LarryH57 Posted February 6, 2015 Share Posted February 6, 2015 I'm working with a friend on a 1942 Hillman Tilly, that should by all accounts be NEGATIVE earth but is actually Positive! As there is not much in the way of electrics in a Hillman Tilly (just lights and ignition and possibly a fuel pump) some have suggested that its an easy job to do. So what's involved in changing it from Positive Earth to Negative Earth? Quote Link to comment Share on other sites More sharing options...
arcot1751 Posted February 6, 2015 Share Posted February 6, 2015 Very easy type into google and it will tell you everything you need to know Quote Link to comment Share on other sites More sharing options...
retriever Posted February 6, 2015 Share Posted February 6, 2015 Swap battery terminals over. Swap over coil connections neg or - or CB to dist Pos + or bat to ign switch, If you hasve a ammeter then change over the connectos on back. Then take a wire from the battery + and touch on the dynamo field connecter for about a second you will get a spark but thats normal thisd will change the polarity of the dynamo.the field connector is normally the smaller one. The car should be now negative earth. Quote Link to comment Share on other sites More sharing options...
Lauren Child Posted February 7, 2015 Share Posted February 7, 2015 Swap battery terminals over. Swap over coil connections neg or - or CB to dist Pos + or bat to ign switch, If you hasve a ammeter then change over the connectos on back. Then take a wire from the battery + and touch on the dynamo field connecter for about a second you will get a spark but thats normal thisd will change the polarity of the dynamo.the field connector is normally the smaller one. The car should be now negative earth. Surely you'd need to change the regulator over as well? Quote Link to comment Share on other sites More sharing options...
fv1609 Posted February 7, 2015 Share Posted February 7, 2015 Didn't seem to be a problem Lauren when I changed my Mini's polarity in the 1960s. It is quite correct to change the terminal on the ignition coil in order to maintain the polarity of the HT so that you still have negative spark. A point often missed is that using a coil designed say for positive earth but wired for negative earth operation will not give quite so much output as a negative earth coil in a negative earth set up. When the points open & the magnetic field collapses inducing an EMF in the secondary winding this is the HT. But there is also an EMF of about 300v released from the primary. The spark at the points is not the battery spark it is this 300v. The windings are so arranged that this 300v is added to the HT. If you use a coil designed for a different polarity this 300v is subtracted from the HT. So in effect you lose 600v of HT compared with a proper polarity coil. May not be significant in a low compression engine but if there are other compromises in the HT system there's no point in introducing a 600v loss. There is one electrical site that is often quoted on Land Rover sites covering many useful topics but the author claims that the coil automatically adjusts for this change. This is cobblers a coil can't rewind itself to the correct phase. This is why there are negative or positive earth coils, they are not universal polarity! Quote Link to comment Share on other sites More sharing options...
Richard Farrant Posted February 7, 2015 Share Posted February 7, 2015 This is why there are negative or positive earth coils, they are not universal polarity! Hi Clive, to add to your comments on coils, another one to watch is those coils marked only SW and CB with no signs of polarity, they do exist, mainly old stock, but determining whether they were design for NEG or POS earth systems might mean checking any part number on it and going through the makers catalogue. If in doubt, replace with one of known polarity. cheers Richard Quote Link to comment Share on other sites More sharing options...
fv1609 Posted February 7, 2015 Share Posted February 7, 2015 Richard yes I see some Lucas coils with a Sellotape type label with polarity, which of course is soon lost. If they can stamp the part number & a date code on the can it seems strange that the design polarity can't also be permanently recorded. Quote Link to comment Share on other sites More sharing options...
retriever Posted February 8, 2015 Share Posted February 8, 2015 Regulators are not normally polarity sensitve and will work with pos or neg earth Quote Link to comment Share on other sites More sharing options...
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