Hello, wanted to share a repair tip. If you are handy with a soldering iron. I have a MM502 I suspect was struck by lightning and the rev counter in the slave ram went bad (also the control board charging circuit burned but that's another repair story). The symptom is as described it the TS...
Torque is right, you can seriously damage the ram if you over retract or extend it. The control board inside provides feedback to prevent this. There are some MM Rams that are self contained with control board inside.