Here are some steps to test the drive unit:
1. Without connecting any power the piston should freely come in and out. This allows for you to use your steering
when auto pilot is off. If this does not move freely I would think the drive is shot.
2. As soon as you apply 12V to the clutch the piston should lock up and no longer move freely as before. If it still moves the clutch is not operating.
3. Keep the connection to the clutch in place while rod is half-way out and then as you apply the 12v dc to the drive connections the piston should move in and out.
Please report back if the procedure helps you and what you find. Cheers.