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Could be a couple/three things. 1, If it's a new 'cheaper' gauge.. it may not have a compensated (dampened ) movement.. and is just quickly responding to the brush noise coming off the armature. Most ammeters are shunt-voltmeters anyway.. and the cheaper the movement the more it moves. 2, you could have worn brushes that are bouncing or not making good contact with the commie.. thus making a bouncy reading. 3, Could have proud mica insulation between the segments.. you can undercut this with a broke hacksaw blade.. etc. Could ahve any combination of these issues. I'd put an analog voltmeter on the battery.. and if the charge votlage is int he 7.x volt range, and doesn't drop with a load, like the lamps on... I'd be inclined to ignore it. I have a cheapy older ammeter on my 660.. thing vibrates like a hummingbirds wings.. but I've never had to jump the tractor off.. so.. I'm not messing with it till I need to... Soundguy
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