DECATUR, Ill. (AP) _ About 3,300 Caterpillar Inc. workers walked off the job today to protest the company's suspension of a union steward's privileges.

The strike began about 3:45 a.m. when about 400 employees refused to report to work at Decatur, said Larry Solomon, president of United Auto Workers union Local 751.

About 1,000 day-shift workers honored the picket lines hours later,. a union spokesman said. Solomon said the local's entire membership of 1,700 was expected to join the strike by day's end.

And by 9 a.m., the strike had spread to about 1,900 workers at Caterpillar's plant in Aurora, a far western Chicago suburb. Machine operator Pat Thorpen, a Local 145 member, estimated that 800 pickets were outside the plant.

The Decatur plant, which makes earth-moving equipment, will stay open today with about 1,000 nonunion workers but will operate at decreased capacity, said Caterpillar spokesman Chuck Hippler. There was no immediate word whether the Aurora plant would stay open.

There was no indication the strike was spreading to other plants. The company has plants in several other downstate Illinois cities, including Peoria and Joliet, and in York, Pa., Memphis and Denver. In all, it employs roughly 14,000 union workers, 12,000 of them in Illinois.

The UAW has been working without a contract under terms of Caterpillar's final offer since April 1992, when a 163-day strike collapsed in the face of company threats to permanently replace all pickets.

The imposed terms allow the company to revoke the privileges of a union steward, Hippler said. But the union said the action, telling a union steward he could not represent workers for 30 days, was an unfair labor practice.

''This is the straw that broke the camel's back. Workers are tired of having their rights violated,'' Solomon said.

The company wants the UAW to put its final offer to a rank-and-file vote. Union leaders say members will never accept it, citing such measures as limits on choice in health care and a lower wage scale for new employees.