One further tidbit of information- - - -to properly check the output of any charging system, but definitely a 3 brush setup, it must be running againat a fully charged battery. A 6V battery with a full charge will have a terminal voltage of 6.3 with no load on it. In order to keep a 6V battery charged, you need to have around 7 volts at the battery posts with the engine running at highway speed. Chances are you're dealing with a partially discharged battery which will accept a charge at a lower than normal voltage.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!