OAKLAND — BART’s new fleet is scheduled to begin carrying passengers next year, but the shipment is already five months behind schedule, and the test cars suffer from design flaws that could impact their long-term reliability.

BART officials have said the faulty parts will be swapped out once a fix is found and that the issues won’t jeopardize passenger safety or the projected rollout of the new $2.6 billion fleet.

But, engineers continue to be stymied by the problems, which afflict the cars’ power supply system, and all of those issues may not be resolved before the first production cars arrive.

BART OK’d an initial order of 10 cars from Bombardier Inc., despite last year discovering problems with their auxiliary power supply equipment, or APSE, that could cause overheating. The system provides electricity inside the car for everything from lights, doors and communications systems, to a pump that replenishes brake fluid on the car’s secondary braking system. Many of the systems inside the car can run safely on battery power, said project manager John Garnham, but the auxiliary units recharge the batteries, as well.

It’s also a piece of equipment that has plagued BART’s existing fleet, costing millions of dollars to replace.

The system is essential for the car to run safely while carrying passengers, said Edward La Guardia, the former chief engineering officer for the Southeastern Pennsylvania Transportation Authority.

“You have all those systems that have to function to run at revenue service, otherwise you have serious liability issues,” he said. “If it doesn’t work, you can’t put those cars in … service.”

But, rather than insist the system be fixed before any cars were delivered, Garnham said staff accepted 10 of the faulty auxiliary power units manufactured by Mitsubishi Electric Corporation so engineers could continue testing other equipment on the first 10 pilot cars and maintain the fleet’s delivery schedule.

“Once we understood the problem and what their solution was, we either could delay the project for eight to 10 months or we could take the interim APSEs … understanding that when they gave us the production units, anything that touched the APSE, they would have to requalify,” Garnham said in a recent interview. “It would allow us to get any bugs out of the rest of the equipment.”

Though that problem was solved, recent testing of the redesigned power supply units has uncovered other problems, including some that could cause the units to shut down, Paul Oversier, the assistant general manager of operations, said in a memo to BART’s board last week.

Garnham admitted there was an issue with a high voltage switch in the unit that causes it to shut down at around 300 degrees Fahrenheit. The problem wasn’t identified earlier because Mitsubishi’s engineers didn’t realize the design could pose a problem for BART’s unique operating needs, Garnham said.

“We don’t know it’s an issue, either,” Garnham said. “We’re trying to ascertain that. … We can’t afford to have a car going up to Pittsburg/Bay Point and shutting down because it’s hot up there.”

The system also shorts when there are too many devices plugged into the electrical outlets in the car, a problem that Chief Vehicle Engineer Henry Kolesar said wouldn’t impact passenger service — since the outlets are only used during cleaning — but needs to be fixed before the pilot cars hit the tracks.

The fact that the issues weren’t discovered until engineers started testing the units came as a bit of a surprise, Kolesar said. “These later problems are just popping up now because they had to go back and redesign (the units),” he said. “Right now, (Mitsubishi’s engineers) are scrambling because the issues came up late.”

Problems with the same system in BART’s original fleet didn’t emerge until the cars had been in service for a number of years.

BART refurbished its oldest cars between 1997 and 2002 but didn’t discover the design flaw in the cars’ auxiliary power system until around 2005, Kolesar said. BART chose to replace approximately one-third of the APSE units in the fleet’s C-cars so maintenance workers could harvest the spare parts after the systems’ manufacturers either went out of business or stopped making the units, said David Hardt, BART’s chief mechanical officer.

BART was supposed to get its first new production car in December of this year, but Garnham said that first car is now expected to arrive around May 2017. BART says it needs to have a certain number of new cars up and running by the time the new Berryessa station in the South Bay opens, projected for late 2018, in order to maintain its current level of service.

The agency should be able to make up some of the lost time by ramping up its production schedule for the new cars. Garnham said BART’s car manufacturer has already agreed to speed up delivery, with the final cars in the 775-car fleet arriving nearly two years ahead of schedule in 2021. But there’s still the problem with the cranky APSE units.

BART could accept some units with known problems and replace the faulty parts once a fix is found, though it is hoping to resolve the issues before the cars arrive, Garnham said. Fixing the cars would require them to be taken out of service, which impacts the overall transit system.

The deadline to receive the final new car is a critical one, Oversier told the board during a Feb. 25 workshop. BART cannot indefinitely operate older train cars that will have exceeded their useful life by 2019. Beyond that point, using the aging cars becomes significantly more risky, Hardt said.

“The one thing we know for certain is the aluminum structure will eventually fail,” he said.

Several BART directors reached this week said they were “concerned” about the recent setbacks but not worried.

Director Robert Raburn compared the testing and commissioning of new cars to delivering a child: “Lots of unexpected things can happen.”

Director John McPartland, who chairs the engineering and operations committee, said he was primarily concerned with safety, the timeline and the cost.

“And I say, ‘No problem’ on safety, and ‘I hope not,’ on the timeline, and ‘There better not be,’ on the additional cost,” he said.

Contact staff writer Erin Baldassari at 510-208-6428. Follow her on Twitter at twitter.com/e_baldi.