De winterized my 1997 mercruiser 350 magnum tournament ski (carbureted) motor last weekend and all checked out great at home. Took it out last night for the first time this season and it started and ran fine, however I noticed my ammeter gauge was not registering at all. Stopped and checked all electrical connections around the motor and battery and all seemed fine. Boat started right up but still no movement of the ammeter needle. After running for 10min or so, the ammeter briefly registered correctly but then stopped after a minute of so. I also noticed that when I turned my blower or bilge pump on that my readings on my water temp gauge and oil pressure guage changed, so I doubt the ammeter itself is bad, but it must be a failing/failed alternator? What do you guys think? Any troubleshooting advice?