ker88839_ch12.qxd
1/9/06
11:32 AM
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Chapter Twelve
Figure 12-27. Daytime running lights. (GM Service and Parts Operations)
the voltage to the headlights and reduces the daytime intensity.
COMMON AUTOMOTIVE BULBS 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. A matching contact in the socket supplies current to the bulb filament (Figure 12-28). A single-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
Figure 12-28. Automotive bulbs and sockets must be matched.
is grounded through contact with the socket. In many 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.
