Federal Home Loan Bank Board Chairman Richard T. Pratt pledged today to try to get federal regulators to suspend a decision allowing financial institutions to pay unlimited interest rates on Individual Retirement Accounts and Keogh plans starting in December.

With the savings and loan industry and the House Banking Committee also opposed to the so-called "wild card" certificates, the pressure will be on the Depository Institutions Deregulation Committee at its next meeting Dec. 15.

Pratt told a press conference following an address to the U.S. League of Savings Associations that he favors instead certificates pegged to money market rates with perhaps a premium of one-half to one full percentage point to avoid both high interest rates and a drain on thrift funds by those switching to higher-yielding securities elsewhere. The league has proposed that thrifts be permitted to offer insured certificates with floating rates based on the rate for three-month Treasury securities. The rate on existing certificates, which would have a maturity of about two years, would change quarterly.

Two weeks ago the House Banking Committee called for suspension of the DIDC decision until it can hold oversight hearings on the controversial body. The first hearing is scheduled for next Wednesday. The precedent for reversing a DIDC rate decision came last month when its chairman, Treasury Secretary Donald Regan, acceeded to the demands of the troubled thrift industry and persuaded his colleagues to rescind scheduled rises in interest rate ceilings on passbook accounts.

Pratt declined to comment on Monday's action by New York State's superintendent of banks, Muriel Siebert, making public the huge third-quarter losses of savings banks in this state and focusing attention on the situation nationwide. Four out of five thirft institutions in the country are now said to be operating in the red.

Pratt said the board expects that 200 mergers--most of them voluntary--will occur this year and that "the pace will increase in 1982," but he refused to elaborate. When asked how many savings and loans are currently on the bank board's problem list, he replied, "We do not have a problem list. There have been official instructions from the bank board that such a list would not serve the managerial interests of the bank board and it is not a management tool for use by the board at this time."

Until last July, when the number of troubled thrifts reached 300, the size of the problem list was revealed monthly. It hasn't appeared since. In the words of Brent Beesley, director of the Federal Savings and Loan Insurance Corp., the government insurer that oversees mergers, the problem list had become "a Dow Jones industrial average of the thrift industry. The reliance placed on it by the public was misleading." He explained that anyone at the bank board could place on the list the names of thrifts he or she thought would fail within two years, but there was no mathematical formula for determining the criteria.

How can the FSLIC do its job without a list of troubled institutions? Beesley was asked. He replied that there are now numerous lists, based on examiners' reports and computer simulations, but that the "real working lists" are in the district Federal Home Loan Banks, not in Washington. He also declined to give any predictions about mergers. He told the conferees that "unless the present slide in short- and long-term interest rates is sustained, there will inexorably be numerous savings and loan associations that will fall into the hands of the FSLIC over the next couple of years."

Beesley said a program of capital maintenance--infusing weak thrifts with enough funds to tide them over until interest rates come down--would not assure their future viability.