What you ought to know about headlights

Just how good are the headlights on your car?

Before you risk a traffic ticket or mishap, learn how to replace and align this vital component yourself. It's a simple job you can do at home to restore good driving visibility while, at the same time, saving $5 to $15 or more for professional parts and labor. Replacing headlights

Begin by referring to the owner's manual in the glovebox of your car. The manual lists the types and sizes of bulbs used in the vehicle.

Depending on where you shop for parts, this information, or any numbers listed on the burned-out headlight, will help you buy the correct replacement bulb. You'll also need a Phillips-head screwdriver and a pair of pliers.

Here's how to do the job:

* Remove the screws that hold the headlight trim ring or molding in place. On some cars, a grille section held on by screws may have to be removed to take out the headlight. Loosen the screws and lift the grille section off.

* Examine the screws around the headlight retaining ring. You'll see some large and small screws. Do not remove the large ones since they are used to adjust the headlight's beam. Loosen the smaller screws and rotate the ring to see if it will disengage; if not, remove the screws completely to remove the ring.

* Carefully pull the headlight forward and disconnect the electrical plug in back. It unplugs just like most appliances. (Some headlights are held in place by indentation-type connections so you may have to push or twist the light before it comes out.)

* Plug in the new headlight and carefully push it back into its normal position. Orient the bulb so it fits into any indentations in the open socket. Make sure it's in place right side up. Some headlight lenses are embossed with an arrow or the word ''Top'' to assist you in the correct positioning of the unit.

* Reinstall the headlight retaining ring and tighten the screws that hold it in place.

* Reinstall the headlight trim ring (and grille section, if necessary) and tighten the screws.

* Test the headlight. If it doesn't work or does not light on either high or low beam, you may have a problem with the headlight switch, dimmer switch, or circuit wiring. Consult a professional mechanic. Aligning headlights

Alignment should be checked once a year, at least, or each time a headlight is replaced.

The process involves two major steps:

* Making an ''alignment screen,'' and

* The actual physical adjustment of alignment screws.

For best results, the adjustment process (Steps 5 through 9) should be done at dusk or after dark when illumination accuracy can best be analyzed. Hence, leave yourself sufficient time to create the alignment screen, which is the most time-consuming aspect of headlight alignment.

To get the most accurate alignment job, make sure the car has a full tank of gasoline. Also, inflate the tires to the limits specified in the owner's manual.

You'll need a screwdriver, tape measure, T-square, and any materials that will help you make lines on a flat surface. This could include such items as chalk, paint, string, or masking tape.

Improvise, as necessary, when performing these steps:

1. Park your car on a flat surface exactly 25 feet away from, and perfectly perpendicular to, a wall, garage door, large piece of cardboard, or any other flat surface that you can use as your alignment screen. (The center of the car's grille must point directly at the center of the screen.)

2. Draw a straight, vertical line through the center of the screen to represent the center line of your car. (A crude, but effective, way to determine your car's center line is to tape a string from the center of the hood to the screen, view it through the rear window of the car for straightness, and then draw a vertical line at the point where the string is taped to the screen.)

3. Using a tape measure, check the distance between the ground and the center of the car's headlights. Mark these heights on the screen for both headlights, and then draw a horizontal line connecting the marks.

The screen now shows a large plus (+) sign. If your car has stacked headlights (one bulb above the other), you will need to draw two horizontal lines.

4. Measure the distance from the center of each headlight to the center of the car. Go to the screen. From where the vertical and horizontal lines intersect, mark this distance on each side of the horizontal line. Now draw vertical lines through the marks from the top to the bottom of the screen. Your screen now shows the height and position of your car's headlights.

If your car has four horizontal (side-by-side) headlights, follow the above procedure to draw four vertical lines.

5. Unscrew the headlight trim rings and look for the large adjustment screws. The top screw moves the light vertically, while the side screw moves it horizontally.

6. Turn on the high-beam lights. Work on one light at a time, covering the others with newspapers or dropcloths.

7. Adjust the lights until their hot spots -- the bright center areas -- are two inches below the intersection of the horizontal and vertical center lines on the screen.

8. Set the lights on low beam. The outer perimeter of the low-beam hot spots should be two inches to the right of the vertical headlight center lines and just touching the horizontal line with its upper edge.

If your car has only two headlights, the low-beam pattern should be correct if the high-beam alignment was done properly.

9. Reinstall the headlight trim rings.