[Mgs] Dynamo output reverses

[Barney Gaylord]

See charging circuit here: http://mgaguru.com/mgtech/electric/circ_c.htm
Notice ground connections inside the generator.  This is parallel 
wound, meaning that both field and armature are connected between 
power and ground.  If you switch the leads on the brushes it will 
reverse output polarity of the armature.  My bet is the brush leads 
have been switched (or brushes swapped in the holders).  When you 
install new brushes, R&R only one at a time so they get connected 
same as original.  This of course assumes the old ones were connected properly.

Barney Gaylord
1958 MGA with an attitude
http://MGAguru.com


At 08:51 AM 3/27/2012 +0100, Paul Hunt wrote:
>.... 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. .... but the owner said it was 
>negative earth.  .... sure enough that showed positive 12v, .... 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.  ....
>....