Hi Barry,
Whilst waiting for Oscar and Brian, here's my 'three penneth'...
Oscar, as I'm not well up on electrics is there any particular reason that I should be aware of for the Dexta to be Negative Earth??
When motor vehicles, tractors etc., were first started being built, it was logical to interconnect everything electrical by taking positive and negative wires to each individual item you wanted to 'power'.
This, of course, meant a lot of wiring until some 'bright spark' thought about connecting everything to a 'common feed' and make everything 'live' then just take negative connections to the things you wanted to 'power'. This reduced the wiring needed by 'nearly' 50 per cent.
Why 'common feed'?
Well again, 'logically' it was believed that negative was earth, was ground, was 'common', and that electricity flowed from the positive to the negative so needed to flow 'to' anything you wanted to 'power' and, anyway, it worked so why not?
Later on, electronics research actually showed that current flows from the negative (which is why it is always 'more minus') to the positive (which is why it is always 'more plus' - but let's not 'get heavy' on semi-conductor junctions and the like). More interestingly, they found that because of this, if they reversed the way things were wired, contacts, armature brushes, releys etc., did not 'burn out' as quickly so, in the sixties, manufacturers started to re-wire vehicles with a 'common negative' or 'negative earth'. They also has a 'political' safety 'lever' applied to their decision (explained later).
This also started to allow them to incorporate new advances like diodes (not welsh money lenders) to produce more efficient charging systems (alternators) but, due to costs, these changes were gradually phased in.
For example, my A35, the first 'real' Minis and my 1200 three bearing crank MK1 cortina were all positive earth with dynamo's (generators). The later Mini's and my much missed 1300 GT Escort (I refuse to refer to it as the MK1 because the first 'Escort' was an Anglia Van) were 'negative earth' with alternators.
Brian, should I revert the Dexta to Positive Earth?? would there be any benefit and if I do will I need to replace anything or just re-set the dynamo.
in my opinion, it's just a matter of logical choice.
The Dexta was originally 'positive earth', if you want to keep 'originality' it should be 'positive earth'.
If your Dexta has an alternator fitted, you'll need to replace it with a generator, re-wire and either replace (if it's missing) or rewire your regulator. Approx. 2 hours work.
If your Dexta still has a generator, you'll just have to re-wire then 'flash' the generator to reverse polarity (see Brians earlier post). Approx half hours work.
If you use your Dexta as a 'tool' then, if it 'aint broke, don't fix it.
Little bit of history...
What was the main prompt to change from 'positive earth' to 'negative earth'? A petrol tanker fire!
Originally, with positive earth, everything had a 'live feed' and just needed a negative connection to complete the circuit. Going back to when I was still in short 'leder hosen', a petrol tanker burst tyres whilst crossing a 'metal'd bridge' to a storage depot. The metal rims of the wheels made contact with the bridge and, as everything was wired 'positive', the 'earth' connection to the metal of the bridge completed the circuit with dramatic results. That was the initial prompt for the change, following political pressure.
My Major is 'positive earth', charges the battery 'like a gud un' and stays that way but, as I said, It's personal choice.
Mike
