Maryland finance companies temporarily have ceased making consumer loans backed by real estate in the wake of an appeals court decision this week that held the firms were charging too much interest. Thousands of potential borrowers may be affected by the move.

The Maryland Court of Appeals ruled that the 18 percent annual interest rate on a $3,500 consumer loan made by Beneficial Finance in 1977 with a Mount Pleasant couple's house as collateral exceeded the state's 12 percent interest ceiling on second mortgage loans. Beneficial was following an opinion of the attorney general that such loans were valid. The company won in lower court, but that judgment was reversed by the higher court.

At a meeting in Baltimore yesterday of the Maryland Consumer Finance Association, a trade group, finance companies heard lawyers say they would advise them within two weeks on how the Beneficial decision affects them. Meanwhile, they have stopped making second trust loans.

The manager of a Finance America office remarked, "as soon as he heard the decision, our head honcho said stop!" Richard Bate, a senior vice president of Benefical Management Corp., who worked on the case, said it would be "imprudent" not to stop until the issue finally is decided.

Six other of the largest of Maryland's 45 finance companies all indicated yesterday they also had ceased this type of lending. None would say, however, how many customers had been turned away. They also declined to estimate how much business they expect to lose.

According to the Maryland commissioner of consumer credit, Alan Fell, 13 percent of all consumer loans made in the state last year were secured by real estate. The companies made a total of 269,462 loans valued at $440 million, of which 7,769 totaling $59 million were second trusts on the borrowers' house or other property.

Fell said he expected Maryland consumers who ordinarily would secure loans over $2,000 with second mortgages on their homes will now have a tough time obtaining loans. He expressed concern that some might be tempted to deal with "illegal lenders" (loan sharks).

The suit was filed in 1978 by Charles Slingluff of Frederick, attorney for Paul and Patricia Schmidt. Now that the case has been remanded to the lower court, that court must decide on the penalty, if warranted. The Schmidts have asked that interest on the loan be forfeited and that they receive three times the interest in damages. If the case had such an outcome, finance companies could expect a raft of lawsuits from those demanding refunds.

The case is being watched closely in other states that have laws similar to Maryland's. Fell feels that consumer loan activities in theses states "could be affected." However, an attorney for Beneficial offered the opinion that his firm's operations in other states would not be affected. Beneficial operates in all states, except Arkansas and Maine, as well as overseas.