15/06/2007
In the intricate world of automotive safety, every component plays a crucial role in preventing accidents. Among these, brake lights are arguably one of the most vital, signalling your intentions to following drivers. While most drivers are familiar with the main rear brake lights, many modern vehicles also feature a 'high-level' or 'third' brake light, typically mounted higher up, either in the rear window or on the boot lid/spoiler. However, what happens when this seemingly simple light starts behaving a bit... 'dodgy'? You might notice a subtle yet perceptible delay between when your main brake lights illuminate and when the high-level light follows suit. This slight hesitation, often just a tenth of a second, can spark curiosity and concern among car owners. But what exactly causes this 'dodginess', and more importantly, what does it mean for your vehicle's safety and its ability to pass the all-important MOT?
The high-level brake light, officially known as the Centre High Mount Stop Lamp (CHMSL) in some regions, was introduced to enhance visibility and provide an earlier warning to drivers behind you that you are slowing down or stopping. Its elevated position makes it easier for drivers in following vehicles, particularly those in larger vehicles like vans or lorries, to see your braking action over the top of other cars in traffic. This seemingly minor addition has been credited with significantly reducing rear-end collisions since its widespread adoption. So, when this critical safety feature exhibits a delay, it's not just an aesthetic quirk; it's a potential safety concern that warrants investigation.
The most common explanation for a delay in the high-level brake light's illumination boils down to the fundamental technology powering the light itself: whether it uses traditional filament bulbs or modern LEDs. Filament bulbs operate by passing an electric current through a thin wire (the filament), causing it to heat up and glow. This heating process, though rapid, isn't instantaneous. It takes a tiny fraction of a second for the filament to reach its operating temperature and emit full light. On the other hand, Light Emitting Diodes, or LEDs, work on a completely different principle. When current passes through a semiconductor material, it emits light almost immediately, with no heating phase required for illumination. This inherent difference in how they generate light is the primary reason for the observed delay.
Consider a scenario where your main brake lights are still using conventional filament bulbs, while your high-level brake light has been replaced with, or originally came equipped with, an LED unit. When you press the brake pedal, the electrical signal reaches both sets of lights simultaneously. However, the filament bulbs take that slight moment to 'warm up' and fully illuminate, while the LEDs in the high-level light switch on instantly. This creates the noticeable lag. Conversely, if your main brake lights are LEDs and your high-level is a filament bulb, you'd observe the opposite delay. Understanding this distinction is key to deciphering the 'dodgy' behaviour.
Comparing LED and Filament Brake Lights
To further illustrate the differences and why LEDs are increasingly favoured for automotive lighting, let's look at a comparative table:
| Feature | Filament Bulbs | LEDs (Light Emitting Diodes) |
|---|---|---|
| Illumination Speed | Slight delay (approx. 0.1-0.2 seconds) as filament heats up. | Instantaneous (virtually no delay). |
| Lifespan | Shorter (average 500-2,000 hours), susceptible to vibration and filament breakage. | Much longer (average 20,000-50,000+ hours), highly durable. |
| Energy Efficiency | Less efficient, a significant portion of energy is lost as heat. | Highly efficient, convert most energy directly into light. Lower current draw. |
| Size & Design Flexibility | Larger, bulky, requires specific housing for heat dissipation. | Smaller, compact, allows for sleek and intricate light designs. |
| Brightness & Colour Consistency | Can dim over time, colour can shift slightly. | Consistent brightness and colour throughout their lifespan. |
| Vibration Resistance | Vulnerable to vibration, which can cause filament to break. | Highly resistant to vibration and shock. |
Beyond the immediate illumination speed, the advantages of LEDs for automotive applications are substantial. Their longer service life means fewer replacements, saving both time and money. Their lower current draw reduces the load on your vehicle's electrical system, potentially contributing to better fuel efficiency over time, albeit marginally. Furthermore, their compact size allows manufacturers greater design freedom, leading to the sleek and distinctive lighting signatures seen on many modern cars. But perhaps the most compelling benefit, especially concerning brake lights, is the enhanced safety they provide. That extra tenth of a second of warning, multiplied across millions of vehicles on the road, can significantly impact reaction times and help prevent rear-end collisions. This is why many vehicle manufacturers are now opting for LED brake lights as standard, including for the high-level unit.
UK MOT Regulations and Your High-Level Brake Light
Understanding the technical differences between bulb types is one thing, but what about the legal implications, particularly regarding the UK's annual MOT test? The rules surrounding brake lights are quite specific and crucial for vehicle compliance.
According to the Driver and Vehicle Standards Agency (DVSA) guidelines for the MOT, vehicles first used on or after 1 January 1971 must be fitted with at least two obligatory stop lamps. These are your main rear brake lights. Now, regarding additional stop lamps, such as your high-level brake light, the regulations state: "Any additional stop lamps fitted and connected must be tested." This means if your car came with a high-level brake light from the factory, or if one was fitted afterwards and is wired up to function, it absolutely must be in working order to pass the MOT. A non-functioning high-level brake light, if connected, will result in a 'Major' defect, leading to a fail.
There has historically been some discussion, and indeed confusion, surrounding the phrase "Where extra lamps are fitted and there is doubt as to whether they are connected, the benefit of this doubt should be given to the presenter." Some might interpret this as a loophole, suggesting that if a high-level brake light isn't working, and the tester isn't sure if it's connected, it might pass. However, this interpretation is often misleading. The spirit of the regulation is to ensure that if a safety feature *is* intended to work, it *must* work. A diligent MOT tester will typically check if a fitted high-level brake light is connected. If it's physically present, it's generally assumed to be connected unless there's clear evidence otherwise (e.g., a completely removed wiring harness, which would be rare for a factory-fitted item). Attempting to deliberately disconnect a fitted high-level brake light to circumvent an MOT failure is highly ill-advised, as it compromises safety and could still lead to a fail if the tester identifies the intentional disconnection.
Therefore, the clear takeaway is: if your vehicle has a high-level brake light fitted, assume it must be working for the MOT. Prioritise its functionality not just for legal compliance, but for the enhanced safety it provides to you and other road users.
Troubleshooting a Dodgy High-Level Brake Light
If you've identified a delay, or worse, a completely non-functional high-level brake light, here are some steps you can take to troubleshoot the issue:
- Identify the Bulb Type: As discussed, the type of bulb (filament vs. LED) is crucial. If your high-level brake light consists of a single or very few (e.g., less than six) distinct light sources, it's almost certainly a filament bulb. If it's a strip of many small, closely packed lights, or a diffused panel, it's likely an LED unit. This will dictate your troubleshooting approach.
- Check for Blown Filament Bulbs: If it's a filament bulb, the most common issue is a blown filament. This is usually easily identifiable by visually inspecting the bulb; the tiny wire inside will be visibly broken or black. Replacing a filament bulb is typically a straightforward DIY task, requiring access to the light unit, usually from inside the boot or by removing part of the trim around the rear window.
- Inspect Connections and Wiring: Regardless of bulb type, loose or corroded electrical connections are a frequent culprit. Access the wiring harness behind the light unit and check for any signs of corrosion, fraying, or loose plugs. Clean any corroded terminals with a wire brush or electrical contact cleaner. Ensure all connections are secure.
- Examine the Light Unit Itself: For LED units, individual LEDs rarely fail, but the entire unit's circuit board or power supply can. Look for signs of water ingress, condensation, or physical damage to the unit. Water can short-circuit the electronics, leading to intermittent operation or complete failure. Some units, particularly those mounted in spoilers, can suffer from cracked housings or seals.
- Fuse Check: While less common for a single high-level light (as it often shares a fuse with other brake lights), it's worth checking your vehicle's fuse box for a blown fuse. Consult your car's owner's manual for the location of the fuse box and the specific fuse for your brake lights.
- Relay Issues (Less Likely for Delay): In some older or more complex systems, a relay might control the brake lights. While a faulty relay could cause a complete failure, it's less likely to cause a subtle 0.1-second delay. However, if you've exhausted other options, consult a wiring diagram for your specific vehicle.
- Professional Diagnosis: If you've gone through these steps and the problem persists, or if you're uncomfortable with electrical work, it's always best to seek assistance from a qualified mechanic. They have diagnostic tools and expertise to pinpoint more complex electrical faults, such as wiring breaks further upstream or issues with the brake pedal switch.
Frequently Asked Questions About High-Level Brake Lights
- Q: Can I replace my filament high-level brake light with an LED one?
- A: Yes, in many cases, you can. Many aftermarket LED replacement bulbs are designed to fit directly into existing filament bulb housings. However, be aware that some vehicles might require a load resistor to prevent 'bulb out' warnings on the dashboard, as LEDs draw significantly less current than filament bulbs. Always ensure the replacement is compliant with E-mark regulations for road use in the UK.
- Q: Is a high-level brake light legally required in the UK?
- A: For vehicles first used on or after 1 January 1971, only two obligatory stop lamps are legally required. However, if a high-level brake light is fitted and connected (whether factory-fitted or aftermarket), it becomes part of the vehicle's lighting system and must be fully functional to pass the MOT. While not strictly *required* to be fitted, if it's there, it must work.
- Q: My high-level brake light works intermittently. What could be the cause?
- A: Intermittent operation often points to a loose connection, a partially corroded terminal, or a failing unit that's sensitive to temperature or vibration. Check all wiring and connectors thoroughly. If it's an LED unit, internal circuit board issues due to moisture ingress are also common causes for intermittent failures.
- Q: How can I tell if my high-level brake light is an LED or a filament bulb without taking it apart?
- A: The easiest way is to observe its illumination. If it lights up instantly with no perceptible delay and often appears as a continuous strip of light or multiple small, bright points, it's likely an LED. If there's a slight ramp-up time to full brightness and you can see a distinct bulb or a small number of bulbs, it's probably a filament type. Also, consider the age of your vehicle; newer cars are more likely to have LEDs.
- Q: Can I just disconnect my high-level brake light if it's faulty to pass the MOT?
- A: While the MOT rules state that if there's "doubt as to whether they are connected," the benefit of doubt is given, deliberately disconnecting a factory-fitted safety device is not advisable. A diligent tester will likely identify a deliberately disconnected lamp. More importantly, it compromises your safety and the safety of others on the road. It's always best to repair or replace a faulty light rather than disabling it.
The 'dodgy' high-level brake light, with its subtle delay, is often a fascinating insight into the technological evolution of automotive lighting. What might seem like a minor flaw is usually a simple case of different bulb technologies reacting at different speeds. However, regardless of the cause, the functionality of this crucial light is paramount. It serves as an additional layer of safety, providing vital warning to drivers behind you. Ensuring it is always in perfect working order not only guarantees your vehicle passes its annual MOT but, more importantly, contributes significantly to preventing accidents and keeping everyone on the road safer. Don't underestimate the power of a clear and timely signal – it could make all the difference.
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