[Mgb-v8] Dynamo output reverses

[James Bown]

If you are really certain that the field winding is OK (not open) then it
would likely be the regulator (control box).
But it is difficult to troubleshoot such things from a distance.
Please let us know what you find the solution to be.
 Regards,
Jim B.
Salt Lake City



> From: paul.hunt1 at blueyonder.co.uk
> To: mgb-v8 at autox.team.net; mgs at autox.team.net; MG-MGB at yahoogroups.com
> Date: Tue, 27 Mar 2012 08:51:30 +0100
> Subject: [Mgb-v8] Dynamo output reverses
>
> 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?