Members of Fairfax County's largest teacher union have voted overwhelmingly to approve a new contract that continues their endorsement of the Washington area's first merit pay plan, but with strong reservations, the union reported last night.

On a ballot with three choices, a 63.2 percent majority of Fairfax Education Association members voted for a "yes, however" option to approve the contract but to express dissatisfaction about several aspects of merit pay. The unconditional "yes" option won 7.2 percent of the vote, and 29.6 percent voted "no."

Officials said 4,343 of the union's 6,700 members cast ballots in voting Monday and Tuesday. Ballots were counted yesterday.

The 1988-89 contract, subject to approval by the county School Board, then by the Board of Supervisors, would give teachers an 8.8 percent raise as part of an agreement with Superintendent Robert R. Spillane. The agreement would provide for an increase in salaries by a total of 30 percent over three years. The union agreed to the pay-for-performance plan last year in return for the large wage increases.

Teacher approval of merit pay has been shaky from the start. Union leadership had endorsed the contract but refused to predict how members would vote, citing widespread teacher morale problems and anxiety stemming from an on-the-job evaluation system that was installed in all county schools this year as the first phase of merit pay.

Although Virginia law gives unions no power over contracts, Spillane has said that teacher endorsement is one reason that his plan will work where others across the nation have failed.

All teacher raises are to be based on classroom evaluations by the 1989-90 school year. The highest rated teachers could get 10 percent bonuses; low-rated teachers could be fired or have their pay frozen.

"People took a deep breath and they realized that in order to maintain a professional status they had to stay in this and see if it works," said Mimi Dash, union president. "This isn't a blank check forever."

Spillane said the vote "says to me that we're still walking and working together, and that has to be the success of any merit system."

The superintendent said he views the "yes, however" vote "as a positive vote." Dash described it as a "tempered protest." The total 70.4 percent approval vote was slightly lower than the initial 76 percent vote in September 1986 to approve merit pay and a 12.1 percent teacher raise as part of this year's contract.

Spillane said the teachers' concerns are under discussion by an advisory group that held its first meeting this week.

"We're willing to discuss it," he said. "It doesn't mean the end results will please everyone."

The only specific teacher objection listed in the "yes, however" option was to a requirement that a teacher's overall job rating be based on his or her lowest rating on the eight performance standards included in the evaluation. There are five possible ratings, from "ineffective" to "exemplary."

But Dash said teachers also are concerned with a rule that restricts eligibility to apply for merit bonuses, and with inconsistent application of the evaluation system across the county.