I stole this from Popular Mechanics a while back...it will give the basics....in a long-winded way.
Proper headlight aim is critical. Check aim with a driver in the car and a full tank of gas. The low-beam cutoff (a) should be slightly below the center of the lens (b)
to keep glare out of oncoming traffic and reduce the light reflection
that occurs during rainy or snowy weather. The bend in the cutoff should
be slightly to the right of the center of the lane, for the same
purpose. This will illuminate the road surface in front of the car, as
well as the curb area to the right.
There is a standard procedure, using a flat and level driveway facing a
gray or dull white wall that is free of any source of peripheral light
(perhaps the back wall of your garage, interior lights off). The open
space on the wall should be at least the width of the vehicle plus an
extra 2 ft. per side. Check the tire pressures—they have to be right.
Confirm that the car is on level ground by using a good carpenter's
level on the side of the vehicle—on an SUV try the roof, and on most
cars, the door frame. Park the car so it's exactly 25 ft. from the wall
to the face of the headlamps—don't estimate. Some exceptions include
Toyota, which specifies 10 ft., Pontiac GTO at 15 ft., and Chrysler,
which specifies 33 ft. on some models. However, the 25-ft. procedure
should produce satisfactory results for most vehicles. Use a tape
measure in any case.
Check the suspension ride height side to side to make sure there's no excessive suspension sag.
Now comes a lot of very careful measuring. First, identify the physical
center of each low-beam headlamp—with today's multilamp housings,
actually turn on the lamps to be sure you have the right lamp. Some
headlamps have a small dot or circle at the center. Make a tape cross
over the center of each lamp, and a vertical tapeline down the center of
the windshield. Using a tape measure, make the corresponding alignment
marks on the wall.
For this part, you might be able to measure at the wall from the ground
up, but it's easier (and good for a double-check) to bring the vehicle
very close to the wall. Then, again with tape, transfer the locations to
the wall (perhaps using a bubble level to span the gap between vehicle
and wall, to ensure an accurate transfer). On the wall, make the
tapelines very wide—basically a single horizontal line for both
centerlines of the headlamps, and a vertical tapeline for the centerline
of the vehicle—and long enough so that it's easy to line up the vehicle
and for the lines to form a cross. Make the centerline vertical
crossing tapes for the headlamps themselves about 2 ft. long on the
wall. The accuracy of any adjustment is only as good as the reference
points. In fact, it's a good idea to lay strips of tape on the ground at
the midpoints of the rear tires, so that when you back up to the
specified distance from the wall, you can be sure you've maintained the
overall alignment of the vehicle to the wall. The tape on the windshield
and the centerline of the wall also should be useful visual aids.
Next, locate the headlamp adjusters. If you're a veteran Saturday
mechanic, this may sound like "are you kidding," but we're serious. The
adjusters used to be on the external rim of the sealed-beam headlamp
assemblies. But with the change to quartz halogen bulbs, they're on the
back of the headlamp assemblies. Some makers simply buried or eliminated
the horizontal adjusters. So find out what you have, and where it is.
We've seen a number of "mainstream" cars (yes, that includes General
Motors) with the vertical adjusters at the bottom of the housings. It's
not a problem if there's nothing in the way of the adjuster, but we've
seen the battery, coolant reservoir, even the antilock brake actuator
behind it. You may have to remove what's behind the housing to confirm
the location of the adjusters, or even to be able to put a tool on the
adjuster.
Once you locate the adjusters, back the vehicle up to the 25-ft. mark
and turn on the low beams—do this on a dark night. Have a passenger sit
in the driver's seat. Block the light from one headlamp, but not by
covering the headlamp assembly (it could get hot enough to melt the
plastic lens). A kitchen chair with your jacket draped over the back a
couple of feet from the bumper works well.
Look at the light pattern on the wall. Vertical aim: The top of the most
intense part of the beam should be at or below the centerline of the
headlamp horizontal tapeline. Horizontal aim: Most of the intense part
of the beam should be to the right of the vertical centerline of the
headlamp assembly.
Specifications vary, and if your state inspection system checks headlamp
aim, it may have its own specifications, which, of course, you should
use. Otherwise, observe manufacturer's specifications (in the lighting
sections of service manuals) if available. With today's brighter
headlamps, you want to be as friendly as possible to oncoming cars
without affecting your ability to see ahead.
Some examples of factory specifications on typical vehicles (measured
below the horizontal centerline of the headlamp): zero distance on
General Motors, less than 1/2 in. on Toyota (at the 10-ft. distance),
less than 1 in. to slightly more than 3-1/2 in. on Nissan vehicles, and 2
to 6 in. on Chrysler Group vehicles. If you don't have specifications
(or a bubble level in the assembly), at least 2 to 4 in. below the
centerline at 25 ft. should be acceptable, although slightly more
certainly would eliminate any complaints from oncoming vehicles. It may
be somewhat difficult to determine the middle of the most intense part
of the beam, but there should be very little of the top edge of the
overall beam above the horizontal line. The kickup is the part of the
pattern where light is projected above the normal cutoff. This is to
illuminate objects to the right, such as road signs or pedestrians,
without throwing glare onto oncoming traffic.
Have to make adjustments? First jounce each side of the front end to
stabilize the suspension, then turn the adjuster a quarter-turn and
look. Horizontal aim of the most intense part of the beam may be given
in the manufacturer's specifications. If not, a friendly setting for
oncoming cars is 2 to 4 in. to the right of the vertical center of the
headlamp assembly. General Motors' ultrafriendly setting is to have the
left edge of the intense part of each beam flush with the physical
vertical centerlines of the headlamp assemblies, and just under the
horizontal centerline. Repeat the procedure for the other headlamp,
trying to get the most intense part of the beam as close to the
adjustment of the first headlamp as possible.
______________
Joel Adams
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