The Virginia Senate voted today to provide Metrorail construction aid to Northern Virginia only if the governor approves of new plans for financing the Metro system.

The conditional grant of $10 million to Metrorail was approved as the Senate adopted revisions to the state's two-year, $7.4 billion budget and sent it back to the House of Delegates to work out minor differences.

The budget bill was passed only after several senators complained that it appears to be based on unrealistic tax revenue hopes and raises state worker expectations of a 4.8 per cent pay increase they may not receive.

The two-year spending plan adopted last March overestimated revenues, according to current projections, by 6 per cent, or $220 million. This budget gap has been closed by spending reductions, tax collection speedups and upward revisions in revenue estimates that have not been endorsed by administration budget officials.

The Metrorail provision is the latest in a long series of maneuvers by Northern Virginians to persuade legislators from other areas and Gov. Mills E. Godwin to support state aid for the $5 billion, 100-mile rail system.

Godwin last year vetoed the $10 million construction grant, but the House in this session again included it in its budget revisions. Sens. Adelard L. Brault and Omer L. Hirst, both Fairfax County Democrats, told the Senate Finance Committee today they were offering the new limitation on the Metro grant in hopes of avoiding another veto.

"We met with the governor," Brault said. "He did not commit himself on this item, but he gave us to understand that tying the appropriation to make it easier for him not to vet it."

The effect of the Senate-approved amendment is to extend until June 30, 1978, the deadline for Godwin or his successor to approve or disapprove the Metrorail grant. Without the change, Godwin would have had to approve or veto the appropriation by next July 1.

Metro officials following the legislation in Richmond said it is questionable whether the process of revising Metro's financial plan, now under way, can be completed before Godwin leaves office in mid-January, 1978.

The $10 million appropriation is small when compared with the total Metrorail cost or even the current obligation of Northern Virginia localties to pay $270 million for rail construction.However, Northern Virginia legislators are trying hard to establish the principle of regular state aid to the system.

Up to now, the state has provided or promised to provide about $130 million in aid to Metro, but only $15 million has been an outright construction grant that reduces the contribution of Northern Virginia cities and counties. It is believed that most if not all of another $35 million being provided out of state gasoline taxes for parking lot and access road construction also will be counted toward the local obligation.

In Maryland, the state government is a paying all construction costs allocated to the Maryland suburbs of Washington.

In another action affecting Metro's fortunes today, a House Finance subcommittee revised the terms of a Northern Virginia gasoline sales tax approved last year. The 4 per cent tax was intended to provide about $22 million over two years to help the area's cities and counties pay their share of Metro operating deficits. However, Fairfax City refused to ratify the tax, thus preventing other Northern Virginia cities and counties from imposing it.

The subcommittee voted to recommend that the tax distribution formula be revised to credit each city and county for all revenues collected within their jurisdictions. The formula in the current law requires that all of the revenues be paid to the Northern Virginia Transportation Commission and then credited to localities on the basis of their share of Metro operating deficits. Under that formula, Fairfax City was expected to receive less than it collected.

The subcommittee, however, refused to remove the requirement that all cities and counties approve the tax before any jurisdiction can impose it. Representatives from other Northern Virginia jurisdictions at the subcommittee meeting predicted that Fairfax City will again veto the tax.

However, they were told Del. Warren G. Stambaugh (D-Arlington) that there is no chance the full House Finance Committee will approve removal of the requirement that all Northern Virginia jurisdictions impos the tax uniformly. The full committee will meet on Monday. The Senate-passed version of the tax would permit the Northern Virginia jurisdictions to impose the levy individually.

The Senate passed the overall budget revisions, 31-7. Sens Joseph V. Gartlan (D-Fairfax) and Wiley F. Mitchell Jr. (R-Alexandria) were among the five who said they voted against the bill to protest what they fear are unsound revenue projections. They said failure to provide adequate revenues runs the risk that Godwin will resort to cuts in school aid to cities and counties to keep the budget in balance.