A Maryland state trooper, convicted last year of sexually assaulting a motorist he stopped on a routine traffic check in Harford County, was dismissed from the force on Thursday, a police spokesman said yesterday. The trooper, Jeffrey Jones, 28, had been suspended without pay since the accusation was made by a New Jersey woman in October 1983.

Neither Jones nor his attorney could be reached for comment yesterday. The Baltimore Sun quoted Jones as saying he maintained his innocence and planned to appeal his dismissal, a disciplinary action rarely used by the State Police.

State Police spokesman Dan McCarthy said that only two other full-time troopers have been dismissed from the department for disciplinary reasons in the past three years.

Jones, a Baltimore resident, is appealing his March 30 conviction in Harford County Circuit Court on charges of committing a third-degree sex assault, battery and two charges of misconduct in office. He was sentenced to 90 days in jail, 18 months supervised probation and 120 hours of community service.

Prosecutors said during the trial that Jones had stopped a New Jersey woman driving after dark on a rural section of Rte. 95 near Aberdeen, Md., for a faulty rear license-plate light. They said he ordered her into his car, tried to negotiate a fine with her, handcuffed her hands behind her back and then put his hands inside her underpants.

According to state police, a department hearing board last month found Jones, who joined the state police in 1980, guilty of violating an officer's code of conduct, insubordination, failing to report an auto stop and conducting an unauthorized strip search.

Department spokesman McCarthy said that Jones was also ordered to forfeit 46.8 hours of personal leave he had accumulated but not used.