Nick, with a properly operating charging system and a good battery the meter should show a fairly high charge after starting, then taper off to just lightly above the center (zero) position. If you turn on the lights or engage the PTO the meter should indicate a moderate charge, but a lot depends on the capacity and condition of the battery. Also, small tractor ammeters are not precision devices so readings sometime vary from one meter to another.
In my opinion the only time to be concerned is if the meter consistently shows a discharge with the engine running at 1/2 throttle or higher.
Stan in CT
The battery is fairly new. I started the tractor this morning. When the motor turns over the needle stays at center even after starting. At a low idle the ammeter remains in the center position. When I put the lights on, it moves negative quite a bit.
I figured it out. I tested the stator and was getting readings in the 25-35 range. I then noticed the middle wire on the regulator had come loose. I fixed it and now when I start the tractor, the ammeter needle is about 1/4 way in on the positive side.
Then you're probably not charging. If you know for sure you are charging, then the meter is reversed. Can't say for sure on the 300's, but my 400 is just a pass through affair, no actual connections. The wire simple gets pushed into a slot on the back of the meter. If that is routed backwards, then the meter swings the other way.
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