Hey everyone.
I see this is an old thread but have the same issue on a Yale 4.3 spectrum. Runs at extremely high idle and if you hit the gas it will set a code 638 and shut down. My sensors are sweeping good on volts.

I took the throttle body off to clean it out and still have the high rpm and fault. I hooked it up to the machine while I had the mixer off and the throttle was off the manifold. I thought, if I remember correctly, when you first key on the truck the throttle will open and close via the stepper motor. The motor is making a high pitched noise but no movement. Seems like it is only working via the accel cable side. I'm assuming my gears are stripped as well. Anyone have the same issue with similar symptoms?

Thanks!