Federal regulators have pledged $2 billion to rescue the largest insolvent savings institution in the nation, American Savings and Loan Association of Stockton, Calif.

The transaction does not involve American Savings and Loan Association in Utah, which is an entirely separate institution from the California S&L, Utah Commissioner of Financial Institutions George R. Sutton said.M. Danny Wall, chairman of the Federal Home Loan Bank Board, said Monday the government fund insuring S&L deposits will provide $500 million in promissory notes and at least another $1.5 billion in cash assistance over the next 10 years.

The Robert M. Bass Group of Fort Worth, Texas, will infuse $550 million in private funds and assume control of the institution, which is owned by Financial Corp. of America, Irvine, Calif. The bank board will own 30 percent of the institution and will receive 75 percent of the tax benefits arising from the transaction.

The transaction, when complete, will be the most costly rescue of a single savings institution. Previously, the biggest was the $1.3 billion infusion last November into Vernon Savings and Loan Association in Texas.

Wall said he expected the deal to be signed within "a few days to a couple weeks" after the IRS approves the distribution of tax benefits in the transaction and California officials approve a state charter for the reconstituted institution.

Wall said regulators had agreed to provide cash assistance through the 10-year agreement and that there was no upper limit on the assistance. He said $2 billion is the bank board's best estimate of its ultimate cost of the rescue after it sells its stake in the S&L, which Wall said should occur within less than five years, and reaps its share of the tax benefits.

Wall said Bass and negotiators for the bank board struck a deal late Friday and the board approved the agreement shortly after midnight on Saturday. He said the board decided to delay announcing it so the news would appear in newspapers on a business day.

Wall said he would reveal details of the agreement after it was finalized.

"We gave some and they gave some . . . I don't think we have given away the store by any means," he said.

FCA, with $30.2 billion in assets at the end of June, is the nation's second-largest thrift holding company, after Los Angeles-based H.F. Ahmanson & Co., and has been S&L regulators' biggest problem for four years.

Financial Corp. first came under regulatory scrutiny in 1984 when it suffered a $6.8 billion run on deposits that sent shock waves through the industry.

Another $490 million deposit run in the first two months of this year prompted regulators in March to promise to protect all of American's depositors, even those with more than the $100,000 insurance limit.

The old management resigned in August 1984 and William Popejoy, the current FCA chairman, was installed. But the company continued to lose money on its portfolio of soured loans, which had accumulated during FCA's rapid expansion in 1983 and 1984. Most of it was secured by real estate in California and the economically troubled oil country of Texas.

In the first six months of this year, FCA reported losses of $223.7 million. Still, with enough federal assistance, American, FCA's largest holding, has been considered a valuable purchase because of its broad access through 186 branches and 23 loan centers to the lucrative California market.

The bank board, which regulates the nation's 3,000 S&Ls, had been negotiating exclusively since April 21 with the Bass Group, the investment arm of a member of a wealthy Texas family.

The exclusive arrangement has drawn criticism from members of Congress and executives of the San Francisco-based First Nationwide Bank, a subsidiary of the Ford Motor Co.

First Nationwide had unsuccessfully sought for nearly a year to acquire American before talks with regulators broke off.

The FCA rescue is the latest in a series of large transactions beginning in mid-August that cleaned up insolvent thrift associations. Wall said earlier this week that the bank board was trying to issue as many assistance notes as it could before the end of the current fiscal year on Sept. 30.