Reversing the polarity.

[Martin Paul]

My 1929 Austin seven has been changed to positive earth in the past. I would like to put it back to original.
At the moment the ammeter is working in reverse. 
Can anyone advise me on how to go about this, and is it a good idea.

[Stuart Giles]

Reverse the connections on the the battery so that the earth is negative. Reverse the low tension connections on the coil. temporarily remove the connection from the dynamo's field terminal. Take a lead from your new positive 'live' and touch it to your dynamo's field connection; there will be a small spark. This repolarises the dynamo and it will now charge the battery once the field wire has been reconnected.

[Martin Paul]

Thanks Stuart 
For the easy to understand response to my question. 
This will be a great help to me.

[Martin Paul]

Stuart  if I change the polarity back to negative will this reverse the rotation of the starter motor?.

[Steve Jones]

Stuart  if I change the polarity back to negative will this reverse the rotation of the starter motor?.

No.

Steve

[Stuart Giles]

Stuart  if I change the polarity back to negative will this reverse the rotation of the starter motor?.

No.

Steve

What he said.... Permanent magnet DC motors will reverse rotation if you reverse the polarity. The A7 (and pretty well all old cars) have conventional copper field windings in starter motors rather than permanent magnets; so they always rotate in the same direction whatever the polarity as long as someone hasn't carried out some internal jiggery pokery.

[Bob Culver]

Those for whom electricity is a bit of a mystery may be interested. The motors in drills, shavers, hedge trimmers etc are essentially much as our starter motors and operate despite the AC current changing direction constantly. (i have been involved with Javelin cars. Along with one other make these use a standard lucas strter but reverse wired. There are many stories of amateurs and professionals puzzling for days over cars with replacement starters which would not start!)

[A G Wood]

Yes, the desired starter armature rotation direction depends on which side of the engine block the starter is intended to be mounted on, iirc [its been a while!] 2 litre I4 Cortina/Transit automatic transmission vs manual transmission starters used to catch people out.
If anyone wants to experiment with reversing rotation its easiest done with a face commutator starter like a Lucas M35J, the local go-kart guys used to turn the brush holder end plate 90 deg on them to make them into wheel drive starter trolleys for their karts.