The Federal Trade Commission has jumped into the national controversy over lenders' computations on home buyers' adjustable-rate mortgages (ARMs) by requiring a major mortgage banker to pay back $500,000 to its borrowers. Some individual consumers will receive as much as $6,900 in cash as part of the settlement.

The FTC's action last week was the first of what could be a series of consent agreements with mortgage bankers. Guild Mortgage Co. in California, with branch offices nationwide, services a $2 billion portfolio of home mortgages and originates more than $600 million new loans a year.

The FTC charged Guild Mortgage with providing adjustable-rate loan applicants "inaccurate, incomplete and misleading information" on the annual percentage rates they could anticipate from the company's adjustable-rate mortgages.

An FTC lawyer who investigated the firm's practices said Guild Mortgage understated the true annual percentage rate on its teaser-discount adjustable rate. By using an improper calculation method, he said, Guild Mortgage's "truth-in-lending" annual percentage rate (APR) disclosures to its borrowers routinely were too low. For example, a one-year adjustable with an annual rate of 7.9 percent might have been presented to borrowers as having a 7 percent APR.

The FTC said Guild Mortgage also violated truth-in-lending procedures by failing to:

Include the cost of private mortgage insurance in the effective rates presented to borrowers.

Provide adjustable-rate applicants with a complete and accurate estimate of their payments in the year ahead. Typically the estimate covered the "discount-rate" period of the loan, when monthly payments would be artificially low.

A spokeswoman for Guild Mortgage, Barbara Dougherty, did not dispute the FTC's charges. "We suffered from the same thing that I know a lot of {ARM lenders} have: We installed a computer system that didn't do the job correctly. I'm sorry to say we didn't catch it ourselves."

The FTC caught it while investigating a consumer complaint against Guild Mortgageon an issue unrelated to adjustable-rate mortgages or truth-in-lending procedures.

"We had {Guild Mortgage's} mortgage documents in hand, so we just thought we'd check out the accuracy of their disclosures," an FTC attorney said. "One look showed they were wrong."

Under the terms of the consent agreement, Guild Mortgage risks future civil penalties of $10,000 per violation if it provides inaccurate mortgage APRs to consumers as part of a "pattern and practice" of its business activities.

One of the five FTC commissioners, Andrew J. Strenio Jr., called the $500,000 penalty too lenient and voted against the settlement.

The Guild Mortgage case is the latest in what mortgage analysts say is a potential tidal wave of consumer and regulatory actions against adjustable-rate lenders.

Four class-action suits against federally chartered savings and loans have been filed in the past two months in Indiana, and lawyers say suits are being prepared or considered in another 20 states.

"Once consumers and lenders understand just how bad the {ARM computation} problem is you're going to see some real fur fly," said M. Scott Barrett, a lawyer in Bloomington, Ind. "There are very substantial dollars involved here on both sides of the table."

Lawyers like Barrett say that the nation's 12 million homeowners who have adjustable-rate mortgages should double-check the lender's computations.

Dougherty said that in the truth-in-lending disclosure area alone, "computer problems" can lead to completely unintentional miscalculations in the lender's favor. The very nature of adustable-rate mortgages, with constantly changing index values and other moving parts, make them highly susceptible to payment errors.

John M. Geddes, an Indiana computer consultant who performed a computation check on 7,000 S&L loans for a federal agency, found 50 percent of adjustable-rate loan payment calculations to be erroneous.

If his conclusions are anywhere near accurate, millions of American homeowners are now overpaying on their mortgages every month.

How to check your ARM for possible errors? Here are three steps:

Pull out your original mortgage documents and jot down all the key ingredients affecting your payment computations: the index rate, the margin (added on top of the index), the definitions of the change dates and any other factors.

Go to your library to consult data on indexes regularly listed in your local newspaper or national business dailies. One-year Treasury notes, for instance, are widely published and easily available.

With the index value plugged in for the mortgage documents in hand, simply do the math. Remember to subtract any escrowed tax and insurance charges included in your monthly payment to get the current principal and interest figure.