After spending months writing legislation to give new powers to banks and savings institutions, Sen. Jake Garn (R-Utah), chairman of the Senate banking committee canceled a drafting session yesterday when he found he did not have the votes to approve his bill.

Garn backed off after a substitute was introduced by Sen. John Heinz (R-Pa.), omitting the most controversial provisions of Garn's bill, including one to let banks get into the securities business.

In announcing the postponement of the markup session, the chairman said he wanted time to study the 30 proposed amendments. But committee sources said Garn simply did not have the votes for his bill. The same sources said Heinz has the backing of a committee majority.

It was not clear yesterday what the two developments portend. If the banking committee cannot agree before the summer recess starting Aug. 20, the chances of getting controversial legislation passed before the November elections diminish.

The House already has passed a version giving new powers to savings institutions such as permitting them to make commercial loans, and giving regulators additional flexibility to arrange mergers of failing institutions.

Instead of a similar bill to give S&Ls short-term aid, Garn wanted to give them new powers.

To do so, Garn worked for months for a consensus among the various financial industries. His bill includes something for everyone. Banks would get new powers, including authority to buy and sell municipal bonds. Mortgage lenders would benefit from preemption of due-on-sale clauses, so existing mortgages could not be assumed by new home buyers. All usury ceilings limiting interest rates for business, agricultural and consumer credit would be eliminated. Depository institutions would get the right to establish mutual funds, and the Truth in Lending Act would be changed at the bankers' request.

However, some of these changes ultimately proved unacceptable and too controversial for committee members. A committee source said Garn could count only three solid votes Tuesday night.

A Heinz staffer, on the other hand, said he has three Republican and six Democratic votes for a simplified bill that concentrates on aid to the stuggling savings and loans.

Besides eliminating most of those controversial provisions in the Garn bill, the substitute directs the Depository Institutions Deregulation Committee to let banks and savings institutions compete with money market funds by creating low-minimum, insured accounts paying market interest rates.

The Reagan administration supports the omnibus bill. Treasury Secretary Donald T. Regan said last week the administation would rethink its position on a bill without at least limited securities powers for banks. But some observers do not expect Regan to oppose any bill giving new powers to thrift institutions.