There might be a question about the condition of your generator.
Making sure the commutator is clean and the brushes good and all the connections
are clean and tight might help. Showing a - reading on the ammeter just means
that all the electric things are drawing more than the generator (or alternator)
is putting out. Both of them vary widely in output with speed so if you are
driving a lot more than idling it is OK to go negative while idling. But if
you sit and idle too long you will eventually run your battery down.
Unless you have a lot of add-on electrical accessories (spotlights, driving
lights and hundred-watt stereo) a good generator will be adequate. If you
go in for the heavy stuff you might need to upgrade to an alternator.
John's right - the Voltmeter really gives more useful info than the ammeter.
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George Downs Bartlesville, Heart of the USA!