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Positive to Negative Earth


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Hi Nick ~

 

I converted my 3A to Negative earth quite easily. Obviously you need to swap the battery terminals around.

The wires at the back of the ammeter need to be swapped and the Dynamo needs to be 'flashed'

inorder to repolarise it. I seem to think that I swapped the wires on the coil as well.

I'm sure that someone will have further information.

 

Tom.

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  • 1 month later...

Hi Guys,

I recently bought a 1973 TR6 and I am trying to eliminate the irregular running by first checking the electrical side of the car (then the PI).

 

I bought an Accuspark EI kit and started to do the easy conversion from points. I would like to say that I had fully read and understood the instructions but was wrong footed by assuming that the car was Neg earth as the battery certainly is. Let me explain, having fitted the new EI kit and run the two leads to the correct terminals + - on the coil I attempted to start the car.....nothing. After checking all the connections and that the rotor arm was in etc I realised that the 12v feed was to the negative terminal on the coil and not the to the positive as the instructions said it should.

 

Q. If the battery is neg earth can I just twist the coil around in its belt and connect up or amI going to blow up the alternator etc etc. I think my rev counter works from the botto,m of my distributor. Secondly I have a sort of plastic collar around the distributor that has a built in plastic bit that slots into the distributor body and as the Accuspark comes with leads that fit through a piece fitting that slot I didn't think it served any useful purpose it had a soft covered wire like the base plate earth attached to it, it that standard?

I think I have fried my EI kit so fitted back the points for now.

Really need the advice on swapping the coil around.

Thanks

Charles

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On a Neg earth car the lead from ign goes to the + on the coil and the - goes to the points/distributor.

Stuart.

Edited by stuart
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Thanks Stuart, as my ignition wire currently goes to the negative side of the coil, can I just switch it around with no ill effects do you thimk?

Charles

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Charles - Its not clear from your description but if you somehow connected the output lead from the Accuspark to the same coil terminal which was connected to battery positive, you may well have fried the unit as it would pull a very large current direct from the battery with nothing to limit it. That lead needs the resistance of the coil primary winding between it and the battery, which is why they are specific in the instructions saying that a low resistance coil is not suitable.

 

Fingers crossed that is not the case.

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Thanks guys will let you know tomorrow......

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This may be of some assistance in checking the current flow through the coil:

 

According to Tony Tranter in Haynes "Automobile Electrical Manual", ignition systems are designed to have negative polarity at the centre electrode and if reversed can be up to 40% less efficient.
Coils for both Positive and Negative earth vehicles were produced, but who knows if anyone has changed a coil for the wrong type in the history of older vehicles, so it is wise to check polarity of the spark (see below for testing polarity using a lead pencil).
With early coils, swapping the CB and SW leads will switch polarity of the spark. Later coils use + and - but the requirements are the same.
To test the spark direction with a lead pencil, interpose the writing tip of a lead pencil between the spark plug top stud and the HT lead, leaving a small gap either side of the pencil tip. If the polarity is correct, a flare between the pencil and the spark plug will be seen (if the flare is between the pencil tip and the HT lead, then the polarity is reversed).
Since all modern vehicles use a negative earth system, almost all replacement coils will expect the vehicle to be negative earth. Almost all in-car entertainment systems are designed for negative earth, as are almost all in-car electronic devices.
There may be validity in keeping a positive ground in an older vehicle for the sake of authenticity, but if it has any modifications, switching to negative earth is to be recommended highly.

See also the MGA Guru’s website:

http://mgaguru.com/mgtech/ignition/ig104.htm

for an even more detailed exposition.

 

Ian Cornish (apologies for mention of the MGA!)

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Just to clarify Ian's post the above, the term '40% less efficient' is a bit misleading - it does not imply that you will have an engine 40% down on power. When connected in reverse it just takes more voltage to initiate the spark and the spark might be tad weaker. An ignition system in good condition has enough margin to cope with that. For normal road use with a standard-tune engine you probably would not notice any difference unless that margin has been reduced by having the plug gap too wide or the points gap is set incorrectly.

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Thanks for all the helpful comments and to you Ian for that link is very helpful in clarifying the problem and giving a practical way to determine the coil direction.

Charles

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Yup I had fried the Accuspark Unit as I followed their instruction but didn't pick up on the fact that the previous owner had the coil live feed on the negative connector so I had inadvertently connected it the wrong way around :( I asked Accuspark sellers iWatford Classics on EBay f they could sell me just the new unit as I had broken the one I bought. They said they appreciated my honesty as most people just send them back as not working and demanding a replacement so they offered to send me a new one with their compliments!

Hopefully I will get this right second time around, maybeI having the coil working the right way around will sort the low rev misfire as well.

Will let you know

Charles

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Roger that

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  • 5 years later...

Newbie here and I may well be missing something - but why bother? Mine is a +ve earth ( 62 TR4 ) which I guess was factory

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If you convert from dynamo to an alternator John, as many of us do to provide more electrical power, you have no choice. 

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Maybe at a considerable price, but why pay over the odds for a non-standard item when converting is so easy ?  You can buy a +ve earth Lucas 11AC at about four times the price of the more modern -ve earth equivalent.    If you are meaning the alternator-in-a-dynamo-case things.....not only are they mega expensive but they are not without problems. 

Edited by RobH
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Don’t plug a sat nav or phone charger into an accessory socket on a positive earth car or your accessory will probably torch itself

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Make sure you have the correct voltage regulator as well. The more modern solid state ones are either + or - earth. The older ones based on the heated coil innards work on either + or - earth cars so don't need changing.

Also check any accessories that may be fitted. I once tried fitting a + earth clock to my - earth car but trying to insulate the earthed metal case from the car body proved near impossible. The electric motor inside the clock also could not be reverse connected without lots of electrickery either. Check the radio as well.

Keith

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  • 4 weeks later...

Looks to retain originality as this is the Revington built police high speed pursuit TR4. Seems he went full on o/e spec wherever poss986759805_Screenshot2021-03-23at13_05_04.thumb.png.b0114acf7af90fa5f397890904fdc9f8.png378638478_Screenshot2021-03-23at13_07_19.thumb.png.47dab120aaa0275fffdd6cba62a8d7a4.png

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