Negative earth coil!

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Joined: Mon Sep 07, 2015 4:38 pm

Negative earth coil!

Post by TonyG » Tue Apr 14, 2020 10:33 am


Further to my recent post regarding ‘fuel going off’, the reply from Luli sent me in the direction of a stronger spark. The logic here is that effective combustion is a combination of an appropriate fuel/air mixture, compression and a good spark to get it going. As the petrol gets old and loses it’s volatility the ability to ignite it with a weak spark diminishes and I think this is what has been happening with my Tourer engine. Changing to fresh fuel resolved the issue but that is only part of the problem.

Knowing that the timing is correct and the plugs, points etc in good condition, I turned my attention to the coil as per Luli’s suggestion. My first thought was that I may have connected it incorrectly when I re-wired the car and changed it to negative earth. However, the new coil was connected negative towards the points and positive towards the ignition, so all ok there. This set me off researching coils to ensure that the simple reversal of coil connections was, in fact, correct when changing the car’s polarity. While this is the perceived wisdom that all retailers of coils like to convey, it is only partly correct and something we need to consider when changing polarity from positive to negative earth.

Put simply, electricity flows from neg to pos and we want the spark at the plug to jump from the centre electrode to the L/earth point. So the power produced by the coil needs to be a negative charge. Although it’s counter intuitive this will occur even in a negative earth situation due to the potential difference in voltage, I.e 20k volts v 12 volts. If the coil connections are left unchanged when we change the car from pos to neg earth the coil will produce a positive charge and the spark at the plug will jump the wrong way. The car will run ok but the plugs, distributor cap and rotor arm will burn out more quickly.
So, simply connecting the coil the other way round as the retailers tell us to solves this? Not completely, the spark is negative but now the internal windings of the coil are compromised. This is because the primary winding is in series with the secondary to create what Lucas refer to in their Technical Service Overseas Training as ‘Auto Transformer Action’ which boosts the output. This is a publication probably from the ‘50s, so relevant to our cars. The solution, according to Lucas, is a Negative Earth Coil, which they used to sell.

Fortunately, I had an old Lucas coil labelled ‘for use on neg earth only’ and was able to fit this to my car. I now have instant starting and expect this to continue even as the fuel starts to degrade.

Where does that leave us when we need a new coil though? Research suggests that most aftermarket coils are based on old designs by Lucas and often made in Africa. These will, therefore, be designed for positive earth installation so ok if you have left your car as originally wired. For those of us with negative earth I’ve yet to find a new coil sold as specifically wound to suit us, so it’s more of a lottery. My car is modified for higher compression so requires an especially strong spark. Standard engines may be ok with a new coil appropriately connected. If not, I guess a higher output coil will be required to overcome the inherent shortfall in power. Research on various forums suggests that the Petromix Flamethrower coil is best suited to negative earth working. I’ve not tried one so cannot recommend. However, if you use a more powerful coil this may impact the condenser/points.

To conclude, the suggestion of negative earth coils is regarded as a bit of a joke, a bit like a left handed screwdriver! According to Lucas they definitely used to make them and I’ve got one. I’ve seen other references to them on line in other forums too. However, retailers will say it’s a simple matter of connecting the coil differently. For most applications this is a solution but it isn’t what Lucas used to recommend.

I hope this helps anyone else who is puzzling around the subject of coils and/or poor starting.

Tony Gilbert

P1 12 Tourer
P2 12 6 Light Saloon
Discovery 3
Discovery Sport

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Re: Negative earth coil!

Post by luli » Fri Apr 17, 2020 5:59 pm

Very nice, Tony. It turns out that the condition of the condenser is also important. I have recently acquired a digital capacitance meter and was supersized by the results. It measures both inductance and capacitance and makes it possible to evaluate the ignition circuit performance. See here I have also experimented with various electronic ignition and got nice results.
Rover 10 1946 RHD
Rover 10 1947 LHD
Rover 12 1947 tourer LHD

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