Erratic ammeter (not Model A)

[mantta]

A dirty comutator or worn rushes will cause that.

[SeaSlugs]

Originally Posted by mantta

A dirty comutator or worn rushes will cause that.

yup- or bent shaft.

My guess and the most common is either A worn bushes or B stuck brushes from carbon buildup or C broken tension springs.

I'm assuming it has a band covering the brushes. you should see how much life is left in them, if they look healthy (not too worn or cracked) blow out the generator with compressed air and focus on the brush holders. If it still looks extremely dirty get a can of electrical contact cleaner from any auto parts store (sometimes called electromotive cleaner) and liberally spray out the brush holders. use a small screwdriver or pick to hold the spring tension off the holder and ensure the brushes slide freely in the holders. Then give it another whirl.

And yes clean the grounding path and check electrical connections from generator to battery to ammeter.