Athearn blue box

[rrinker]

Make sure you get the motor isulated from the frame.

I'm going to guess this is where you went wrong, the motor wasn;t totally isolated. The AT harness clips get rid of the stock lower clip's tab that sticks out to make contact with the frame, but you still need to fill up the space under the motor with a layer of electrical tape so there's no chance for the motor, tab, or the solder point on the AT harness tab from contacting the frame. Plain clear tape won't do it, just the motion of putting the motor back in place will cut right through it.
Also check the motor, a motor with a damaged winding might run but that partially shorted winding will make a high current pulse every rotation of the motor which can fry the decoder as well.

--Randy