General Motors Corp. announced a tentative agreement in a 3-day-old strike by 9,000 United Auto Workers members that halted truck production at three Pontiac, Mich., plants. Members of Local 594 scheduled a ratification vote for today. Approval would allow production of GM's new full-size pickups and popular sport utility vehicles to resume Monday, the company said. Details of the accord were not released, pending ratification. The UAW contended GM violated the local contract by subcontracting union jobs to non-union workers and by reducing job classifications at the plants. More than 1,000 grievances were cited by the union, including allegations of health and safety violations and illegal assembly-line speedups.