Preventative maintenance is the key to keeping your car healthy. While many automobile service problems are the result of faulty design and improper assembly, most mechanics agree that the single most important cause of today's astronomical repair bills and the premature death of automobiles is the failure of owners to conduct regular maintenance checks.

Here's a rundown on what to look for in some of your car's most important systems:

The Cooling System

How to check it: Once a month or every 1,000 miles, whichever comes first, check the level of the coolant in the radiator. Whenever you tow a trailer or boat, check the level daily.

Coolant level should be checked on a cold engine. If there is a chance the engine is hot, cover the radiator cap with a heavy cloth. Turn it counterclockwise to the first stop and let the pressure release. The pressure is gone when the hissing stops. Push down on the cap and turn it all the way around to remove it. (Note: When adding antifreeze/coolant, the proportions should be 50 to 70 percent antifreeze and 30 to 50 percent water. Do not use undiluted antifreeze as it comes from the container.)

Checking the hoses: The top radiator hose (which generally needs more frequent replacement) is the easiest to check. It generally returns hot coolant from the engine, so it is subject to the highest temperatures. The radiator's bottom hose, which generally handles cooler water, is the supply hose. It supplies radiator-cooled coolant to the engine.

With the engine cold, check each one by squeezing it along its entire length. If a hose is soft and mushy or hard and brittle, then have it replaced.

Hoses should be pliable and free of cracks or swellings. You may need a flashlight to see some of the hard-to-get-at hoses. Look at the heater hoses, the smaller hoses that run back toward the fire wall (the wall at the rear separating the engine from the passenger compartment). Clamps holding the hoses should appear strong, secure and free of corrosion.

The Electrical System

Most of the components in your electrical system are too complicated to attempt do-it-yourself repair on except, of course, the lights. A few years ago any car owner with a screwdriver could change any light on the car, including the headlights. That is still the case for taillights and overhead interior lights. But check your owners's manual or a local service station regarding headlights. The new aerodynamic designs of today's cars are making headlight changes out of the reach of the average driver.

How to check it: Check the fuse box whenever anything electrical in the car fails to operate. Like the fuses in your home, the fuses in the car protect the system from overloads and short circuits. The fuse box is usually located under or near the dash and in newer cars each fuse is labeled. Know what type of fuses your car needs (check your owner's manual) and keep extras in your glove box -- they could be a lifesaver.

The Battery

If you have a battery with caps on the top, lift off the caps and check to see if the fluid comes up to the bottom of the filler neck.

Check the battery cables. A common mistake is to buy a new battery when a new set of cables is all that is needed. Look for frayed or cracked cables and check for white, dusty corrosion around the connections. Finally, make sure the battery is clamped tightly in place.

Turn off the headlights and other power accessories when starting the car. Check the water level once a month. If it is low, add water (preferably distilled water). (Note: Some of the newer batteries are permanently sealed and will not need water.) If the temperature is below freezing, add water only if you are planning to drive the car immediately.

To clean the battery terminals, first remove the battery cables. Always disconnect and remove the negative connection of the battery before working on the positive connection. Do not allow the negative connection to touch metal while the positive cable is connected.

Clean both the terminals and end of the cables with a stiff brush, steel wool, or sandpaper. You can use a solution of two tablespoons of baking soda dissolved in a cup of warm water. If the cables are corroded, clean them the same way. In addition to cleaning the terminals, cleaner will also neutralize any acid on the outside of the battery. Wipe off the cleaner with paper towels and immediately dispose of them safely so the acid deposits do not contaminate or damage anything else.

Replace and tighten the connectors in the reverse order of their removal. Battery posts should be shiny when cables are reconnected.

Apply a light coating of grease, petroleum jelly, or protective spray designed to retard corrosion.

The Brakes

A simple test will signal problems. (If you have power brakes, turn on the engine to do the test.) Push the brake pedal down and hold it down. The pedal should stop firmly about halfway to the floor and stay there. If the stop is mushy or the pedal keeps moving slowly to the floor, you should have your brakes checked.

The most important item in brake system maintenance is periodic checking of the brake-fluid level. Check the level monthly, or at least at every oil change. Many new cars have translucent brake fluid reservoirs in which the level can be checked without removing the cover.

Brake linings will finally wear out from slowing and stopping thousands of miles. When that happens, get a qualified service center to replace the necessary parts. Remember that the master cylinder, wheel cylinder, and all other components of the system should be checked at the same time. They may also be due for service when the linings are replaced.

The Transmission

An automatic transmission is a very complicated item and expensive to replace. Checking your fluid level is easy and can prevent an expensive repair job. The transmission fluid dipstick is usually at the rear of the engine and looks like a smaller version of the oil dipstick.

To get an accurate reading, the engine should be warmed up and running. If the fluid is below the ADD line, pour in one pint at a time. Be sure not to overfill the reservoir.

To check a manual transmission: Be sure the clutch pedal is loose enough to push down a half inch to an inch, depending on the car, before the clutch engages. It should require more pressure yet to push it to the floor. Have your clutch adjusted if the amount of play exceeds one inch.

Fluid levels in both the transmission and differential should be checked with each oil change, or when you notice erratic or rough gear shifting.

The Oil System

To add oil, remove the cap at the top of the engine and use whatever type of oil your owner's manual recommends. Be careful not to put in too much. Overfilling can cause the oil to foam, depriving important parts of the engine of proper lubrication.

On most cars, the manual-transmission lubricant does not require changing but should be replenished.

Suspension Systems

Shocks wear out gradually. If the car sways or turns or tends to dip when you step on the brakes, the shocks have worn to the dangerous point and you should replace them. Worn shocks can also cause the tires to wear quickly and unevenly.

If your shocks are more than 15,000 miles old, bounce the car up and down hard at each wheel. Once it's going good, let go and see how many times it bounces. Good healthy shocks will stop after one bounce.

The Air Filter

You can usually tell the filter needs changing simply by looking at it. If it appears dirty, change it. If you are not sure how clean your filter is, try the following: When your engine has warmed up, put the car in park or neutral and, with the emergency brake on, let the car idle. Open the filter lid and remove the filter. If the engine begins to run faster, you need to change the filter:

1.Remove the air filter housing cover by hand or with a wrench or screwdriver, depending on fittings.

2.Remove the old air filter. If it's only slightly dirty, it can be gently tapped on a hard surface to dislodge dirt. If it is extremely dirty, discard it.

3.Check the small crankcase breather filter, if fitted. If it's dirty, replace it also. Unsnap the crankcase filter hose from its retaining clip on the filter neck.

4.Install a new crankcase filter. Snap the hose connector back on the filter neck.

5.Wipe the inside of the filter housing clean and install the new air filter. Replace the air filter cover.

The Belts

Check belts by pressing down on the middle of each belt. A belt shouldn't give more than about half an inch. If it does, it needs to be tightened. Check for wear and cracks.

To check the alternator belt tension, grasp the vanes on the front of the alternator and try to turn them without using a lot of force. If they turn easily the belt is too loose.

The Tires

When to replace your tires: Insert the top of a penny into a tread groove. If all of Lincoln's head is visible, it's time to replace the tire. While this old rule of thumb is still valid, today's tires have a built-in wear indicator. A series of smooth horizontal wear bars will appear across the surface of your tire when the tread depth reaches the danger zone.

Reprinted by arrangement with Perigee Books from How to Make Your Car Last Almost Forever;(c)1987 Jack Gillis; North America/The Times of London Syndicates.