Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

268 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.
be matched.
Automotive bulbs and sockets must
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.