Headlamp Control Module (HCM) [2010-2011]: Design

BI-XENON HEADLIGHT SYSTEM 

Courtesy of VOLVO CARS CORPORATION

1. Left headlamp control unit (LHCU)/Right headlamp control unit (RHCU) (vehicles with active headlamps only)
2. Motor, headlight levelling
3. Lamp contact, high voltage
4. Bi-Xenon lamp
5. Internal headlight edge
6. Motor, high and low beam

Bi-Xenon, a headlight system with moveable reflector, is based on gas discharge technology. The system combines high and low beam into the same lamp.

Due to legal requirements (regarding low beam) for this type of lamp, the vehicle must be equipped with automatic headlamp levelling.

Xenon vs. Halogen 

Xenon:

• higher color temperature, which produces a whiter light
• better reflection of road signs and road markings
• lower power consumption (approx. 65% lower).

Good to know: 

• Daylight has a color temperature of about 5000 °K. The closer to natural light, the less strain on the eyes. Standard H4 bulb: 3200 °K. Volvo's gas discharge bulb: about 4200 °K.
• With the Bi-Xenon system, high and low beam generates the same light color. The human eye thus has an easier time adjusting to switches between high and low beam.

BI-XENON LAMP 

Courtesy of VOLVO CARS CORPORATION

The light source consists of a discharge tube surrounded by a glass that filters out harmful UV radiation.

• The discharge tube is filled with a blend of chemical compounds, including the inert gas Xenon.
• An electric arch is created through an electrical discharge between two tungsten electrodes.
• Because the lamp does not have a filament, it is less sensitive to bumps and vibrations.
• Bulb designation: D1S  .
• Power consumption: 35 W.

POSITION SENSOR, HEADLAMP LEVELLING 

Courtesy of VOLVO CARS CORPORATION

Vehicles without Four-C (Continuously Controlled Chassis Concept) have two position sensors for automatic headlamp adjustment. The headlamps are adjusted vertically based on load and road conditions to reduce the risk of dazzling.

One position sensor is located next to the rear axle and is connected to the left-hand rear control arm by a linkage system. The other position sensor is next to the right-hand front axle and is connected to the right-hand front control arm by a linkage system. This linkage system affects the sensor, allowing the angle of the car to be gauged.

The position sensors are directly connected to the headlamp control module (HCM). Each position sensor has three connections. Two are used for power supply and ground. One is used for signals on vehicle angle. When the ignition switch is in position II, level control is active and the sensors are used to calculate the average vehicle angle. At about 4 km/h (2.5 mph), dynamic level control is activated, which also makes quick corrections during acceleration and braking. In addition to vehicle speed in excess of about 4 km/h (2.5 mph), darkness is required.

The position sensor can be calibrated using the diagnostic tool. The calibration is saved in the headlamp control module (HCM) and must be performed again upon replacement of the control module or position sensor.

Vehicles with Four-C (Continuously Controlled Chassis Concept) use the signal from the suspension module (SUM) instead.

There are diagnostics for the position sensors.

LEFT HEADLAMP CONTROL UNIT (LHCU)/RIGHT HEADLAMP CONTROL UNIT (RHCU) (VEHICLES WITH ACTIVE HEADLAMPS ONLY) 

Courtesy of VOLVO CARS CORPORATION

A control module is mounted beneath each lamp housing on vehicles with active headlamps.

The left headlamp control unit (LHCU) and right headlamp control unit (RHCU) handle headlamp levelling and active control of the light beam. Active control means that the light beam is turned out to the left or right. The light beam is turned out at varying degrees and rates depending on vehicle speed and steering angle.

Information is transferred between the left headlamp control unit (LHCU), right headlamp control unit (RHCU) and headlamp control module (HCM) via serial communication.

The lamp housing contains a sensor that indicates whether the light beam has been turned out to the right or left. The left headlight control unit (LHCU) and right headlight control unit (RHCU) send this information to the headlight control module (HCM).

If there is no communication with the headlamp control module (HCM) or there is a fault in the left headlamp control unit (LHCU) and/or right headlamp control unit (RHCU), headlamp levelling and active control are disabled. The light beam then assumes its "limp-home" position.

The left headlight control unit (LHCU) and right headlight control unit (RHCU) can be diagnosed.

HIGH VOLTAGE UNIT 

Courtesy of VOLVO CARS CORPORATION

High voltage is required to light the Bi-Xenon lamp.

• A high voltage unit is connected to each Bi-Xenon lamp. This transforms 12-V voltage to the approximately 24, 000 V required to light the lamp. Once the lamp is on, voltage is lowered to the approximately 100 V required to keep the lamp lit.
• Each time voltage is supplied to the high voltage unit, an attempt is made to light the Bi-Xenon lamp. If this voltage does not exceed 9.5 V during a time period of 200 ms, the lamp does not light. If voltage is too low, such as after a voltage-consuming cold start, the lamp does not light just because the engine is running and the alternator begins charging. A new attempt must be made to start the lamp by turning the light switch to the "0" or parking light position and then back to the low beam position.