American carmakers are screening auto loan applicants more carefully and steering them toward shorter term debt as they try to spur lagging sales with the lowest interest rates offered since 1987.

What that means for would-be new-car buyers is a potpourri of financing options, not all them the bargains they seem to be. And people at the bottom of the credit ladder may have a harder time borrowing money this time around.Chrysler Corp. has announced interest-free financing on two-year loans for new cars and some trucks. Both Ford Motor Co. and General Motors Corp. are already offering new car buyers 2.9 percent financing for the same type of loan.

The three companies also lowered rates for three-year loans to 5.9 percent, and to 6.9 percent for four years, significantly less than what banks and credits unions charge. As an alternative to the low-cost financing, the carmakers are offering cash rebates of $300 to $4,000.

But the latest round of incentives - aimed at reducing inventory as 1989 sales fall 12 percent behind last year's pace - comes as some manufacturers report a higher rate of loan defaults and repossessions. The problem is blamed in part on incentive programs within the past three years that relaxed credit requirements.

GM officials downplay the 23 percent increase in the number of vehicles the company repossessed last year, saying the current repo rate is satisfactory and not unlike that experienced by the company in the 1970s and early '80s.

However, a spokesman says, GM now is encouraging larger down payments and "we're looking at (applicants) more closely than we had."

So is Ford, whose 1988 profit fell 17 percent from the previous year partly because of a higher number of repossessions.

Don Cook, executive vice president of Ford Motor Credit Co., Ford's financing operation, says the company has hired more people to crack down on loan delinquencies and now takes "a little bit more time with the applicant to check them out a little bit more.

"That might require some people to come up with a bigger down payment or go to a lower priced car," he says.

Banks, whose higher loan rates discourage marginal applicants, haven't shared the experience of the automakers' finance companies and aren't changing lending practices.

An American Bankers Association survey, however, found that delinquencies for direct and indirect (purchasing of loans from car dealers) auto financing were up slightly in 1988 from 1987.

Manufacturers are also trying to attract better credit risks by tying their lowest-cost financing to shorter-term loans. Ford and GM offer interest rates of 9.9 percent on five-year loans; Chrysler 10.9 percent. All three have four-year loans at 6.9 percent, and three year-loans at 5.9 percent.

Even at Chrysler's zero interest for a two-year loan, a buyer financing $12,000 would have to pay a hefty $500 a month for the car.

The person who benefits the most, industry sources say, is the one who would have paid cash for the car to avoid finance charges but now can pay in installments and earn interest by putting that money away.

Carmakers attribute the slump in sales to rising interest rates and consumer concern over the economy, but some acknowledge that the longer-term loans may be affecting the market. The average term of a new car loan was 56.2 months in 1988. The average price of a new car was $14,410.

Most car buyers, 42 percent, get five-year financing. Another 30 percent spread payments over four years. Some lenders offer up to 10 years of financing.

"That's probably causing some problems in the business in general," says Cook. "A person owes 2 1/2 more years on a car, takes that car to a dealership and finds out it's worth less money than he or she owes on it." Some just stop paying.

Huey Mayronne, executive director of the American Recovery Association, which represents repossessors, says, "There's a definite trend of increased repossessions (and) one reason is the long-term financing that we've seen today."

Ron Tonkin, head of the 20,000-member National Automobile Dealers Association, says the automakers' incentives are "phony," confuse customers and should be eliminated.

Tonkin charges that the cost of cash rebates and low interest rates is built into car prices. Both manufacturers and dealers would do better, he says, if prices were lowered and rebates dropped altogether.

Competing lenders also note that the carmakers' dazzling interest rates don't make good deals automatic. Often, they add, buyers are better off taking the rebate and shopping for financing elsewhere.

"By taking the rebate they lower the price of the car and need to borrow less money," says Philip Heck-man, of the Credit Union National Association.

For example, the Chrysler buyer who's financing $12,000 interest-free for two years and pays $500 a month could take the $2,000 rebate instead and finance only $10,000. Even at a 13 percent rate, the buyer's monthly payments would be $475.42.

The buyer's total cost with the zero percent financing would be $12,000. With the rebate and 13 percent interest, the buyer would pay about $1,400 in finance charges but the total cost of the car would still be lower: $11,410.08.

The Big Three say it's too early to tell how the incentives are working. Chrysler's incentive program runs indefinitely. GM's incentives expire June 5, Ford's on May 31.