted down to their end position and then up again to normal position in order to check the stepping motors.
In case of a system malfunction, the headlamps will be tilted down to a safe position somewhat below the normal setting and the control module will turn on an indicator lamp in the main instrument unit. This position corresponds to the headlamp position that the headlamp levelling system sets when the car is fully laden.
The load angle sensors are supplied with B+ when the ignition is ON. The sensors convert the current position of the front and rear axles to information on the angle of the axles. This signal is a 200 Hz pulse width modulated voltage that the load angle sensors send to pin 6 of the headlamp/AHL control modules. This voltage of this PWM signal varies between 0 and 12 volts.