The Federal Trade Commission voted 2 to 1 yesterday to modify a rule requiring retailers to make product warranties available to consumers before purchase.

As a result of the changes, retailers may choose either to post a copy of the warranty near the product or to provide the information when a customer requests it.

Under the existing rule, retailers may choose any one of four methods of providing the information -- posting the text of the warranty on or near the product, on the product package, on a sign or in a loose-leaf binder.

Yesterday, the commission substituted that rule with the requirement that the information be posted near the product or be readily available. The amendments adopted by the commission also require that the retailer place signs "reasonably calculated to elicit the prospective buyer's attention" in prominent locations in the store or department advising consumers of the availability of warranties upon request.

The FTC's acting chairman, Terry Calvani, who voted against the changes, said he did so because of the requirement for signs telling shoppers that the warranty information is available.

"I cannot . . . support the pointless requirement that retailers post signs stating the obvious," he said. "The rule-making evidence demonstrates that the overwhelming majority of American consumers are fully aware of the availability of product warranties."

"It's not expensive and not intrusive," said FTC Commissioner Patricia P. Bailey. Bailey, who made the motion to revise the rule, noted that similar signs are required under the current rule. Some consumers are unaware of that the warranties are available, she said. "The evidence that the staff had accumulated would not support deleting the signs requirement," she said.

The FTC action grew out of a request by the National Mass Retailing Institute in 1980 for changes in the rules. The rules are an outgrowth of the Magnuson-Moss Warranty Act, which requires the FTC to ensure that shoppers have access to product warranties before making a purchase.