INSTANT EXPERT
Like virtually all parts of the modern motor car, lighting has become more complicated but Rob Marshall finds that owners are not banished entirely from repairing, maintaining and upgrading from home.
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If you were asked what is the biggest cause of MOT failures in the UK, what would your answer be? Suspension? brakes? tyres? The truth is that lighting (albeit incorporating reflectors and electrical equipment) accounts for almost three and three quarter million MOT fails, virtually twice as many caused by tyres. These recent findings, from the AA, will be of particular interest to DIYers, because many lighting issues are relatively easy to rectify.
Types of lighting
While the science and Type Approval legislation that governs automotive lighting are convoluted topics, the range of light sources is fairly straightforward.
Filament bulb
While the pictured bulb type tends to be used in taillights and direction indicators, alternative designs suit other applications, such as those within the interior.
A ‘standard’ filament bulb works by passing an electrical current through its internal coiled wire filament, made from tungsten, chosen because of the metal’s high melting point. Hella describes such bulbs as ‘temperature radiators’, because the tungsten filament glows as a result of heat produced from electrical energy. While this makes the bulb relatively inefficient, the required design and manufacturing accuracy ensure that the light output is directed correctly to the appropriate reflector within the lamp. This is one reason why replacement bulbs are required to be e-marked, which signifies legal compliance. A vital factor is oxygen being banished from within the glass envelope, which would encourage the filament to burn away and fail relatively quickly. Even without oxygen, the tungsten filament does not last forever. As it evaporates, small particles become deposited on the glass envelope. Should you notice any blackening on the glass, the bulb is reaching the end of its useful life and it is worth renewing it, before the filament ‘blows’ completely.
Halogen bulb
Manufacturers realised that adding minor quantities of halogen gases (such as iodine and bromine) increases filament life considerably. These additions facilitate vaporised tungsten particles to be redeposited back on the filament but this self-regeneration cycle needs the bulb’s external temperature to reach around 300°C to work effectively. This is one reason why the glass bulb must be as close as possible to the filament. It also explains why these bulbs risk causing severe skin burns, if not allowed to cool, before being handled.
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