Lamp/Light Bulb Information

All lamps or light bulbs have letters and numbers stamped on them that typically indicate their part number and often the operating voltage and power consumed. For instance, in a bulb marked 12V/21W, the filament will consume 21 watts of power when 12 volts is applied across the filament. While the wattage is not necessarily an indication of light output, it can be generally assumed that the higher the wattage, the greater the light output.

Dual-filament bulb.
Bayonet-style bulbs.

Lamps and light bulbs come in a variety of configurations to fit the various applications within a vehicle. One designation is how many filaments the bulb has Figure 39-7. Single-filament bulbs are common for use as courtesy lights, dash lights, and warning lights. Dual-filament bulbs have two filaments of different wattage; one filament emits a small amount of light, and the second filament emits more light. These bulbs work well as a combination taillight and brake light. Headlights can also be dual-filament bulbs. In some cases, the low beam filament is lower wattage than the high beam filament, but not always. In a headlight, the filaments are positioned to give a different profile of light. Low beams emit light closer to the vehicle and angled slightly toward the side of the road, while high beams tend to focus farther down the road and straight ahead.

Another feature that differs among lights is the type of base on the lamp—in other words, what type of socket it is retained in. Bayonet-style bulbs have been around for a long time. They get their name from the two retaining pins on the side of the base Figure 39-8. The pins follow slots in the side socket and at the bottom, and the slots turn sideways into a small pocket. The pins are retained in the pocket by the spring-loaded base in the bottom of the socket, which pushes the bulb upward. This design resists vibration very well. Removal of the bulb requires carefully pushing in on the bulb and rotating the bulb slightly counterclockwise and pulling it out. One or two electrical contacts are built into the bottom of the bulb’s base. If the bulb is a dual-filament bulb with two contacts on the base, then the pins will be unequal height so that the contacts will be registered properly with the contacts in the socket

Wedge-style bulbs.
FIGURE 39-10
Festoon-style bulbs.

Many newer bulbs use a wedge base either made from the glass bulb itself or with a built-in plastic base Figure 39-9. The bulbs are pushed straight into the socket, and tension from the socket retains the bulb. The electrical contact on the glass-based bulbs is made by wires extending from the base of the bulb and bent over opposite sides of the wedge.

Dome lights use festoon lights, which have a base on each end of a cylindrical light bulb Figure 39-10. Each end of the filament is connected to one of the bases. Generally, the bases fit in spring steel contacts, which hold the light bulb securely in place.