Hey jak,
Just for info, what were the model #s of the Nikons. Like I said the
lens on the 4100 just popped back into place when I drove the motor
from an external battery. It had been dropped so hard the shutter
closure mechanism on the front of the lens was just gone. No other
camera I work on will recover like that there will just be little
broken pieces ratteling around in the lens barrel. Come to think of it
there was a little piece that fell out but the lens seemed to function
normally.
If the lens if jammed and you power up the camera most I've worked on
will try to bring the lens to the retracted position. Usually there are
optical sensors on the main zoom element that sense this. If it dosen't
get the end of travel signal or too much power is consumed the camera
will give a system error or shut down.
Richard