A federal agency that oversees highway safety is investigating the cause of several bus engine fires that occurred in Montgomery County’s Ride On fleet in the past three years.

The National Highway Traffic Safety Administration began an investigation March 22 into fires originating near the transmissions of some of Navistar, Inc.’s International 3200 buses to determine if there is some kind of manufacturing defect in the vehicles, according to the administration.

Montgomery County has 54 Champion International buses — whose chassis are built by Navistar — in use, said Esther Bowring, a spokeswoman for the Montgomery County Department of Transportation. At the end of last year, Ride On had 339 buses.

The county bus drivers’ union, which represents more than 600 bus drivers, said there have been six fires aboard these buses in the past three years and the union has filed a grievance with the county demanding they be removed from service.

“The County Executive’s office should step up to lead and pull these buses from service,” Gino Renne, president of the union that represents Ride On bus operators and mechanics, said in a statement. “Quit playing Russian roulette with the safety of our bus operators and the riding public.”

The traffic safety administration says it currently is aware of three bus fires that have occurred on those model buses in the past three years. No fatalities or injuries have been reported.

Part of the drivers’ union’s grievance is a claim that Montgomery County did not formally report the problems to the federal safety agency on its own. The statement attributed the discrepancy between the traffic safety administration’s number of fires and their own to this lack of reporting.

Bowring wrote in an email that representatives of Montgomery County’s Department of Transportation have notified the federal safety agency about the problem and plan to remove the buses from service within the next six months.

Nelvin Ransome, a drivers’ union leader and bus operator, said that is not soon enough, as the vehicles present a hazard for drivers and riders.

“A lot can happen between now and six months,” he said.

A summary of the investigation by the traffic administration states that an inspection of the buses shows that the parking brake, behind the transmission, applied while the vehicles were in motion, may have caused the fires. Drivers of the buses say the parking brake can unintentionally be applied while the vehicle is moving.

Administrators with the county department of transportation could not be reached Monday.

Champion’s line of International Defender buses made between 2007 and 2008 were recalled in November 2010 due to a defect in the model’s evaporators that also caused fires, according to traffic safety administration records.

In January, Navistar recalled more than 300 of its International 3200 models nationwide after reports of fires in its commercial bus line due to a defect in its brake line, according to traffic safety administration records. The first report of a fire in the model was a November 2011 report out of Montgomery County. The report does not indicate whether or not the bus was part of the county’s Ride On fleet.

The county pulled the diesel-fueled Champions out of service in September 2009 after it was discovered the rear brakes were overheating, Bowring wrote in an email to The Gazette. They were returned to service in July 2010 after repairs were made.

Representatives of Navistar, Inc. did not return a message requesting comment Monday.

aruoff@gazette.net