The Arlington County Board last night approved a far-reaching overhaul of its pay scales that will raise salaries for most of the county's 3,500 full- and part-time employes.

The board has set aside $2 million to pay for implementing the pay plan in the first six months of 1988 although the costs could rise to $3 million, according to a study for the county. The final costs of the change are unknown, in part because a reclassification of positions will not be completed until October.

Michael E. Brunner, the board's sole Republican, voted against the plan, saying the raises are too high and the costs uncertain.

The plan, which passed on a 4-to-1 vote, will put Arlington employes in the top 25 percent of a pay range determined by averaging salaries for comparable jobs in Fairfax, Prince George's and Montgomery counties and Alexandria. Arlington, with a total payroll of about $90 million, now ranks in about the middle of that range.

Members of the board's Democratic majority said the plan gives the city manager enough discretionary power to modify some pay increases to keep costs within the budget.

The pay overhaul is the first since the 1950s. It will create a single pay schedule with 24 grades and 14 steps. There are currently six pay plans totaling more than 100 grades. Board members said they wanted to make the pay scale more competitive with other jurisdictions and to make salaries for different jobs equitable.

Some employes at the public hearing urged larger increases and opposed a proposed limit on the amount a worker's salary could increase. Others said the pay plan would penalize more senior employes, who would get smaller annual raises while workers starting out would get larger annual increases.

The board voted not to impose the salary cap but gave the county manager the discretion to impose one and voted to distribute annual raises among employes more evenly.