The Metro Transit Authority and the union that represents its 4,800 bus drivers, subway operators, mechanics and others announced jointly last night that they had been unable to negotiate a new contract and were submitting to blinding arbitration.

The contract between the authority and the Amalgamated Transit Union Local 689 expires at midnight Sunday. The blinding arbitration agreement means that there will be no strike and that both sides will accept as the final contract the ruling of an arbitration panel.

There have been occasional wildcat work stoppages by Metrobus drivers in the past in similar situations, however. Both union and management sources said they do not expect such difficulties this time. Nonetheless, they did not announce the decision to arbitrate until after the Friday evening rush hour.

Under the arbitration procedure, each side chooses one arbitrator, and those two agree upon a third.

Major issues have not been resolved in negotiations that began March 13, according to representatives of both sides. "Every single substantive issue in the contract is still on the table," one union negotiator said.

A key issue for the Metro board has been a desire to hire part-time drivers for use on rush-hour runs, a practice the present contract forbids and one that is traditionally opposed by transit unions. Metro also was seeking other productivity gains through changes in work rules, general manager Theodore Lutz said.

According to the joint statement of Lutz and George Davis, president of the union local, "it now appears that these negotiations will not result in agreement."

The union is seeking improvement in salaries beyond the cost of living increases that have been guaranteed in previous contracts, it was learned.

A fully experienced bus or subway operator receives $8.16 an hour and, with overtime and extra pay for working certain shifts, can make about $20,000 annually.

Salary schedules for other employes are figured as a percentage of the bus-driver salary.