Col. Wilbert T. Travers Jr., who rose through the ranks to head the Maryland State Police, announced yesterday he is retiring from the agency after 28 years on the force.

Travers, 51, was named state police superintendent three years ago by Gov. Harry Hughes, and his tenure has been plagued by dissension in the ranks over the agency's promotional and hiring practices. The headaches associated with resolving those longstanding problems were said to be a factor in his decision to retire.

Though Hughes had been urged by some legislators and others in state government to fire Travers over his failure to correct alleged promotional abuses, the governor did not ask for his retirement, according to sources.

In March, the Maryland attorney general's office was critical of Travers for not correcting abuses that reportedly included the rigging of promotional evaluations to favor selected individuals for promotions.

The attorney general's report followed a 1981 investigation that uncovered similar problems. A panel chaired by the Baltimore police commissioner is studying the more recent allegations of abuse in promotions that the attorney general was unable to specifically prove or disprove. A high-ranking officer in the agency refused to cooperate with the attorney general's investigation, citing the police officer's bill of rights, which holds that a law enforcement officer can be investigated only by another officer.

Travers has also been faced with internal dissatisfaction over his plans to increase the number of black officers in the Maryland State Police by 1987. The Maryland Troopers Association recently issued a vote of no confidence in Travers and Hughes because of that policy, which was adopted in June.

Hughes, in announcing Travers' retirement today, said the superintendent had a "long, honorable and distinguished record" of service to the state. "During that time, he has displayed courage, dedication and determination to guide his agency through a difficult transition."

Travers has agreed to stay on past his retirement date of Sept. 1 until Hughes finds a replacement.

Del. Timothy F. Maloney (D-Prince George's), who chairs a legislative subcommittee that oversees the state police, said it was a "safe bet" that the new superintendent would not come from within the agency.

"Red Travers is a very good cop and an enormously decent human being," said Maloney, "but the situation he inherited would have been difficult for an insider to cure. It's like a convent. It's very clannish. It will take an outsider with a vacuum cleaner or a broom to clean it up."