The Senate voted 89 to 0 last night for a series of changes designed to restore public faith in the way the Pentagon buys weapons.

One senator after another said constituents are demanding that Congress do something to keep the Defense Department from purchasing $7,000 coffee-makers and weapons that do not work.

"Nothing can be much worse than continued erosion of public confidence in procurement," Sen. Dale L. Bumpers (D-Ark.) said in typifying the "do-something" mood pervading the Senate as it took its first votes on the fiscal 1986 defense authorization bill.

The measure, which would put appropriation ceilings on most but not all defense programs, would authorize $231.8 billion. Money for military pay and construction is authorized in separate legislation, bringing the total request to $302 billion -- enough to offset inflation but far short of President Reagan's original request for a 5.9 percent increase after inflation. Reagan has agreed to support the Senate bill.

"I'm tired of having to go home and apologize about scandalous waste in the Defense Department," Bumpers said, brushing aside warnings by Sen. John H. Chafee (R-R.I.), a former secretary of the Navy, that Congress might be sorry if it intrudes into the procurement process.

"We've got Congress in the middle of contracting and that isn't good," Chafee said in opposing an amendment by Sen. David H. Pryor (D-Ark.) that would have imposed stiffer competitive contracting requirements than those in the bill that passed.

"It puts Congress back in our oversight function that we have abdicated over the last 30 years," Pryor said, calling his changes "hardball" and those recommended by the Armed Services Committee "softball."

After Pryor's amendment was rejected, 67 to 22, the Senate unanimously approved the changes in the committee amendment, which would:

* Forbid Pentagon employes working in procurement from discussing future employment with defense contractors unless they disqualify themselves from working on the prospective employer's contracts.

* Require the secretary of defense to use two companies to produce weapons under major contracts, starting in fiscal 1987.

* Set minimum standards of education and training for civilian executives and military officers who run large procurement programs.

* Establish regional centers to help small businesses compete for defense contracts.

Sen. Dan Quayle (R-Ind.), chairman of the Armed Services subcommittee that studied procurement and recommended most of the changes approved yesterday, said they "should help restore public confidence that the integrity of the procurement process is being maintained."

Meanwhile, Deputy Defense Secretary William Howard Taft IV told two House Armed Services subcommittees sitting jointly that the Pentagon has instituted changes but would welcome others that would ensure that the taxpayers get their money's worth.

Taft generally endorsed a House bill that would crack down on contractors passing on unjustified overhead costs to the Pentagon. One change the Pentagon has put out for public comment, Taft said, would require an executive no lower than divisional vice president to sign requests for reimbursement for overhead. Another change being circulated, he said, would ban payment by the Pentagon for corporate aircraft unless the contract stated that such transportation was required. General Dynamics Corp. has been under fire for charging the government for executive vacation trips by corporate aircraft