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Automatic headlamp levelling, detailed description

Automatic headlamp levelling, detailed description

Cars with Xenon lighting are equipped with automatic headlamp levelling (AHL).


The headlamp adjustment system ensures that the headlamps are always correctly angled on acceleration, braking and different loads. When the front of the vehicle is raised under strong acceleration, the headlamps are adjusted down and on strong braking, when the vehicle front is lowered, the headlamps are adjusted up. Also when the car is heavily loaded the headlamps are adjusted to the correct angle. The drive brake and vehicle speed influence the reaction time of the adjustment system.



The adjustment system is fitted with two load angle sensors placed on the front and rear axles. The sensors detect the height of the front and rear of the car. The sensors are connected to an AHL control module that interprets the information from the sensors together with information on vehicle speed and brake pedal status. Each headlamp contains a stepper motor that is controlled by the AHL control module.

The system is calibrated automatically once the engine has started. The stepper motors are activated to the lowest position and then go up to the correct position based on the height/inclination of the vehicle chassis.

Upon a system fault, the headlamp beams are adjusted to a safety level slightly below the normal level. At the same time, the message "Headlamp levelling not working. Contact workshop" is displayed in SID.

The load angle sensors are supplied 5 V from AHL control module (664) pin 8 to front sensor (704F) pin 5 and from control module pin 9 to rear sensor (704R) pin 5. Pin 1 of the sensors is grounded in AHL control module pin 7 for the front sensor and pin 10 for the rear sensor. The sensors provide information to the control module in the form of a PWM signal. This signal comes from pin 6 of the sensors. The AHL control module receives the information from the front sensor on pin 5 and from the rear sensor on pin 6.


The AHL system's adjustment reaction depends on the following:


• Information from the sensors 704F/704R

• Light switch position

• Vehicle speed

• Brake pedal status


The adjustment system reaction is described in the table below.

Headlamp levelling system reaction

Vehicle status Headlamps On/Off Adjustment system reaction
Stationary car Off No reaction
On No reaction
Stationary car with load change Off Slow reaction
On Quick reaction
Vehicle moving at constant speed Off Slow reaction
On Relatively quick reaction
Vehicle speed change (acceleration/retardation) Off Slow reaction
On Reaction based on acceleration/retardation