GRJ, the original poster said this was a PS1 loco, and I didn't think the SAW board had speed control. I know that two motors wired in series acts like an open differential. But as John Acton said, most of these locos are such strong pullers, the risk of losing traction is acceptable for home layout-sized trains. In most cases wiring the motors in series gives the loco slightly better slow-speed performance. (The real solution would be a lower gear ratio and/or back-drivable gears, but we know that's never going to happen 
)
I wholeheartedly agree that he should determine and resolve the root cause of his component failure.
