Sears, Roebuck & Co., continuing its 3-year-old diversification efforts, announced yesterday that it plans to test a new combination credit/financial services card later this year.

The card could be used at a wide range of retail, travel, entertainment and service companies, including Sears retail and catalogue stores, which also will continue to honor the original Sears credit card.

Additionally, Sears said the new card will incorporate a package of financial products and services, including a family savings account.

Credit-card experts speculated that this service would differ markedly from traditional credit-card services. Instead of charging a customer interest on an unpaid balance, they predicted, this service would pay consumers interest for funds kept under the card's savings plan.

"The formation of the Sears financial network in 1982, followed by the establishment of 302 Sears financial network centers in major markets throughout the nation, has clearly positioned Sears as one of America's most innovative financial institutions," said Sears Chairman Edward R. Telling.

"We view the introduction of a new credit/financial services card as the next logical step in our strategy of providing our customers nationally with a broad range of high quality and affordable financial products and services from an institution they trust," Telling added.

The entrance of Sears, the nation's largest retailer, into the credit-card field "could really give banks a lot of trouble," noted Spencer Nilson, publisher of an international newsletter for credit-card executives.

"In the short run -- during the next three to four years -- the banks won't know much of a difference. But five years from now, Sears could become a major competitor in the credit-card world," Nilson said.

While Sears' potential competitors such as Visa and American Express Co. knew few details of the retailer's plans, they agreed with Nilson's assessment.

"Sears is a strong and efficient organization with a large and well-established card base, and we're sure that they will provide formidable competition to everyone in the industry," said Charles T. Russell, president of Visa USA.

Sears has about 57 million cards outstanding; MasterCard, 58.4 million and Visa, 71.3 million, according to Nilson. However, because Sears' cards are used only at its stores, the amount of business done with the Sears card is far less than that under the bank cards -- $13.6 billion a year with the Sears card, compared with a total volume of $116 billion for the two major bank cards.

Nilson predicted that Sears' decision to enter the market could further escalate the war between banks and retailers and other non-banks entering the banking business.

With retailers unregulated, banks have been pressing Congress to place curbs on ventures into financial services by Sears and other retailers.

Sears said its new card would be issued by the Greenwood Trust Co. of Greenwood, Del., which Sears acquired earlier this year.

Sears declined to release further details about the card -- such as where it would be tested.

Heading the venture will be Raymond A. Kennedy, who has been vice president-credit of Sears Merchandise Group.