Steering spins to lock at idle

I put in a refurbed firm feel box, and when the wheels are off the ground and engine is idling, the wheel will spin all the way to lock. The mechnanic I took the car to for suspension work says its not supposed to do that - says the box is bad. I just wanted to make sure that diagnosis is right as I've never heard of this. He's a great mechanic, but a GM guy and not all that familiar with Mopars - so I wanted to verify this info.