Here's a general procedure:
1. Park on a flat surface: Find a level area, ideally against a wall or garage door about 25 feet away.
2. Measure headlight height: Measure the distance from the ground to the center of each headlight. You'll need this for reference later.
3. Locate adjustment screws: Open the hood. The headlight adjustment screws are usually located on the back of the headlight housing. They're small screws, often with a vertical and horizontal adjustment. You may need a screwdriver (Phillips or flathead, depending on your truck). Sometimes you'll need to remove the access panel to reach the screws. Your owner's manual should show where they are. If you can't find them, a quick online search for "1998 GMC [your trim level] headlight adjustment" with images might help.
4. Mark the wall: With your headlights on low beam, mark the center of the headlight beam on the wall or garage door. This is your starting point.
5. Adjust the vertical aim: The vertical screw typically adjusts the up-and-down aim. Turning the screw clockwise usually raises the beam, and counterclockwise lowers it. Have your helper check the beam height on the wall as you make small adjustments. The goal is to have the beam level, or slightly angled downward (so it doesn't blind oncoming drivers).
6. Adjust the horizontal aim: The horizontal screw adjusts the left-and-right aim. Again, make small adjustments and have your helper guide you. The beam should be aimed slightly to the right (in the US), to compensate for the vehicle's lean when driving.
7. Repeat for both headlights: Make sure both headlights are aimed consistently.
8. Check from a distance: Once you're satisfied with the aim at 25 feet, check it from further away (e.g., 50 feet) to ensure the beams converge properly and maintain a consistent cutoff.
Important Considerations:
* Owner's Manual: Consult your 1998 GMC truck's owner's manual. It should have a diagram showing the headlight adjustment screws and recommended aiming procedures. This is the best place to start.
* Professional Adjustment: If you're unsure about adjusting your headlights yourself, it's best to take your truck to a qualified mechanic or auto repair shop. They have the tools and expertise to ensure proper headlight alignment.
* Burnt-out Bulbs: Make sure your headlight bulbs aren't burnt out before you start adjusting. A burnt-out bulb will make aiming inaccurate.
* Safety First: Always work safely, and be careful not to damage any components around the headlights when accessing the adjustment screws.
Remember to make small adjustments at a time to avoid over-correcting. Take your time and be patient to achieve the best results.
