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Automatic headlamp levelling, xenon headlamps

Automatic headlamp levelling, xenon headlamps


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.

Diagnosis


If the input signal to pin 6 on the headlamp is missing, the control module will turn on the xenon lighting system indicator lamp in the main instrument unit by grounding pin 5 on the headlamp.

DTC B3410 will be set if there is a break or short circuit in the cable to the front load angle sensor and the beam length will be controlled by using only the rear level sensor. The xenon lighting system indicator lamp in the main instrument unit will also come on in this case.

DTC B3420 will be set if there is a break or short circuit in the cable to the rear angle load sensor. Headlamp levelling will be deactivated, the headlamps will be adjusted to their safe positions and the xenon lighting system indicator lamp in the main instrument unit will come on in this case.

DTC B3435 will be set if there is a break or short circuit in the cable to the level control motor. The headlamp levelling motor will stop in the position it was in when the fault occurred and the xenon lighting system indicator lamp in the main instrument unit will come on in this case.

Load angle sensors


Cars with automatic headlamp levelling are fitted with a front and rear load angle sensor.

The front sensor is fitted between the subframe and the suspension arm and receives information on the wishbone movement in relation to the subframe. This information is sent on to the headlamp/AHL control module as a pulse width modulated signal.

The rear sensor is fitted between the subframe and the lower transverse link and receives information on the movement of the transverse link in relation to the subframe. This information is sent on to the headlamp/AHL control module as a pulse width modulated signal.

The headlamp/AHL control module governs the angle of the headlamps using the information provided by the two load angle sensors.