Headlights - Condensation/Misting

91bmw12

Group 63
Lights


Bulletin Number
63 07 91 (3425)


Woodcliff Lake, NJ
November 1991
Product Engineering


SUBJECT: Condensation Inside Headlights

MODELS: All





Situation: Customers have complained of headlights misting over on the inside in extreme weather conditions.


Cause: The headlight housings are made mostly of plastics that allow moisture to diffuse through them. When these plastics become warm, they give off moisture again (not just outward, but also inward). The lens of the ellipsoid principle headlights is heated in the middle by the bulb, whereas the outer part of the lens is virtually at ambient temperature due to the cooling effect of the airflow.

This can lead to condensation forming on the cold parts of the lens. There is nothing wrong with a thin film of condensation which temporarily covers the inside of the lens. For this very reason, headlights are not completely airtight.





Outline of systems in use:

There are ventilated and aerated headlights:





The following headlights are ventilated:

High beam headlights: E30, E32, E34
Low beam headlights: E30 up to 09/87
Front fog lights: E30 from 09/87, E36





Aerated headlights include:

Low beam headlights: E30 from 09/87, E32, E34
Headlights: E31, E36 (figure)
Lighting strip: E31
Flashing turn indicators: E31, E32, E34, E36


The aeration openings are so positioned that there is a deliberate movement of air due to differential pressure inside the headlight. This means that when a lens is misted over, it clears again.


Example

The car is driven in extremely humid conditions (rain, fog) at a low ambient temperature. It is then driven in stop-and-go traffic or parked.

Result

The headlight housing is warmed from behind due to the heat given off by the engine. The air inside the headlight is unable to take any more moisture so it condenses on the colder lens.

At a relative humidity of 60%, a temperature difference of 18~ F is sufficient to cause the lens to mist over; at 80% relative humidity, a difference of just 9~ F is sufficient.

The lenses clear again depending on how the weather (humidity) changes when:

- the vehicle is parked and the engine has cooled off;
- the car is driven with no lights on (approx. 10 - 100 miles); or
- the low or high beam headlights are switched on for several minutes.

Larger droplets on the inside of the lens or water of more than a depth of 2 mm in the bottom of the headlights casing indicate that the headlight is no longer watertight. The most common cause is damage to the headlight lens caused by tire-thrown rocks.

Note: Headlights with lenses which are only slightly misted over do not leak and there is no reason to replace them.