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

264 Chapter Twelve
end of an electrical conductor that produces a
spark. Light is produced by an arc that jumps
from one electrode to another, like a welder’s
arc. The presence of an inert gas amplifies the
light given off by the arcing. More than 15,000
volts are needed to jump the gap in the electrodes.
To provide this voltage, a voltage
booster and controller are required. Once the
gap is bridged, only 80 volts is needed to keep
the current flow across the gap. The large light
output of the HID allows them to be smaller in
size. HIDs will usually show signs of failure
before they burn out.
Headlamps are mounted so that their aim can
be adjusted. Most circular and rectangular
lamps have three adjustment points, as shown in
Figure 12-19. The sealed-beam unit is placed in
an adjustable mounting, which is retained by
a stationary mounting. Many cars have a decorative
bezel that hides this hardware while
still allowing lamp adjustment, as shown in
Figure 12-20. Composite headlamps use a similar
two-point adjustment system, as in Figure
12-21, but require the use of special adapters
with the alignment devices.
Headlamp Location
and Mounting
State and federal laws control the installation of
headlamps. Automotive designers must place
headlamps within certain height and width
ranges. In addition, two- or four-lamp systems
must follow one of the patterns shown in
Figure 12-18.
Figure 12-19. Most sealed-beam headlamps have
vertical and horizontal adjusting screws. (GM Service
and Parts Operations)
Figure 12-18. The law requires that headlamps be
arranged in one of these patterns. The same requirements
apply to rectangular lamps.
Figure 12-20. Headlamps are held in an adjustable
mounting which is generally concealed by a decorative
bezel.