My RK was re wired in the 80's to the (then!) fashionable Positive earth. As I am "going through" the wiring I thought I might go back to the original negative earth and at the same time try & stop the ammeter reading the wrong way.
I am also taking the opportunity to make the (accessory) Lucas Rotary Isolating cut off switch work properly and to put an in line fuse just after the isolating switch in the lead from the battery to the ammeter(I have a new 1930's 25amp Lucas small fuse-box that will do nicely I think).

I remember changing the polarity on a 1961 Mini by, I think, doing something with a dynamo lead. My RK's dynamo has two leads into it's terminal block, and both look the same thickness so I don't know which is which! One
I think will end up on the middle terminal (shunt?) on the ammeter & the other to the warning light, but looking at the dynamo I cannot tell which is Pos & which Neg?
David

Location: Stratford upon Avon

You don’t say what dynamo/cutout combination you have which would make it easier to comment specifically to your car. But The two connections on all the coil dynamos are output (D) and field (F) the “F” terminal is the one that needs to be “flashed” to change the dynamo's polarity.

Easiest way I can think of to identify which connection is which is: if the ignition warning lamp is wired on your car, whatever cutout you have, the dynamo's "D” output has to be teed to the "D" terminal on the middle of the back of your switch panel so that would identify the “D” connection.

The dynamo's "F" terminal is the one you need to "flash" to change the dynamo's polarity. It's the one that doesn't have the teed connection mentioned above. Once you've changed over the battery connections to negative earth, disconnect the wire from the "F" terminal and touch a wire from the battery positive to the dynamo "F" terminal, there is usually a small spark, and the dynamo should charge once you've reconnected the "F" wire to it again.

Location: N W Kent

David, I posted this in an earlier thread under the title of Dynamo Polarization.

To re-polorise your dynamo,remove both wires from the unit,connect a long wire to the live side of the cars battery,taking care not to earth out the other end of this wire,flick the live end of the wire to the "F" terminal on the dynamo.THIS WILL SPARK! That's ok as the field coil will now be correctly polarised. Remove fly lead from battery,reconnect "F" & "D" leads to dynamo.Job done.
FIRE WARNING.SPARKS AND BATTERIES DON'T MIX.Keep live fly lead away from battery.

Remember the coil will be connected around the wrong way afterwards so swop the terminals over.

Location: Piddle Valley

With the car switched of and not running.
1. disconnect temporarily the 'D' brush and Field 'F' wires from dynamo noting which wire went to which terminal (label with tape if necessary till job done).
2. disconnect the battery, and reverse the battery cable connections
3. take a 'flying' lead (length of wire)from battery 'positive' and 'stroke' the dynamo Field 'F' terminal a couple of times to repolarize the dynamo, you should see a bluey white flash.
4. Remove flying lead, reconnect the wire D and F from where they came and BOb's your Mothers Brother.

This page may help you understand the Dynamo layout.
http://www.austin7.org/Technical%20Articles/A7%20Charge%20Circuit/

Steve V.

Location: Polegate, East Sussex, United Kingdom

If a fuse in the battery lead fails, the battery is isolated, system voltage rises and will burn out bulbs and possibly the dynamo field winding.
Whilst moderns are extensively and individually fused, in older cars it was considered safer not to fuse headlights.

(PS When tinkering with engine stopped, it is important the cutout is not in any way operated and permitted to stay locked in the operated state. And do not try to follow the muddled and wrong polarisng method in the Williams Special book!)

Location: Auckland, NZ

Thanks to you all. I think I follow! Dynamo, cut out etc all look remarkably original, but...!
I will pull the switch panel out & check where the various wires go by doing some continuity checks. There are a lot of (un-labelled & black)wires under the dash, but heavens, there are not many places for them to go!
Watch this space (& the small blue flash!)
David

Location: Stratford upon Avon