Common Automotive Bulbs

Figure 12-27. Daytime running lights. (GM Service and Parts Operations)

the voltage to the headlights and reduces the daytime intensity.

Sealed-beam and composite headlamps are very specialized types of lamp bulbs. The other bulbs used in automotive lighting circuits are much smaller and less standardized. Each specific bulb has a unique trade number that is used consistently by all manufacturers.

Most small automotive bulbs are clear and are mounted behind colored lenses. Some applications, however, may call for a red (R) or an amber (NA) bulb.

Small automotive bulbs use either a brass or a glass wedge base. Bulbs with a brass base fit into a matching socket. The single or double contacts on the base of the bulb are the insulated contacts for the bulb’s filaments. Amatching contact in thesocket supplies current to the bulb filament (Figure 12-28). Asingle-contact bulb contains one filament; a double-contact bulb has two filaments. The ground end of the bulb filament is connected directly to the base of the bulb, which is grounded through contact with the socket. Inmany cases, a separate ground wire leads from the socket to a ground connection. All doublecontact bulbs are indexed so that they will fit into the socket in only one way. This is called an indexed base.

Figure 12-28. Automotive bulbs and sockets must be matched.