What role does the Body Control Module (BCM) play in controlling contemporary rear lamps?

The Body Control Module (BCM) controls and monitors the light functions of contemporary rear lamps. It processes the input signals originating from various switches and control units and activates the relevant light function. Depending on the vehicle architecture, the BCM also monitors light function control for short circuits, open circuits or excessive current consumption and saves any detected faults in the fault memory.

Why is it often no longer enough to measure the voltage of contemporary rear lamps?

Simply measuring the voltage is often not enough, as contemporary rear lamps are controlled and monitored electronically. As a result, it is not always possible to assess the actual operating status with certainty. The fault memory, parameters and actuator tests must also be analysed as part of reliable diagnostics. Depending on the control method, it may also be useful to check the signal with an oscilloscope.

How can you tell whether a faulty rear lamp is caused by a fault in the control unit or in the light itself?

Systematic diagnostics help to pinpoint the cause of the fault. The fault memory, parameters and actuator tests show whether the control unit is controlling the rear lamp correctly. If the plug connector is being provided with the correct signal, the fault frequently lies in the rear lamp. If there is no control signal, check the control unit, power supply, ground connections and wiring.

Why can malfunctions occur after having replaced a rear lamp?

Depending on the vehicle, additional steps may be required after having replaced a rear lamp. Not every control unit automatically identifies a new lamp or automatically configures it. Encoding, parameterisation or initialisation may be required depending on the vehicle architecture. If these are not carried out, fault code entries may remain saved, or certain light functions may be restricted.

Can you replace individual LED segments in a rear lamp?

In most cases, repairing individual LED segments is not intended. Contemporary rear lamps are designed as compact assemblies comprising a light source, electronics and optical components. As many lamps are permanently sealed or glued in place, the entire lamp unit is usually replaced in accordance with the manufacturer's specifications once repairs are required.

What kind of impact can damaged plug connections or corrosion have on the operation of the rear lamps?

Damaged plug connections or corrosion do not always lead to the rear lamp failing completely. In many cases, sporadic malfunctions, variations in brightness or isolated failures of light functions initially occur. For this reason, plug connectors must always be checked for secure fit, corrosion, damage and any ingress of humidity during diagnostics before replacing lamps or control units.

Contemporary rear lamps – design, technologies and diagnostics

Contemporary rear lamps: Rear lamps have evolved from simple incandescent bulb systems into electronically linked lighting systems that also meet requirements relating to convenience, safety and design in addition to their signalling function.
Contemporary lighting technologies: LED, OLED and FlatLight technologies, paired with a range of optical concepts, enable homogeneous light patterns, customisable light signatures as well as targeted light distribution, requiring little mounting depth.
Electronic networking: Contemporary rear lamps are controlled by body and lighting control units, their interconnected nature enables dynamic light functions as well as additional comfort and safety features.
Typical diagnostics clues: Damage, humidity, faulty plug connections, variations in brightness or failed LED segments already provide important indications of potential electrical or electronic faults during visual and functional checks.
Systematic troubleshooting: The diagnostics system assists workshops in pinpointing faults in the control system, power supply, wiring or control unit communication by retrieving fault codes, analysing parameters and controlling components.

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.

Rear lamps are amongst vehicles' most important signalling units. They tell other road users the vehicle’s position, braking manoeuvres and changes in direction, thereby making a significant contribution to road safety.

Over the past few decades, rear lamp technology has continued to evolve. Simple incandescent lighting solutions have evolved into contemporary lighting systems featuring LED technology, electronic control and customisable lighting effects. This has not only enhanced the possibilities in lighting technology, it has also changed the diagnostics and troubleshooting requirements.

The following article traces the development of contemporary rear lamp technologies, from bulbs to LED and OLED systems as well as the latest lighting concepts. It also describes the structure, wiring and diagnostics of contemporary rear lamps in day-to-day workshop operations.

Requirements for contemporary rear lamps

The demands placed on rear lamps have continuously changed in recent decades. In addition to the actual signalling function, vehicle design, installation space and the integration of additional light functions also play an important role nowadays. As a result, contemporary rear lamps are significantly more complex in design than earlier, bulb-based systems using bulbs.

At the same time, the demands placed on rear lamp design are increasing. Distinctive light signatures are increasingly shaping the appearance of contemporary vehicles, leading to the development of new lighting concepts, such as light guides, three-dimensional light units and two-dimensional lighting systems.

At the same time, rear lamps are dominated more and more by electronics. Contemporary lighting systems are often integrated into the vehicle network and are controlled and monitored by control units. This results in additional possibilities in terms of light functions and diagnostics. At the same time, however, the demands placed on fault-finding in day-to-day workshop operations are also increasing.

Rear lamp technology developments

Over the past few decades, rear lamps have evolved from simple signalling devices into complex lighting systems. Whilst bulbs were predominantly used for the various light functions in the past, a variety of lighting technologies and optical concepts are employed today. Developments range from traditional bulbs to LED and OLED systems as well as modern, flat-panel lighting technologies.

The first LED rear lamps

LED technology was introduced in the late 1990s, marking the beginning of a fundamental change in rear lamps. Whilst bulbs had previously been the dominant technology, the first light functions were increasingly implemented using light-emitting diodes.

The first LED rear lamps often consisted of several individual LEDs that were arranged so that they were clearly visible. This resulted in the dot-like appearance typical of many vehicles. Compared with bulbs, LED technology offered new possibilities in terms of the design and arrangement of light functions.

The demands placed on light distribution have also increased as LED technology has become more widespread. The aim was to make individual light sources less visible and create as homogeneous a lighting effect as possible. This led to the development of various optical systems, such as light guides and two-dimensional lighting concepts.

Light guide technologies and LED modules

As LED technology has continued to evolve, new possibilities for the design of rear lamps have emerged. Instead of individual visible light dots, the focus increasingly shifted to homogeneous light patterns and continuous light outlines.

For this purpose, LEDs are frequently combined with optical fibre systems in contemporary rear lamps. The light generated by the LEDs is guided through transparent optical elements and distributed evenly to create the desired light pattern. This makes it possible to develop narrow light strips, three-dimensional light patterns or large illuminated areas.

At the same time, optical systems have also been continuously refined. Prism light conductors, Glowing Body concepts and so-called EdgeLight systems, amongst others, are used nowadays in addition to traditional reflector designs. The aim of these developments is to achieve the most homogeneous light distribution possible as well as enabling a high degree of design liberties for contemporary rear lamps.

The optics used influence both the rear lamps' appearance and light distribution. However, the LED remains the actual light source at the heart of all these systems. The optical components play a key role in determining how light is guided within the lamp and distributed to create the desired light pattern.

Optical concept	Functional principle	Typical lighting effect
Reflector concept	Light distribution with reflective surfaces	Dot-shaped or segmented light signatures
Prism light conductors	Guiding the light within a rod-shaped light guide to emit light via prisms	Linear light signatures
Glowing Body	Homogeneous light distribution across a three-dimensional light units	Three-dimensional surfaces or units that light up evenly
EdgeLight	Guiding the light within light guide with light emerging at the leading edge	Narrow light outlines and accurate lines of light

Dynamic light functions

New possibilities have opened up for the design of light functions with the increasing use of LED rear lamps. As LEDs can be controlled electronically, individual light segments can be selectively activated or their brightness adjusted.

This made it possible to implement dynamic light functions for the first time. A well-known example is dynamic direction indicators, in which the flasher function is achieved by sequentially controlling several light segments to indicate the desired direction of travel. Electronically controlling contemporary rear lamps has also made it possible to create customised lighting designs and lighting effects when unlocking or locking the vehicle.

In addition to design criteria, dynamic light functions help to make vehicles more visible on the road. At the same time, the demands placed on the electronic control system and the integration of rear lamps into the vehicle architecture increased.

OLED rear lamps

With the introduction of OLED (organic light emitting diode) technology, the development of contemporary rear lamps has been enhanced to include yet another lighting concept. Unlike LEDs, where light is distributed by optical systems, OLEDs themselves generate an even, diffuse light output.

This makes it possible to create particularly homogeneous light patterns paired with shallow mounting depths. Individual OLED segments can also be controlled separately, enabling a variety of light signatures and animations. This opens up additional design possibilities, whilst simultaneously placing greater demands on rear lamps' design and electronics.

FlatLight technology

FlatLight technology has led to the development of a new optical concept for LED-based rear lamps. In this process, FlatLight technology uses micro-optical structures to accurately distribute the light generated by LEDs across the illuminated surface within a particularly slim optical element.

The micro-optical structures used for light distribution are smaller than a grain of salt. This makes it possible to evenly distribute the light across the illuminating surface within the slim optical element.

Unlike in OLED technology, the LED remains the actual light source. FlatLight therefore employs the well-known properties of modern LEDs and combines them with a particularly slim optical design. This makes it possible to create homogeneous light patterns paired with shallow mounting depths.

FlatLight technology is now also used in series-production vehicles. One example of its use in series production is the Nissan Qashqai, featuring rear lamps with FlatLight elements. FlatLight thus represents a further stage in the development of contemporary LED rear lamps.

From light source to electronic system

Networking of contemporary rear lamps

Nowadays, contemporary LED rear lamps form an integral part of vehicle electronics. They are usually controlled by a body control unit, such as the Body Control Module (BCM). It processes the various input signals and controls the individual light functions to match the driving situation.

Input signals include elements, such as brake pedal actuation, use of the direction indicators or reverse gear. Control units also exchange information using vehicle bus systems. For example, signals from the ABS/ESP control unit can be used to activate the Emergency Stop Signal (ESS) function. Depending on the vehicle equipment, other control units may be involved in controlling individual light functions. As well as controlling the light functions, the control unit network also enables targeted diagnostics of electrical and electronic faults.

Input signals	Control unit information	Rear lamp functions
1 Direction indicator switch	5 Engine control unit	8 Stop light
2 Reverse gear switch	6 Airbag control unit	9 Reversing light
3 Light switches	7 ABS/ESC control unit	10 Tail light
4 Stop light switch		11 Direction indicators
		12 Rear fog lamp

Comfort and safety features

As contemporary vehicles have become increasingly connected, the rear lamp functions have also been enhanced. As well as traditional light functions, such as tail lights, stop lights, direction indicators and reversing lights, contemporary LED rear lamps now support a range of convenience and safety features. These are controlled depending on the current driving situation and the information provided by superordinate control units.

The Emergency Stop Signal (ESS) function is a widely used safety feature. If the system identifies sudden braking, the stop lights, for example, light up brighter or flash to draw other road users attention (depending on the vehicle manufacturer) to the braking vehicle and additionally warn following traffic. The specific implementation of this function depends on the particular vehicle and system architecture as well as the applicable legal requirements.

In addition, many vehicles are equipped with convenience features such as Welcome and Leaving Home functions. Selected light functions are automatically activated or accompanied by brief lighting animations when the vehicle is unlocked or when occupants get out. A host of vehicles are also fitted with dynamic direction indicators. In this process, several LED segments are activated in sequence, resulting in a dynamic flashing pattern.

Modern lighting technologies, such as OLED and FlatLight also make it possible to create more complex light signatures and lighting animations. This enables a combination of additional light functions with standard rear lamp light functions.

Diagnostics in the daily workshop routine

Visual and functional check

Diagnosing contemporary rear lamps starts with a visual and function check of the vehicle. At this stage, many fault patterns can already be identified without using of measuring or diagnostic units. Apart from checking the lamp housing for damage, it is also necessary to check plug connections, lines and seals for damage and any ingress of humidity.

Subsequently, all light functions must then be checked. These include elements, such as the tail light, stop light, direction indicators, reversing light and rear fog lamp. For vehicles with advanced light functions, it is also advisable to check that the dynamic direction indicators, Welcome and Leaving Home functions as well as any lighting animations are working correctly. Irregular light patterns, variations in brightness or individual LED segments that have failed may be the first signs of electrical or electronic faults.

Play

In this video, we take a look at various rear lamps and show how lighting technologies differ in terms of their design, light distribution and appearance.

Diagnostics using the diagnostic system

Contemporary rear lamps are integrated into the vehicle electrical system and, depending on the vehicle architecture, are controlled by the vehicle electrical system control unit or other lighting control units. A diagnostics system can be used to check fault memory entries, current operating statuses and individual light functions. This makes it possible to pinpoint the cause of a fault before carrying out any further electrical or electronic tests.

Reading out the fault memory

Fault codes saved in relevant control units can be read out and cleared using the diagnostics system. Fault memory entries provide initial indications as to whether there is a fault in the rear lamp control system, power supply, wiring or control unit communication. Before clearing the fault memory, entries must be documented and assessed in the context of the vehicle complaint.

Reading out and assessing parameters

Check parameters to display current operating states and input signals for the rear lamp control system in real time. Depending on the vehicle manufacturer, it is possible, for example, to read out the operating states of tail lights, stop lights or rear fog lamps, as well as other status information. By comparing the parameters with the activated light functions, it is possible to check whether the control unit is processing the relevant signals and controlling the rear lamp correctly.

Controlling components

Individual lamps or groups of lamps can be selectively controlled. This allows you to check whether the main control unit is correctly controlling the relevant outputs and whether the rear lamp is working properly. Actuator diagnostics help to pinpoint faults in the control system, wiring or rear lamp.

Conclusion

The evolution of the rear lamp illustrates the shift from a simple signal light to a connected electronic system. Contemporary lighting technologies, such as LED, OLED and FlatLight, enhance the possibilities of lighting design and enable additional comfort and safety features. As the digitalisation and connectivity of vehicles continue to advance, rear lamps will, in the future, be even more closely integrated into the vehicle's architecture and will increasingly form part of intelligent lighting and communication systems. As a result, they are steadily becoming a key component of contemporary vehicle concepts.

We have provided further information on vehicle lighting systems, lighting technologies and diagnostics procedures on our "Vehicle Lighting" page in HELLA Tech World:

https://www.hella.com/techworld/de/technik/beleuchtung/

The various diagnostics options have been illustrated as examples using the mega macs S20 diagnostics unit. The respective test depth and variety of functions can differ from vehicle manufacturer to vehicle manufacturer and depend on the respective system configuration of the control unit. Exemplary representations, pictures and descriptions serve to explain and illustrate the document text and cannot be used as a basis for vehicle-specific repairs.