My son ran a Futaba am 75 mhz radio on his electric car all season. Speed control worked fine. took the car out to use it one day and the throttle channel would not work. Plugged a Futaba servo in the throttle channel and it worked fine.JR servo would not move in this channel. Steering channel works the JR servo and speed control.
What gives?

Sorry, we'd not be sure what would have happened. If a Futaba servo is working correctly, and other Futaba servos work correctly, then the receiver may not be at fault. If the speed control is a Futaba unit, then we'd consider it necessary to check both the speed control and the receiver. If only non-Futaba equipment has a problem, then there may be nothing wrong at all.

Without a comprehensive checkup, we really can't give you a positive answer.