I am Puzzled working on a MTH FT A Unit Diesel 20-20630-1 motor spins wildly

It could also be the PS/3 board.

Thanks John,

I try another board and post back.

Remove the shell, hold the wheels and try to turn the flywheel.  I have seen slipping worm gears on the motor shaft, and slipping worm driven gears in the truckblock.

What Jon G said.

The symptoms seem to be consistent with the motor slipping.

I changed the board and still the same. 

I tried what Jon suggested and do not detect slippage. 

Could it be this slider switch?  How should I check it? 

No.  I agree a motor issue, so when you say the tach motor spins wildly do the wheels spin too?  If so the Tach motor is not balanced with the non tach motor.  I bet it has a much lower winding resistance and it takes all the current.  BUT spinning wildly also means the tach reader is not working ( do you get rpm ram up sounds?).  So you have possible secondary problem too.

You need to be very specific with the observations of how they respond and sounds you hear.  Run it elevated so you can observe decoupled motor action.

I had one of these on a new PS-3 diesel, tach motor spun and non did not. But it was not run away speed.  Though non tach motor was bad and replaced it with same symptom.  When I measure resistance of motors, individually the non tach was normal, and the tach was way low.  Replaced it and engine ran fine.  G

Thank you George.  Yes it was motor.  In the first post I was not clear on which motor I had already replaced.  I replace the tach motor first which prompted me to post.  As it did not solve my problem.  As George suggest I put the unit up on rollers and it took a lot more voltage to move the front motor.  I had a new non-tach motor and I put it in and both motors turning insync on the rollers and ran like a champ on the track. 

You all are very helpful