Enhancing the performance of your headlights while preserving their original look can significantly improve night driving safety and overall usability. This technical article provides a comprehensive guide on upgrading your headlights, including a brief development history, different bulb designs, essential regulations, and a straightforward step-by-step DIY approach to rewiring your car using the original loom.

An often overlooked yet incredibly vital part of your vehicle is the wiring. In fact, the failure of electrical components and wiring is one of the leading causes of vehicle fires. High-current draw items, such as headlights, not only cause increased heat within the wiring but can also cause wear to other electrical components. Overloading the wiring system by installing stronger wattage aftermarket bulbs or additional spotlights is easy.

Unfortunately, the headlight wiring on the E9 is a rather convoluted and messy affair, let alone the differences in models in both European (1) vs. US schematics (2). To make matters worse, only the high beam has a relay, whereas the low beams draw current through the headlight switch and indicator stalk. Combine this with metres of 50-year-old oxidised wiring, and you’re looking at a recipe for disaster. Voltage drops, decreased lighting performance, increased heat, and excessive switch wear are all common occurrences. There’s a big chance you’ll see evidence of this on your headlight switch connector, as many will have melted connectors and/or damaged insulation (3) – both key indicators of problems looming on the horizon. 

A BRIEF HISTORY
Over the years, automotive lighting has evolved dramatically due to the constant desire to improve safety and visibility.

The Early Days
When cars began to make their appearance in the early 20th century, acetylene gas lamps (4) became the first widely used headlights. These lamps, often taken from horse-drawn carriages, burned acetylene gas by mixing water and calcium carbide. Although they provided reliable illumination while performing well in poor weather, they required manual ignition and regular maintenance. Unfortunately, reflectors were still rudimentary at best in those days, leading to uneven or dim light output.

A Move to Electric Lighting
Significant developments during WW1 led to the introduction of electric headlamps by the early 1920s. Using the car’s battery revolutionised automotive lighting, eliminating the need for manual ignition. The early designs used incandescent bulbs with tungsten filaments and glass lenses to focus the light. However, despite their advance, they were still susceptible to water ingress and lacked durability. Constant maintenance and bulb replacements remained a constant problem.

The Sealed Beam
First introduced in the United States, sealed beam headlights became mandatory in 1939, setting a new benchmark for lighting consistency and reliability. These units combined the reflector, lens, and filament into a single, factory-sealed unit, standardising performance and simplifying maintenance. Instead of replacing individual components, headlights could now be swapped out entirely, making maintenance and repairs far easier and quicker. Dominating the global market for over three decades, sealed units became the foundation for modern automotive lighting.

The Halogen Revolution
It would take until the mid-60s before the next major breakthrough entered the stage: Hella’s halogen bulbs (5). Using a tungsten filament enclosed in halogen gas, these bulbs produced a brighter and whiter light while increasing energy efficiency at the same time. Thanks to their improved visibility and affordability, they became the new global standard by the 1970s, including on our coupes.

Modern Technologies
The next development emerged in the 1990s with the introduction of high-intensity discharge (HID) lamps, offering significantly higher brightness and energy efficiency when compared to halogen bulbs. Using a xenon gas, HID bulbs have a blueish tint, and their superior light output quickly made them the favourite for high-end cars.

The final advancement in light bulbs came in the form of LEDs (6). Their compact size, long lifespan, and low power consumption revolutionised the industry and allowed designers more freedom in headlight (and taillight) design. With a continuous push for increased safety and visibility, these lights now offer tailored beam patterns, customisable light temperatures, and adaptation to driving conditions (such as automatic high beam dimming).

LIGHT BULBS EXPLAINED
In this article, we will focus on improving the light output in the E9 by using halogen bulbs that align with the original design. As briefly mentioned at the start, we will avoid discussing using upgraded light bulbs such as xenon or higher-wattage bulbs due to the added safety and overloading issues. The benefit of halogen is that it requires no additional modifications to the car, is widely available and affordable, and is a simple plug-and-play installation. Despite their shorter lifespan and lower brightness than modern options, they still provide adequate lighting for everyday driving. However, we will briefly discuss LEDs shortly and provide you with suitable options as more owners are starting to use them as suitable alternatives.

H1 (EU) vs H4 (USA)
When upgrading your E9’s headlights, it’s essential to understand the differences between H1 and H4 bulbs. The primary distinction lies in their construction and application. H1 bulbs feature a single filament, designed to serve a single lighting function. As a result, European-spec cars that use H1 bulbs require separate units for high and low beams, allowing for better control over the beam pattern and improved road illumination. This separation also enables finer tuning of beam angles to reduce glare and improve visibility.

On the other hand, H4 bulbs feature a dual-filament design, which allows them to provide both high and low beams in a single unit. This design was commonly used in US-spec cars with sealed beam conversions or reflector housings. While the H4 system simplifies wiring and reduces the number of individual bulbs needed, it can sometimes compromise beam pattern sharpness. When selecting bulbs for an upgrade, it is crucial to match the correct type to your headlight housing to maintain proper beam patterns and avoid glare for oncoming drivers.

Modern Halogen Bulb Options
Several modern halogen bulb options are excellent choices for those seeking to maintain an OEM appearance while enhancing performance. They offer improved brightness without requiring modifications to the existing wiring. Bulbs such as the Osram Night Breaker Laser and Philips X-tremeVision Pro150 deliver a whiter, more powerful light while maintaining the period-correct look of the original halogen setup.

The LED Option
We have noticed that an increasing number of E9 owners are considering LED upgrades. LEDs, as already mentioned, provide a higher light output at a lower current, resulting in a double win. However, this comes at a slight drawback in that additional cooling is required. As such, all LEDs will come with a cooling fan attached. Thankfully, our headlights provide ample space for these to fit correctly. Ensure that the LEDs you’re considering meet local regulations and are legally permitted for use in your country. One such option is the H1 or H4 LED conversion kit by Classic Car LEDs (7) (www.classiccarleds.co.uk), which also provides LED alternatives for rear lights, indicators, and interior lighting.

A Legal Note
Different regions have specific regulations governing headlight modifications, and ensuring compliance is crucial before making any upgrades. We’ve included the European and US regions as they form the biggest markets for the E9. However, as covering every other country or region becomes nearly impossible, we recommend checking your local regulations before modifying your headlights. This will prevent legal issues and ensure safe operation, as well as compliance with road safety standards.

In Europe, bulbs must be E-marked, signifying that they meet EU safety standards. This includes ensuring proper beam alignment and incorporating additional components, such as automatic self-levelling systems and headlight washers, if you decide to upgrade to HID or LED systems.  Failure to comply with these regulations can result in fines, failed vehicle inspections, and the need to revert to factory specifications. Thankfully, none of this is required for our cars, provided we continue to use the standard halogen bulbs.

The Department of Transportation (DOT) sets specific headlight standards in the United States. Compliance with DOT regulations ensures that headlight upgrades meet legal brightness levels, beam pattern requirements, and minimal glare output. Modifications that fall outside these regulations, such as improperly installed LED or HID systems, can result in citations and may cause issues during vehicle inspections.

COMMON PROBLEMS
Several common issues can arise when upgrading E9 headlights, often due to the age of the vehicle’s electrical system. One of the most frequent issues is voltage drop due to aged wiring and long cable runs. Over time, oxidation and a build-up of internal resistance significantly reduce headlight brightness. This can make nighttime driving difficult and unsafe, particularly on poorly lit roads.

Overloaded circuits form a major problem and likely fire hazard. The stock wiring on the E9 was not designed to accommodate higher-wattage bulbs or additional lighting accessories. For instance, the stock wiring for the headlights is only 1.5mm. A modern multi-strand wire of equal size  can handle a maximum of about 8 amps at the current length from fuse to furthest headlight. That means a bulb of more than 96 watts (or two 48 watt bulbs if in series) would overload the circuit (8). Mind you, our wires are 50 years of age, and will have decreased significantly in performance. Our standard Halogen headlights are 55 watts each, which leaves sufficient safety margin, even with the older wires. Pushing the system beyond its intended capacity can lead to melted connectors, burnt-out switches, and, in extreme cases, electrical fires. Ensuring the electrical system is adequately reinforced before upgrading bulbs is essential to prevent such hazards.

Poor grounding is a common issue that affects headlight performance, as well as your other lights. Over time, corrosion at the grounding points can lead to dim or flickering lights. Checking and cleaning these connections during a headlight upgrade can prevent this issue and ensure consistent performance.

Lastly, improper beam alignment is a frequent problem that arises when installing new headlights. If the beams are not correctly adjusted, they can create excessive glare for oncoming drivers or fail to illuminate the road effectively. Proper alignment ensures that the headlights provide optimal visibility without causing a safety hazard. We strongly recommend visiting a specialist to properly align your headlights. However, the original handbook has a (rather complicated) procedure listed in case no specialist is available.

BEFORE WE START
A modern wiring approach should be considered to maximise headlight performance while maintaining a clean and professional installation. One of the most effective methods is upgrading the wiring harness. Replacing old, oxidised wires with new multi-strand automotive-grade cabling (9) significantly improves electrical conductivity and minimises voltage drop. It also increases safety as it reduces the chances of short-circuiting and potential fires. Using new wires also ensures that the headlights receive maximum power, enhancing brightness and overall performance. Be aware, though, that this can be(come) a lengthy process if you decide to replace more than just the headlight wiring, and once you start, you can’t stop midway.

Besides upgrading the wiring, installing dedicated relays for low beams is strongly advised, as the high beams already come with a factory-installed relay. A relay (10) serves as an intermediary switch that enables the headlights to draw power directly from the battery, bypassing the original headlight switch. This approach alleviates stress on the factory switches, avoids overheating, and prolongs the lifespan of electrical components. It is crucial to use high-quality, automotive-rated relays for dependable performance. Even better is to opt for appropriately fused relays.

Another crucial step in the rewiring process is selecting high-quality connectors. Heat-resistant and weather-sealed connectors (11) prevent corrosion, ensuring long-term reliability, however, they don’t fit into the connectors. Instead, you’ll need to use locking spade connectors (11). Now is also a good time to clean and reinforce grounding points, as weak or corroded grounds can lead to dim or flickering lights. This simple step can significantly improve overall headlight performance by ensuring a solid electrical connection.

Our last recommendation is the addition of fuse protection as a vital safety measure. Integrating inline fuses or using fused relays (12) (as mentioned above) protects the system against short circuits and potential electrical failures. A properly fused system ensures that any issues are contained, preventing damage to other components and reducing the risk of fire.

THE DIY SOLUTION
Thankfully, following the steps below makes fixing the problem of voltage drops and increased currents going through the components quite simple, even for those with a minimum understanding of auto electrics.

Maintaining the Factory Look
The most effective solution is to add a relay to the low-beam circuit. The E9 has three relay slots next to the battery (13): one for the high beam, one for the horn, and (if fitted) an A/C fan relay. For those without the A/C, we can use this slot for our new relay, keeping the engine bay in a complete factory appearance. If your car has an A/C, we can use a small bracket to place the new relay behind the headlight units. This solution offers a clean and remains close to the factory appearance.

Using Relays
Essentially, a relay (14) is an electrical switch that uses a small amount of current to control a larger current. That way, it helps protect your car’s wiring and switches while allowing a power connection directly from the battery to the component. For a relay to function correctly, we require four elements:

Power supply
This is the main power feed to the output from the battery and is connected to pin 30.

Power output
This is the main power output that will connect to your low beams and is connected to pin 87.

Control input
This is the input from the original harness, which is controlled by your light switch and is connected to pin 85.

Control ground
This is connected to the car’s ground point and completes the control circuit. It is connected to pin 86.

Using the original harness to trigger the relay can significantly reduce the load on the wiring loom and other components involved. The new power supply will be much shorter, reducing the resistance and voltage drop along the wiring, resulting in a brighter light output.

Choosing the Correct Wiring
We cannot stress enough the importance of using high-quality multi-strand cabling rated to the correct level. The table on the left  (15) provides an overview of copper wire gauges vs. amps vs. distance and should give you a good idea of what size of wiring you’ll need. In our case, we need 1.5mm (14 AWG), as the maximum distance will be less than 2m / 6ft.

The Right Tools
In addition to suitable wiring, we need a few essential tools (16) to get the job done. Thankfully, these can be picked up at the local DIY hardware store for not too much money. You’ll need a good ratcheting crimper, spade connectors, wire strippers (or a sharp knife), side cutters, flat screwdriver, and a volt meter. The only element not available in any stores, and of which you’ll need quite a bit, is patience.

Getting to Work
Now that we have the right tools and materials for the job, it’s time we get to work. Carefully remove the harness protective covering tape close to where you’ll place the new relay and identify the wires powering your headlight low beams. Depending on your model, it could be any of the following colours:

Yellow/White (L & R) (EU, pre ‘73)
Yellow (L & R) (USA)
Yellow/Black (L) & Yellow/Blue (R) (all, ‘73 onw.)

In essence, there will be two wires, one to the left light and one to the right light. This leaves you with three options:

1          Each wire controlling a separate relay (17)
2          One wire feed will control the relay, the other is
            terminated (18)
3          The relay replaces the soldering joint in the pre
            ‘73 EU cars (yellow/white wires) (19) 

Once you’ve chosen your preferred option, cut the wire(s) to length and crimp a relay terminal connector, which will be installed into pin 85 of the relay connector block. A correctly crimped terminal has insulation crimped on the first portion and cable strand on the second (20). If you’ve chosen option 2, you can now terminate the other wire with heat shrink to prevent shorting against other wiring or the vehicle body.  You may wish to add two relays (option 1), one for each light, to have redundancy in case of a relay failure. In that case, you can install the second relay behind the headlight to maintain a factory-looking engine compartment and use the second wire again in pin 85 of the relay block.

It is also good practice to run a new wire from the relay to the headlights, as the old ones may have become brittle or damaged over time. If possible, use the correct-coloured multi-strand wire to maintain the factory appearance and the ability to trace the wire in the future if needed. The terminals (21) can be removed from the headlight connector with a small flathead screwdriver or pin-removal tool.

By installing a relay, we now require a fuse as the original fused circuit only protects the circuit’s low current (trigger side). A clean way to install a fuse into our new circuit is to fit a fused relay (22). Another option is to install an inline fuse (23) in the power supply to pin 30. The latter option will result in additional solder or crimp joints. We recommend a 20A-rated fuse, which will suffice for most needs. The relay grounding can be easily achieved by connecting the control ground (pin 86) to the car’s earth, conveniently located close to the battery (24).

The final step is to wrap the new loom with the original tape if it is still usable or replace a section with new, matching tape (25).

Before testing the headlights by reconnecting the battery, it’s worth testing the circuit using a voltmeter to ensure everything works correctly. Using the resistance function, you should be able to check the connections (26). Once you’re happy, connect the battery and check if the lights work correctly.

And that’s it in essence. This simple solution provides an excellent upgrade to any coupe headlights while improving night-time visibility, overall safety, and the components’ longevity.

CONCLUSION
Upgrading the headlights on your E9 is a simple and practical improvement that enhances safety and night-driving confidence. By selecting the correct bulb type, ensuring compliance with legal standards, and implementing a relay wiring upgrade, you can achieve better lighting performance while preserving the original aesthetics of your coupe.

Once you’re all done, there are no more worries about staying a bit longer with family and friends before driving home in the dark.