Ever since Congress started taking the deduction out of auto loans, more and more Americans have used their houses to buy cars. According to the Federal Reserve Board, 28 percent of those who take out a home-equity loan - on which the interest can be tax deductible - used some of the money to buy a car.

But experts warn that while you may pay less tax by doing that, there are risks. Your house is the guarantee on the loan, not the car. There are closing costs on the loan. Interest rates are not fixed, but variable, and they often change on a monthly basis.So, should you get a home-equity loan to buy a car instead of a car loan? The answer depends on whether you get as good a price on a car loan, whether you want to gamble on rates and how disciplined you are about money.

One major risk is that, with a large line of credit, you might buy a more expensive car than you would be able to afford with a regular car loan. What you are borrowing against is your savings - the equity in your house. "It depends on whether you will be tempted to buy a more expensive car than you normally would, and whether the savings going for the car will be replaced," said Stephen Brobeck, executive director of the Consumer Federation of America.

"It is a value judgment," said Robert Johnson, director of the Credit Research Center at Purdue University. "If you can get it cheaper on a home-equity loan and pay it off at your own speed, that may be better than going in and paying a market rate . . . The discounts on car loans may be phonies because they have to build it in someplace."

Brobeck cautions that "there is still an open question" whether you can negotiate the sales price of a car if you are taking the discount offered on an auto loan. But with home-equity cash in hand, you should be able to negotiate a better deal, he said.

The deductibility feature that attracts people reduces the after-tax cost of a 13.5 percent home-equity loan to about 9.72 percent for those in the 28 percent tax bracket, and 8.99 percent for those in the 33 percent bracket. The tax law allows you to only write off 20 percent of the interest on car payments made in 1989, 10 percent in 1990, then nothing. Car loans are not that cheap unless automobile companies are underwriting special rates.

Don't let the tax break blind you to a better deal from a dealer. Rate discounts may apply to a limited number of models, but if they are the ones you want, that is usually a better deal than using a home-equity loan.

Also, because auto loans are for a fixed term, three to five years, you know how much they cost every month and when they will be paid off. On the other hand, some banks allow you to pay only interest on your home-equity loan for several years without repaying the principal, unless you specifically set up a payment program.

There are other problems. If you draw more money from the credit line - to improve the house, go on vacation or send the kids to college, to name three popular uses - the repayment of those loans may obscure the car loan. You may go from owing $12,000 for the car to owing $28,500 for several things, and it is not difficult to lose track of the car loan as a discrete entity that you want to pay off separately. You want to avoid a situation in which, 10 years from now, you are still paying for a car you traded in six years earlier.

In addition, closing costs on many equity loans can add up to $1,000 or more, including points, origination fees, title searches and the like. You can pay cash or the bank will deduct the fees from the credit line, so you begin paying for them immediately, on top of the car loan. That way, if you're taking out a $50,000 credit line, you have $49,000 available and already owe $1,000.

Keep in mind that you are buying a car but borrowing against your home. If you default on a regular car loan, the car gets repossessed, and that almost always ends the bank action. If you default on home-equity loan, your house could be foreclosed.