The Fairfax County Board of Supervisors, after pondering the issue of what to do with the county's controversial new formula for valuing older cars, voted last week to return to its old system with a slight modification.

The action will be held in abeyance until the state attorney general verifies that the board's action was legal.

Earlier this month, county assessors riled hundreds of taxpayers when they mailed out assessments that dramatically increased the previous valuations on a large number of older cars. The resule, as supervisor Marie B. Travesky (R-Springfield) noted irately last week, "literally tied up our office lines" with taxpayers' complaints.

County assessors say the new valuations were issued after they discovered the assessed values of some older model cars were no longer in line with their fair market values.

Under the old system, cars manufactured before 1972 declined rapidly in value until a $100 floor was reached. Under the new system, the decline was far slower and resulted in some cars being reassessed from as little as $100 last year under the old formula to $1,950 under this year's new formula. Supervisor James M. Scott (D-Providence) said the valuation on his 1969 Dodge Dart jumped from $325 last year to $560 under the new formula.

However, car values that dropped under the new formula would remain at those new levels, the supervisor said.

What especially irked the supervisors was the fact that county assessors had originated the new system without first clearing it with the nine-member board. That fact prompted board Chairman John F. Herrity to remark, "Maybe we should decrease the budget so the people up there (the county assessors) don't have time to come up with screwy formulas like this."