[Mgs] Dynamo output reverses

["Paul Hunt" <paul.hunt1@blueyonder.co.uk>]

During a track-day a pal noticed his dynamo had stopped charging.  We removed
the leads to do a dynamo output test and I connected the meter leads and
noticed a small negative output.  "Stupid boy", I thought, it's positive earth
(I'd connected the meter as I would on my alternator-equipped cars) but the
owner said it was negative earth.  Checked my meter on the brown lead at the
fusebox and sure enough that showed positive 12v, but on the dynamo output
terminal (no wires connected to either terminal) I still had this small
negative voltage.  It should be a small positive voltage, as that is fed back
to the field winding by the control box, which generates a bigger output,
which fed back to the field generates even more output and so on until the
control box connects the output to the battery to start charging.  Pal thought
it was brushes, but a resistance check on both field and output showed a very
low resistance as I would expect.  Even tried flashing the field from the
brown at the fusebox but it made now difference.  Subsequently pal changed the
brushes but that made no difference either.  Any ideas?

PaulH.
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