Home-repair trend drives auto-parts salesmen up the wall

Americans are keeping their cars longer these days than ever before, and sometimes this can lead to big problems. When the oil companies began the switch from full-service to self-service facilities a few years ago, a whole generation was forced to learn what a dipstick is. Follow this switch with a recession that forced many motorists to economize on car repairs, and the size of the problem becomes clear.

A large number of American drivers now are taking the do-it-yourself route to car repairs - and not always successfully.

For a look at how large this number is, visit any automotive-parts store and see what happens. These facilities existed initially to service garages and trained mechanics, only occasionally seeing a weekend mechanic.

The people who work in these places have similar complaints. Few of the new generation of back-yard mechanics really knows what he's talking about. He simply doesn't know what he's working on or how to do the job.

The experience is painfully similar in five parts warehouses investigated:

''Is that a 350 or a 455 engine?'' the clerk asks the customer. ''Yes, I have to know.''

''Does it have regular shocks or automatic-leveling shocks?'' another would-be mechanic is asked. ''You don't know? Well, you'd better find out.''

''I have two alternators listed for that year: a standard one and a heavy-duty one,'' a customer is told. ''Which do you want?''

Often the counter people have to play diagnostic mechanic to the customer. ''Let me see if I've got this right,'' he says. ''You think you need a new transmission because it's slipping and missing shifts, but you can't afford to buy one. Has your car been losing fluid? Have you checked the cooler lines? It could be that or the vacuum modulator.''

Few counter people have the time to lend that much help. They're paid to sell parts, not play mechanic.

Most of the parts-house people agree: If the car owner would take the time to find out what his or her particular car is equipped with, it would clear up a lot of the confusion.

The owner's manual or the papers that came with the car should give this information. If they're missing, ask at the parts house or at a car dealership for a suitable service manual for the relevant year and model car. You can then identify your car from the vehicle identification number.

Because of the boom in home auto repair, there are a lot of repair manuals and do-it-yourself handbooks on the market. Look through several of them. Find one that fits your level of expertise. If you are an absolute neophyte, chose one with lots of descriptions, drawings, and photos that you can understand.

People who have done it before know what bleeding a brake system after repairing it means. If you don't, you want to read about it, step by step.

Some of the manuals, such as the factory ones sold by the manufacturers, are meant to be used by experienced mechanics. They may list this step: ''Remove distributor.'' You may need to know how to remove it.

The factory manuals contain a wealth of information, and for usually less than an hour's labor charge, but it may still be too much for the neophyte. Another of the drawbacks of the factory manual is that often it will tell the reader how to completely disassemble a component to repair it. In many cases this is only possible with specialized tools available to the dealer.

The more generalized repair manual will explain how to identify a faulty component, how to remove it, and how to install a new or used part.

Used parts are becoming very popular and, as a result, the salvage and junk-yard business is booming. If you choose that route, ask about guarantees. Most places dealing in used parts will replace a component that proves faulty.

Another point to consider is tools, mechanics and counter people agree. Don't buy gimmicky or cheap tools. Most repairs can be accomplished with a good set of 3/8-inch drive sockets, a ratchet, and a few extensions; a set of box and open-end wrenches; a quality set of screwdrivers; and a few types of pliers.

Obviously, there are some jobs that will require other tools. You may find you need a wheel puller or a torque wrench. Often the parts house will rent these items. Try your local rental center as well.

One last thing to consider: It's small solace, but even the experienced mechanic sometimes skins his knuckles.