Becton Dickinson & Co. yesterday sued Sun Co., claiming Sun's purchase of 34 percent of B&D's stock was illegal. The suit further confused Sun's effort to fainncontrol of the big health supply company.

The lawsuit charged that the purchase of the shares amounted to an illegal tender offer and violated numerous securities laws.

B&D asked the courts to prevent Sun from voting its shares, which are enough to control the company effectively.

Sun should be required either to sell th shares or to offer all other shareholders the $45 a share it paid a small group of institutional owners, the lawsuit said.

Several B&D shareholders already have sued Sun, and the sale is under scrutiny by the New York Stock Exchange, the Securities and Exchange Commission and top members of the Senate Bankingl Committee. An aide to Sen. Harriosn Williams (D-N.J.) said the purchase appeared to violate takeover legislation that bears the senator's name, the Williams Act.

Details of the lawsuit could not be obtained yesterday because the blizzard closed down the federal court in New York where it was filed, along with offices of B&D and its lawyers.

Three weeks ago, Sun. Co. - it used to be Sun Oil Co. - spent an estimated $300 million to acquire more than one-third of the shares of B&D, a New Jersey maker of thermometers, syringes, needles and other health care products.

The stock was purchased through Salomon Brothers, the investment baking house, from a group private and institutional accounts. Among the sellers was Fairleigh S. Dickinson Jr., former chairman of the board and son of the company's founder.

Dickinson, who has a long association with Salomon Brothers, last year was ousted from his post in a struggle with B&D management.

Sun offered the sellers $45 a share for the stock, then trading at about $33, and bought the shares without a public announcement.

B&D's lawsuit and other critics of the deal insist Sun was required by the Williams Act to make a public tender offer for the stock. The 1966 law requires that all shareholders be given an equal opportunity to sell their shares when a company's stock is acquired.

Other shareholders had no chance to profit from Sun's takeover, because the massive block of stock was purchased before Sun announced its plans.

Salomon Brothers asked the New York Stock Exchange to suspend trading in B&D because of a possible takeover. The exchange is investigating complaints that the takeover had been completed before the request was made.

The Securities and Exchange Commission's staff already was looking into deal informally when it received a complaint from the New Jersey senator and two colleagues influential on securities matters, Sen. William Proxmire (D-Wisc., and Rep. John Moss (D-Cal.).