Condensation in headlight

If complaints are received about the above, this does not necessarily mean there is a fault in the lighting. When the cover lens fogs up, the photometric output area should dry within a certain period after the light bulb has been switched on. However, this process can happen more or less quickly depending on the ambient temperature and relative air humidity. This process is completely in line with normal physical laws and completely harmless from a technical point of view, since the reflector is protected against the influence of fogging.

When the light bulb is switched on, the air inside the light heats up. The expanded, heated, and dry air is displaced out of the housing of the rear combination lamp through the ventilation slits. After the light bulb has been switched off, the air in the rear combination lamp slowly cools down again. This causes saturated humid air from outside to be “sucked” into the light interior. This can lead to condensation on the inside of the cover lens on account of high air humidity and greater temperature differences inside the light. This condition occurs more frequently in the cold months and in humid weather. However, if there is so much fogging that water drops form on the cover lens (see Figure 1) or water even accumulates in the lower area of the light (see Figure 2), the seal should be checked for damage and replaced if necessary.

Equally, any “blockage” of the light ventilation openings should be checked. The light can be blasted with oil-free compressed air to dry it out. If water still accumulates in the light after this process, the light has to be replaced.

The capillary effect in lamps is a less well-known subject. This phenomenon is often responsible for water penetrating into a light and accumulating in it. The capillary effect describes the property of liquids to spread to different extents in narrow tubes or cracks.

In the case of an electric cable, the capillary effect is that water molecules and molecules of the cable sheathing gravitate towards one another. The tighter a capillary is (capillary = tight cavity), the greater this gravitational effect. For the capillary effect to have any effect at all, water has to get into the cable. This is often due to a plug connection not being waterproof. Simple blade terminals, incision-type connectors (current thieves) etc. do not offer the cable sufficient protection against humidity. This means that water can penetrate the cable through poorly or non-insulated cable areas under the cable sheathing (insulation) (see Figure 3).

Through the capillary effect in the cable, humidity penetrates between the copper wires and cable sheathing into the light.

For this reason, waterproof connectors and cable connections such as Superseal connectors (see Figure 4) should always be used.

Failure of a light function is often the reason for a visit to the workshop.

In the case described here, the left stop light and one side marker light have failed. The reason becomes obvious when dismantling the lamps. The rear combination lamp is not sealed against the plastic body, so that there is nothing stopping dirt and water penetrating the light (see Figure 5 and 6). The bulb holder, the bulb sockets, and the crimp contacts are corroded so heavily that they have to be replaced completely. In addition, the cable duct into the body is not sealed, so that water penetration has left clear traces here, too.

For these reasons, a certain assortment of installation material is required for professional repairs. You can see a selection in Figure 7.

The same problems can be seen after dismantling the side marker light. Here, sealant has been used generously in the past to stop humidity penetrating in (see Figure 8). Unfortunately, this measure was not successful.

The crimp contacts are completely oxidised, which in the end led to the light failing.

Generally speaking, recessed luminaires with fitted light bulbs should never be fitted or sealed using sealant or adhesive. If one of the bulbs were faulty, the light would be extremely difficult to disassemble, if at all possible. In such cases, the only solution is “complete reconstruction”!