GMC Yukon 2007 Owner's Manual page 504

To adjust the vertical aim, do the following:
1. Open the hood. See Hood Release on
page 460 for more information.
2. Locate the aim dot on the lens of the
low-beam headlamp.
3. Measure the distance from the ground to the
aim dot on the low-beam headlamp. Record
the distance.
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4. At the wall measure from the ground
upward (A) to the recorded distance
from Step 3 and mark it.
5. Draw or tape a horizontal line (B) on the wall
the width of the vehicle at the height of the
mark in Step 4.
Notice: Do not cover a headlamp to improve
beam cut-off when aiming. Covering a
headlamp may cause excessive heat build-up
which may cause damage to the headlamp.
6. Turn on the low-beam headlamps and place a
piece of cardboard or equivalent in front of the
headlamp not being adjusted. This allows
only the beam of light from the headlamp
being adjusted to be seen on the flat surface.