Conversion to Negative Earth

[Anonymous]

My Slough Light 15 was, naturally, designed as +ve earth. I thought that it would be a good idea to fit a USB charger for phone and satnav (sorry for the bad language) but assumed (correctly I think) that they will all be -ve earth as all new vehicles are -ve earth.

So far so good – re-polarise the dynamo, get the clock converted to -ve earth while it is being repaired and away we go.

Reading the Lucas training manual on ignition systems, I discovered some things about coils that I did not know before. Coils are designed for either +ve or -ve earth. In addition, according to the prince of darkness, it takes 10% less voltage to spark across a plug with +ve earth than with -ve earth. I assume that this is because the electrons will be dragged from the pointed, central, electrode more easily than from the flat electrode.

I am now starting to wonder whether I have made the right decision. Maybe I should modify a USB socket, insulating it as necessary, and stick with +ve earth. Converting the clock back to +ve earth will not be cheap but the alternative is getting another coil.

Also, I don’t think the capacitors are polarity sensitive but am worrying that they may be.

Any advice from anyone who has made the conversion or decided not to, would be appreciated.

Roger

[Anonymous]

I did it on my Light 15, the only thing that was different was that the ammeter reads in reverse. It was a matter if switching the wires to it but I was too lazy to bother. Nothing else changed in any way. I just turned the battery around. The only reason I did this was to fit electronic turn signal flashers in conjunction with the trafficators.

[Anonymous]

Thanks for the feedback. I am being convinced that there is nothing to worry about. Sometimes the theory is not so bad in practice. 🙂
Roger

[Anonymous]

Hi Roger,
Electrons jump better from a hot surface to a cold one than the other way around. The centre electrode is much hotter than the outer one so you won’t get such a good spark if you don’t preserve current direction in the spark plug. So, ideally, if you swap the battery polarity, you should swap the two low voltage connections around on the coil. You don’t need to change the coil. Neither end of the low tension circuit is earthed.
I’d recommend changing to -ve earth – can’t imagine driving around without a satnav these days.
Chris

[Anonymous]

Thanks Chris,
I will make sure my sparks are jumping the right way.

Regards,
Roger

[Anonymous]

There is an argument that a positively earthed car creates more electro-chemical reaction with it’s surroundings and consequently develops more rust than a negatively earth vehicle, but I do not think there is substantial proof of this.
I would disagree with Chris though: ignition coils are designed to be either positive or negative earth, whilst it is true that swapping the LT connections on a positive earthed coil will work on a negative earthed car, it is better to swap it for a modern negative coil. I am not aware that a condenser is polarity sensitive, but again I would swop it for a new one.

[Anonymous]

On my Light 15, I didn’t do anything to the coil and it worked fine.

[Anonymous]

I believe from the Lucas tech information that there is a difference between +ve and -ve coils. I need to check on the theory when I have time but, basically, when you break the primary circuit a reverse voltage is developed in the primary winding and, if the polarity is correct, it boosts the secondary voltage. If incorrect, it opposes it. I am not sure whether swapping the coil connections sorts this out or not. As I said, I must have a look at the theory.

However, it seems that, in practice, the difference is not enough to worry anyone. I will continue with -ve earth and worry about it if I have a problem.

Thanks to all for the feedback.

Roger

[Author]

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