Sounds like you may have bad connection of the sensor wires either at the arm or the back of the 920. This happened to me and that was my problem.
I got mine sensor wires like this: Black(or brown) on GND and blue on PULSE. Make sure not to the +5V.
Do you know anyone that may have a arm for the C Band dish? If so ask if you can borrow it. Get a 3 or 4 foot of cable that have two sensor wires (small wires) and 2 bigger wires (red and black) for the motor. With the 920 off, hook the sensors wires to the receiver and arm motor, and the motor wires to the receiver and arm motor. If you look inside the cover that comes off the motor arm you would see how the sensor and motor wires are hooked up.
Turn on the 920 and go, say Satellite G1 to C3. Make sure the arm will not strike anything when it expand out! Watch the count. It should go from 5000 (G1) to whatever the count is on C3.Go on over a satellite or two, and back to G1. Watch the count. If it is counting then it means your arm sensor is not working or like I said, bad connection.
I hope this will help you!