By a voice vote and without debate, the Senate yesterday decided to suspend strict limitations it had voted two years ago on the amount of money senators can earn from speeches or other outside employment.

It was a case as Majority Leader Robert C. Byrd (D-W. Va.) put it, of the Senate 'having its cake and eating it too." Byrd put it that way two years ago. Yesterday he absented himself from the floor just before the voice vote.

"They took the money and ran," said David Cohen, president of Common Cause, the lobbying organization that originally pressed for the strict limitations on outside income.

Two years ago, the Senate voted to limit this outside income to $8,625 a year, or 15 percent of a senator's $57,500 annual salary. The limit was to go into effect Jan. 1 of this year, but yesterday's vote suspends it at least until 1983, leaving in place a previously legislated $25,000 limit that applies only to outside income from speeches.

The action two years ago came just after the Senate voted itself a 22.9 percent pay increase, and Byrd said then the raise and the limit on outside earnings were a package.Taking both the raise and higher outside income would be having the cake and eating it too, he said then, adding:

"The present situation which allows senators to earn up to $25,000 a year in honoraria [for speeches] is intolerably high, and contributes to doubts in the minds of the public as to why individual groups give so must money to a senator for his speeches when he is being paid by his constituency to represent their best interests in the Senate."

Some senators bitterly opposed the outside income limitation (which controlled income from employment and speeches, but not from private wealth or book royalties), but it was adopted on a 62-to-35 roll-call vote.

There was no roll-call vote yesterday, and only three senators spoke against the change. One, Sen. Howard Metzenbaum (D-Ohio), initially called for a roll call, but then dropped the idea after several colleagues urged him to do so.

Yesterday's action means that senators can now earn up to $25,000 a year the limit established by the Federal Election Campaign Act of 1976.

The suspended limit included a proviso that a senator could earn no more than $1,000 from a single speech. The vote yesterday has the effect of doubling that figure to $2,000, the maximum fee allowed by the 1976 election act.

Yesterday's action was the product of a stealthy backroom maneuver conducted by Sen. Ted Stevens (R-Alaska), the minority whip and a passionate opponent of the income limitations since they were first considered. Very few Senate aides knew anything about Stevens' maneuver. Even some senators claimed ignorance.

Stevens arose at 7:08 p.m. Wednesday to introduce his resolution to a nearly empty chamber. It called for suspending the implementing date of the new income limits from Jan. 1 of this year until Jan. 1, 1983. Sen. Daniel Patrick Moynihan (D-N.Y.) rose to support Stevens, and Majority Leader Byrd arranged for a vote to be held at 12:30 P.M. yesterday.

The vote took place as scheduled. Only Metzenbaum, Max Baucus (D-Mont.) and Dennis DeConcini (D-Ariz.) registered opposition to the Stevens proposal, but none of them invoked their privilege of calling for a roll-call tally. "It wasn't worth it," DeConcini said later, noting that Stevens had lined up the votes. "We were bushwhacked," DeConcini added.

In the Capitol yesterday, aides and employes -- and a few senators too -- grinned or groaned at the speed and stealth with which the Senate had voted to enrich its members. (Most senators have little trouble lining up lucrative outside speaking engagements.)

In an interview by telephone, Stevens heatedly defended his move as fair and reasonable. He said senators who had been elected to six-year terms in 1974 or 1976 had the right to anticipate a high level of outside earnings, but the 1977 limits imposed an unexpected, unjust limitiation. He said that seemed unconstitutional to him, because it violated the Constitution's bar on reducing the pay of government officials during their term.

Asked if the $10,700-a-year pay raise the Senate granted itself in 1977 had in any way mitigated the unfairness he perceived, Stevens sid it did not. Why hadn't he asked for a rollcall vote? "I didn't think one was really necessary," Stevens replied.

He added that in his opinion, the public would take his side on this issue because "working people" would see the justice in giving senators who have no "unearned" income (that is, the proceeds of private wealth) an opportunity to earn additional money.

Yesterday morning a reporter sent Sen. Byrd a copy of his 1977 speech defending the 15 percent limitation as absolutely necessary. "Any lifting of that ceiling will cause serious doubts in the public perception as to the intent of the members of this body to be full-time senators and public servants," Byrd said then.

Had anything happened in the intervening two years to change Byrd's mind, the reporter asked in a note?

Through his press secretary, Byrd replied: "I'm convinced that as long as there is sufficient disclosure [of outside earnings, which is required by existing law], there should not be a specter of conflict of interest."