Hi, this post is a little old but I try my luck.
I have the same problem as you and I can not solve it.
I tested several engines with or without the additional power supply.
Never result.
The problem does not come from the code, I tried with the examples of Adafruit always without result.
I like you, no tension on the terminals of Shield.
I am also using version v2.3 with arduino Mega R3 2560.
I have no idea ...
If anyone ever used this Shield with a Mega, please help me.