Hi everyone.... I need help with my 19 year old John Deere 318. When I turn on the PTO switch, the mower deck does not engage. I have adjusted the air gap at the 3 points on the clutch to .018" and still nothing happens. The light comes on the dash but that is about all that happens. Do I need to replace the clutch field coil or do I need to replace the entire clutch? Any help or suggestions will be greatly appreciated.