Pos to neg earth. A simple question.

[Grumpy21]

I do like a gadget so I’d prefer my traveller to be negative earth

I’ve never done this before but understand it’s quite straightforward to carry out the conversion. Swap the coil leads, battery leads and repolarise the dynamo by flashing the field terminal.

It the last bit that I’m not 100% certain about. I assume the field terminal is the spade connector and flashing means I briefly connect it directly to the positive of the battery.

Possibly the blower motor leads may need to be swapped too.

Missed anything?

[svenedin]

On the Lucas C40 dynamos there are two male spade (Lucar) connectors, D and F. The Field terminal, F, is the smaller spade connector. D is the larger terminal.

It is very simple to flash a dynamo to NEGATIVE earth. Disconnect the connectors to D and F on the dynamo. Ensure your battery is correctly connected for NEGATIVE earth; negative terminal connected to earth (body of the car). Find a suitable length of wire. Connect one end of the wire to the POSITIVE terminal of the battery and now touch the other end to the F terminal on the dynamo. There will be a small spark. Remove the wire from the F terminal immediately and touch it again briefly another couple of times. Reconnect the connectors to D and F. That's it.

As to the heater motor it may turn the wrong way. I say may because Smiths used different designs of motors that look exactly the same from the outside. Some have permanent magnets and will rotate backwards with reversed polarity and some have field coils and will still turn the same way with reversed polarity. The best thing to do is to test the blower before you change polarity. Then when you have converted to negative earth test the blower again. If you think the output is reduced, switch over the connections.

You mentioned the coil, yes swap the connections round.

On a standard car these are the only things to change. If there is anything electronic that was added then this will probably either not work or be destroyed by reverse polarity. Examples of electronic additions are electronic versions of the SU fuel pump (standard ones are points and polarity is irrelevant), electronic ignition, some LED bulbs.

Stephen

[Grumpy21]

Thanks for the clarification ‘smaller spade’

May I ask if ‘D’ is a typo? “ Remove the wire from the D terminal immediately and touch it again briefly another couple of times.‘

My car has a ‘Hardi’ pump which I believe isn’t polarity sensitive?

[svenedin]

Thanks for the clarification ‘smaller spade’

May I ask if ‘D’ is a typo? “ Remove the wire from the D terminal immediately and touch it again briefly another couple of times.‘

My car has a ‘Hardi’ pump which I believe isn’t polarity sensitive?

Yes it's typo! I'm tired

I don't think Hardi pumps are polarity sensitive.

Stephen

[Grumpy21]

Cheers.

[geoberni]

On the Lucas C40 dynamos there are two male spade (Lucar) connectors, D and F. The Field terminal, F, is the smaller spade connector. D is the larger terminal.

It is very simple to flash a dynamo to NEGATIVE earth. Disconnect the connectors to D and F on the dynamo. Ensure your battery is correctly connected for NEGATIVE earth; negative terminal connected to earth (body of the car). Find a suitable length of wire. Connect one end of the wire to the POSITIVE terminal of the battery and now touch the other end to the F terminal on the dynamo. There will be a small spark. Remove the wire from the F terminal immediately and touch it again briefly another couple of times. Reconnect the connectors to D and F. That's it.

I am deliberately not referring to Pos and Neg in the following, because the procedure is identical for Positive or Negative earth vehicles.


All of Stephen's information is correct, however the above instructions, whilst being the most commonly repeated, are by no means the easiest. Getting to touch a cable onto the F terminal on an installed Dynamo can be quite tricky, especially if you have large hands. Also, if you've already got it connected to the Battery, there's a good chance you might touch it to something else that is connected to Earth, like the Engine Block, the Dip Stick, the Dynamo mounting bracket, etc. Which will make you jump...

The reason to 'Flash the Field' terminal is because the Dynamo relies upon a magnetic field to operate.
Back in the day, any mechanic fitting a new dynamo would automatically 'flash' the new one before fitting it, to ensure the residual magnetism was the restored to the correct polarity.

As Stephen says, this is done by flashing the 'Supply' from the battery while the dynamo casing is connected to Earth/Ground.
There is no need for the flashing connection to remain on the Field terminal at all, just a few quick strokes to send a pulse of the desired polarity through the Field Windings.

So, back to an installed Dynamo.
The Field Terminal is directly connected to F on the Voltage Regulator.
The Battery 'Supply' connection is connected to A on the Voltage Regulator.

The easiest way to flash the Field is simply to remove the Connection from F and 'stroke' it across the A connection a few times.

Don't hold it on, because that runs the risk of it welding itself to the Terminal since it's got the full power of the battery connected to it.
Hence why the instruction is to 'stroke' it, so it's moving rather, than saying 'Dab' it, which implies making a definite connection.

See this Moss Video, they use a separate length of cable, but you can just use the lead from the F connection.
https://youtu.be/PPUfQtVEJGg

[Bill_qaz]

Working smarter Berni
After refurbishment we used to flash them, then run on test rig.
Last step brown luggage label saying pos or neg earth before putting in stores. With a mixed fleet in 1970 so we had some of each.

[Grumpy21]

All sorted thanks.

Needed to change the fuel pump polarity too otherwise very straightforward and took less than an hour.

[Bill_qaz]

Well done, Stephen will think you have gone over to the dark side

[geoberni]

All sorted thanks.

Needed to change the fuel pump polarity too otherwise very straightforward and took less than an hour.

Ah yes, I've seen that some Hardi have 2 terminals....

[svenedin]

Well done, Stephen will think you have gone over to the dark side

I will but I know why people do change to negative earth. It is not very convenient keeping positive earth when adding gadgets, many of which will only operate with negative earth. It really reduces choice carrying on with positive earth. I always think about changing when I replace the battery as that would be an ideal time to switch to a battery with the terminals the other way around. What stops me is that I do I have some electronics specifically for positive earth (fuel pump and electronic internals in the RB106 regulator) and it would be expensive for me to change polarity now.

Stephen