HEADLAMP BULBS AND BEAMS

Know your enemy

Headlamps are the main lights on the front of a vehicle that illuminate the way for a driver to enable them to drive at night-time or in reduced visibility, also allowing that vehicle to be seen by others. Headlamps are therefore important safety features, and as such are regulated by various laws.

The earliest light sources for headlamps were from burning oil or acetylene gas, created by adding water to calcium carbide, but when vehicles were fitted with batteries, the incandescent light bulb became the sole method of headlamp technology for over a century until LED and High-intensity Discharge (HID or Xenon) technologies started to emerge.

The incandescent bulb is essentially a thermal radiator, only around 8% of the energy input being used to produce light radiation, the rest being heat radiation.

In contrast, an HID lamp converts about 28% to light radiation, 14% to UV and 58% heat radiation. LEDS are about 75% more efficient than an incandescent bulb. Bulbs require a method of directing the light beam, either with shaped reflectors, projector lenses or more usually in classic cars, prismatic lenses that ensure the correct beam pattern.

Headlight Law

The Highway Code stipulates the legal requirements for drivers: headlamps must be used during the hours of darkness, except on roads with street lighting (usually 30mph), and should be used in built-up areas at night and dull days, and in reduced visibility. It’s also a legal requirement to prevent dazzle or discomfort to any other road-user. In addition, there are Type Approval, European Approval (E mark), Road Vehicles Lighting Regulations (1989) and other laws/standards governing the use, design, and fitment of headlamps.

For most, the MOT test will provide the benchmark for legality and functionality, the gov.uk website providing the testers’ manual for perusal given the number of requirements for different ages and types classic vehicles.

Many problems are due to a loose control cable or detached electrical wire, which may be easily found. Still, it's always a good exercise to know your enemy before engaging it.

Consult the workshop manual to see how the controls interact with the system. Many 'faults' are in fact misunderstandings of how to work the system correctly, so don't skip over the basic instructions on use in the handbook, either.

Heater matrix

In all systems, hot water from the cooling system passes into a heat exchange matrix made of copper, brass or aluminium. Air passes around the hot tubes and fins on its way to the cabin. Recirculation

Self-explanatory symbols often denote recirculation or fresh air, but on older classics, controls may be more cryptic. A suffocating cabin and no airflow suggests 'recirculation' is selected. Bypassed heater

It's not uncommon to find heaters bypassed. Reconnect, but you might quickly discover why this was done. Maybe the heat won't turn off, or the matrix or water valve is leaking. Repair.

Temperature regulation

There may be a valve in the hot water supply to the matrix. In air-blending heaters, the matrix is permanently heated. Flaps regulate hot and cold air quantities, which give perfect heat output. Perplexing designs

Some heater designs are wilfully obtuse – so have a good look at the whole unit. In the car pictured, for hot air you turn on the heater valve – but must also flip a lever beside the clutch pedal. Not a fan any more?

Fans are usually reliable. Direct your suspicion toward a dirty or blown fuse, a loose switch wire or a disconnected motor earth wire (most heater boxes are plastic). Blown fuse: investigate.

Recirculating air

Early heaters simply heated air already in the cabin. Later designs took in fresh air. Modern classics often do either via a fresh air/recirculate control. This helps with screen demisting. Optimise demist

To maximise demisting, direct as much air as possible to the windscreen. Turn on the fan and close other vents, either directly or via a dash -mounted control. Trapped wind?

A cold heater can be the result of an airlock. If a bleed valve is fitted, open it to 'burp' air out. But if the problem returns, it's quite likely the head gasket is leaking.