Why might an LED lamp be diagnosed as faulty even though the light source is working?

The light source is just one component of the complete system. Faults in control units, bus communication or the sensor system may result in individual light functions being deactivated or restricted. For this reason, the diagnosis should always take into account not only the electrical tests but also communication with the relevant control units and also the stored diagnostic parameters. A fully functional LED module does not rule out the possibility of a system fault.

How important is temperature management for the service life of an LED headlamp?

The light output and service life of LEDs depend largely on their operating temperature. Heat sinks, fans or heat-conductive components ensure that the permissible barrier layer or junction temperatures are not exceeded. Dirt, damaged fans or restricted heat dissipation can lead to reduced performance, premature wear or to the failure of individual LED modules. That is why checking the cooling system forms part of a systematic diagnosis.

Why might it be necessary to calibrate the LED lighting system following work on the chassis?

Work carried out on the chassis, axle geometry or vehicle height alters the vehicle’s spatial position. As a result, the reference values for the automatic headlamp levelling system and other light functions may no longer correspond to the actual vehicle data. Calibration ensures that sensors and actuators are once again operating at the correct setpoints and that the light distribution complies with the manufacturer’s specifications.

What role does bus communication play in modern LED headlamps?

LED headlamps form part of a networked vehicle system. The control units continuously exchange information with the vehicle electrical system and also with other systems, such as the camera, steering and driving dynamics control units. Faults on the CAN or LIN bus can therefore affect light functions, even though there is no fault with the lamp itself. A bus diagnostic check is therefore an essential part of thorough troubleshooting.

Why do control units or LED headlamps often have to be registered or initialised after replacement?

New components do not initially have the vehicle-specific parameters for the vehicle in question. It is only through registration, initialisation or parameter setting that they are correctly integrated into the vehicle architecture. Depending on the system, identification data, configuration values or calibration data are then taken over. If these steps are not followed, malfunctions, diagnostic entries or restricted light functions may occur.

How can you tell whether a fault lies with the LED driver or with the LED module itself?

A fault in the LED driver often manifests itself as a complete failure of a light function, sporadic flickering or the appropriate diagnostic entries. Faulty LED modules, on the other hand, often display individual segments that have failed or a permanently reduced light output. A definite identification of the problem is revealed by means of voltage testing, diagnostic parameters, actuator tests and also a comparison with the manufacturer’s specifications.

Why is mechanical lamp adjustment alone often not sufficient after a repair?

In modern LED lighting systems, electronic control units, sensors and driver assistance systems influence the light distribution. Even if the basic mechanical settings are correct, missing calibration data or components that have not been initialised can lead to incorrect adjustment. Following certain repairs, therefore, both mechanical adjustment and also electronic calibration and functional tests are all required.

Why do diagnostic and calibration procedures differ between the various vehicle models, despite the use of similar LED technology?

Although the basic LED technology is similar, vehicle architecture, software versions, control units and networking vary depending on the manufacturer, model and equipment variants. As a result, diagnostic functions, calibration procedures and the necessary initialisation steps also vary. The vehicle-specific repair and service information, and also the relevant diagnostic procedures are therefore always the guiding principles.

LED headlamps in Asian vehicles – useful information for the day-to-day workshop routine

Today LED headlamps are now standard equipment in many Asian vehicle models. Hyundai and Toyota vehicles, in particular, use LED lighting systems for low beam, high beam, daytime running lights and also for adaptive light functions. Compared with halogen or xenon systems, LED lamps offer high luminous efficacy whilst consuming little power and boasting a compact design.

At the same time, however, the demands placed on diagnostics, calibration and repairs are increasing.

During the day-to-day workshop routine, in addition to electrical tests, knowledge of control unit communication, temperature management and driver assistance systems is becoming increasingly necessary. Modern LED headlamps are often linked to cameras, radar or control unit systems and form part of ADAS functions. The following information is presented using a Toyota LED headlamp as an example.

LED headlamps as networked systems:
Modern LED headlamps in Asian vehicles perform a variety of light functions and are often networked with control units and driver assistance systems, a situation which significantly increases the demands placed on diagnostics and servicing.

Hybrid lamps with shutter technology:
Hybrid headlamps combine LED and halogen technology, with low beam and high beam generated by the same LED light source and switched from one to the other in a matter of milliseconds by means of a movable shutter.

Communication via the vehicle electrical system:
The lamp control units communicate with the vehicle electrical system and other control units via the CAN or LIN bus, and process signals from, for example, the light switch, steering angle sensors, camera sensors and rain sensors.

Systematic diagnosis required:
Typical causes of faults range from moisture and faulty LED driver control units right up to communication faults in the vehicle electrical system; therefore, a visual inspection, electrical measurements and an evaluation of the error memory have to be carried out systematically.

Observe calibration requirements after repairs:
Following the replacement of control units or headlamps, registration, initialisation, adjustment or calibration may be required; in such cases, the manufacturer’s specifications and also appropriate diagnostic and calibration procedures must be strictly adhered to.

The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for use by suitably qualified personnel only.

LED headlamps consist of several electronic and optical components that work in coordination with one another. Unlike conventional halogen lamps, the light is generated not by a filament but by semiconductor component parts.

The key components of an LED headlamp are LED modules for low beam and high beam, electronic control units, heat sinks, projection or reflector systems, daytime running light and direction indicator modules, headlamp levelling, and also the sealed housing.

Design and function of a hybrid headlamp

The hybrid headlamp shown below combines LED and halogen technology to provide a range of light functions.

The low beam, high beam and daytime running light functions are produced by LED components. The direction indicator is powered by a halogen bulb.

LED projection module with shutter function

The LED projection module generates both the low beam and the high beam using the same LED light source and the same projection lens. Switching between the two light functions is not achieved by means of a second light source, but by means of a movable aperture, the so-called shutter.

Low beam mode

In low beam mode, the shutter is positioned in the beam path and obscures the upper part of the light beam. This ensures that the light is precisely restricted before it is projected through the lens. This creates the characteristic cut-off line, which illuminates the road perfectly whilst at the same time preventing oncoming traffic from being dazzled.

High beam mode

When the full beam is switched on, the shutter is operated by an electromagnet and moved out of the beam path. This allows the LED’s complete light beam to be emitted, which can then be projected onto the road via the projection lens. The light pattern now extends much further into the distance and also illuminates the area above the cut-off line.

Hybrid headlamps — Hybrid headlamp 1. Lamp unit for daytime running light 2. LED - projection module for low beam and high beam 3. Lamp unit for direction indicator light

An illustrative diagram of the shutter function for low beam and high beam. A. Low beam B. High beam 1. Lens 2. LED 3. Reflector 4. Light beam 5. Shutter

Switching between low beam and high beam

The switch between low beam and high beam takes place within a few milliseconds. As the LED light source generally remains switched on permanently during the switching process and only the position of the shutter is changed, the change in function takes place virtually immediately.

Depending on the lamp design, the control of the LED modules is adapted in the high beam mode so that these operate at a higher electrical performance. This increases both the light intensity and the range.

The schematic diagram illustrates, by way of example, the network topology of a hybrid headlamp

Communication within the vehicle electrical system

The individual light functions of the LED headlamp are controlled by the control units mounted on the headlamp housing. Internal vehicle communication is achieved by networking the lamp control units with the control unit of the vehicle electrical system and also by linking up with the control units of the individual driver assistance systems via the databus system. Depending on the vehicle architecture, this takes place using CAN or LIN bus.

Among other things, the control unit processes signals from the light switch, from the vehicle’s speed, from the steering angle, and also information from the camera and rain sensors.

In the Toyota Corolla and Toyota RAV4, depending on the vehicle equipment, the lighting systems are networked with driver assistance systems such as High-Beam Assist, Lane-Keeping Assist and Adaptive Cruise Control.

Causes of failure and effects

The most common causes of faults include moisture in the lamp housing, faulty LED driver control units, and also interruptions or short circuits in the power supply. Other relevant factors include external damage or mechanical faults, damaged plug connections, thermal overloads in the LED modules and communication errors in the vehicle electrical system.

Possible symptoms include flickering LED segments, the failure of individual light functions, error messages on the instrument cluster or faulty adaptive light distribution.

Diagnosis of modern LED lighting systems

A systematic approach is required when diagnosing faults in LED headlamps. In addition to the lamp itself, the electrical peripherals and the control system linked to the vehicle’s electrical system are also to be checked for possible faults.

At the start of the diagnostic process, the customer’s complaint should be recorded in detail. Here it is important to establish which light function is affected (e.g. low beam, high beam, daytime running lights or direction indicators), whether the fault occurs continuously or intermittently, and whether one or both sides of the vehicle are affected. This information helps to narrow down the scope of further troubleshooting.

Visual inspection

At the start of the diagnostic process, the housing, plug connections and cables should be checked for damage, moisture or corrosion. Similarly, heat sinks and fans should also be checked for dirt or mechanical damage.

Electrical test

All this is followed by a check of the power supply, including earth connections, fuses and voltage drops under load. Depending on the vehicle model, the vehicle electrical system voltage is typically between 12 and 14.5 volts.

In this function, the required parameters can be selected, read out and evaluated.

System-specific circuit diagrams can be taken from the vehicle information and used for troubleshooting. Here, for example, the PIN assignment on the lamp can be read and used for further troubleshooting.

Control unit diagnostics

The system functions of the LED lamps are continuously monitored. Any occurring errors are stored in the control unit's error memory and can be exchanged by using a suitable diagnostic device. Depending on the system, additional parameters can be displayed and used for troubleshooting. And also depending on the system and diagnostic device, it is possible to read out error codes, carry out actuator tests, and to check software versions and calibration data.

Typical error codes include, for example, B2430 (LED lamp malfunction) or B2412/2413 (fault in the lamp circuit).

However, the actual error codes must always be considered on a vehicle-by-vehicle basis.

Calibration and basic setting

After replacing a control unit or an LED lamp, it may be necessary that vehicle-specific adjustments have to be carried out.

Depending on the vehicle model, the lamp design and also the control unit that has been fitted, additional work using a suitable diagnostic device is required in order to ensure that the lighting system is functioning correctly.

In the repair instructions issued by the vehicle manufacturer Toyota, these procedures are often referred to as adjustment, calibration, initialisation or registration. Such measures serve to ensure that new or replaced components are correctly integrated in the vehicle and that they function properly.

In this context the terms can be described as follows:

Registration
During registration, a new control unit or component is identified in the vehicle. This enables the vehicle to recognise the fitted component and match it to the existing system functions and vehicle data.

Initialisation
Initialisation is the first setup of a new or replaced component. This involves creating basic operating data and integrating the control unit or component into the vehicle system.

Setting
During setting, existing parameters or functions are adjusted to suit the vehicle’s specifications. This includes, for example, setting the luminous range or adjusting certain system values following a repair.

Calibration
During calibration, the system determines reference values and matches sensors or actuators to defined set values. This ensures that functions such as automatic headlamp levelling and LED lighting systems operate precisely.

Calibration may be required following the removal or replacement of a headlamp, after accident repairs, following work on the chassis or after changes have been made to the vehicle’s height.

So as to guarantee correct calibration, the vehicle must be parked on a level surface. Tyre pressure, vehicle level and vehicle electrical system voltage must be checked before any work begins. In addition, all the manufacturer’s specifications are to be strictly observed.

Work on LED headlamps requires careful handling of electronic components. A stable power supply, suitable diagnostic equipment and ESD protective measures are essential for carrying out repairs to a professional standard.

In the case of vehicles fitted with driver assistance systems, additional system tests and calibrations often have to be carried out following repairs.

Depending on the vehicle model and lamp type, control units, fans, headlamp levelling motors or brackets can be replaced separately. Faulty LED modules or damaged circuit boards, on the other hand, often mean that the entire lamp unit has to be replaced.

Modern LED headlamps in Asian vehicles combine lighting technology, electronics and driver assistance systems to form highly integrated, complete systems.

The Toyota RAV4 illustrates the increasing integration of modern LED headlamps into the vehicle architecture. The light functions are coordinated via various control units and are closely linked to the driver assistance systems.

For the workshop, this means that increasing demands are placed on the staff as regards diagnostics, calibration and repairs. Systematic troubleshooting, compliance with the manufacturer’s specifications and also the use of appropriate diagnostic and calibration equipment form the basis for safe and professional repairs.

The information, illustrations and examples of work processes presented in this article are intended solely to explain technical concepts and to promote a general understanding of modern vehicle lighting. They are not a substitute for vehicle-specific repair, testing or adjustment instructions.

Work on lighting systems, driver assistance systems and their associated sensors and control units is to be carried out exclusively in accordance with the relevant vehicle manufacturer’s specifications and only with suitable diagnostic and calibration equipment. Throughout such work, the applicable legal provisions and also the relevant manufacturer’s instructions are to be strictly observed.

Vehicle equipment, system functions and diagnostic procedures may vary depending on the manufacturer, model, year of manufacture and software version. The vehicle manufacturer’s current repair and service information is therefore always the authoritative source.