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Coil to Distributor wiring?


Senior Member
I read somewhere that you must run the negative wire from the coil to the distributor, NOT the positive or you will shorten the life of the points. Anyone know if this is correct?


Jedi Knight
Assuming that the car is negative ground then I believe you are correct.


Great Pumpkin - R.I.P
I don't believe it has anything to do with actual wear of the points. What happens is that it's easier for the spark to jump the plug gap in one direction; so if you get the spark polarity backwards, the engine will misfire easier than with it right. But in a sense, that would correspond to shorter point life as it will start "needing a tuneup" sooner with the coil wired backwards.

As noted, if the car is negative ground, then the negative terminal of the coil goes to the points. Positive ground, positive terminal to points.


Luke Skywalker
If you want points to last until the spring wears out (10-12K) use an MSD6 unit. No pitting, no dwell issues, not gap sensitive.


Jedi Knight
TR3driver said:
What happens is that it's easier for the spark to jump the plug gap in one direction...

Can one of the professors here explain <span style="font-style: italic">why</span> this would be the case? I see two pieces of metal, separated by an air gap. Why does the spark plug care which direction the electrons jump that gap? Honest question... I don't understand...


The gap the electricity jumps is the spark gap. The effort increases with increased compression.

Used to be an old autoshop test to tell if the polarity was correct was to take a pencil, and hold it between the plug wire and plug end cap closely. Watch where the flash of electricity occurred. If it was above the pencil it was one polarity, if below the pencil the other way. or some such. It's been so long now I forget....

Might find it interesting, and informative to do a search for it.. Electrical scientists found that it slightly easier for the electron flow, in a compressed environment(combustion chamber) with negative ground....


Great Pumpkin - R.I.P
Geo Hahn said:
Did you mean 'points gap'?
Nope, plug gap. Ideally it doesn't jump the point gap at all (which is the main function of the condenser).

The theory at least is that it's easier to pry electrons from the center electrode of the plug (which is a special alloy that releases electrons easily) than from the (usually iron) ground electrode. Electrons are of course negative, so making the plug terminal negative means electrons jump from the center electrode to the ground electrode.

Some sources claim as much as 40% more voltage is required to fire the plug in the 'wrong' direction. Personally I think the difference is much smaller than that, but I've never actually tried to measure it so I don't really know.

But it's worth noting that many modern distributorless engines fire half of their plugs backwards all the time. I had an old Honda motorcycle that worked that way too, and it seemed to run OK. By using a single coil to fire 2 plugs in series (one on each side of the high tension winding), these "waste spark" systems get by with half as many coils as would be otherwise required. Eg, that 2-cylinder Honda had only one ignition coil, with two high tension terminals.


Great Pumpkin - R.I.P
Oops, forgot to add: In the old pencil lead test, you should be able to see brighter sparks between the pencil tip and the plug, than between the pencil tip and the plug lead.


Great Pumpkin - R.I.P
prb51 said:
If you want points to last until the spring wears out (10-12K) use an MSD6 unit. No pitting, no dwell issues, not gap sensitive.
:iagree: However, on a TR2-4, the spark from the MSD will toast the center conductor in the distributor cap! BTDT; used a paper clip to get home :banana:

PS, I got a lot more than 12K miles out of those points. Never changed them again after installing the MSD6; had to be at least 75K or so. But I did reset the gap a few times, as the rubbing block wore a bit.

Had to go colder on the plugs, too.


Darth Vader
The spark in aviation engines with a magneto ignition system flows from the center electrode to the ground electrode in half the spark plugs. The spark flows from the ground electrode to the center electrode in the other half. This alternates as you go down the firing order. Like:

Cyl 1: center to ground
Cyl 3: ground to center
Cyl 4: center to ground

When the electrons jump off of one electrode, they take a piece of material with them. When they hit the other electrode, they knock off a bigger piece. So the positive electrode will wear faster. To get more life out of a plug, you put them back into the next cylinder in the firing order after you service them. This will reverse the flow through the plug and over time the electrodes will wear at the same rate and you'll get more hours out of your plugs because you have to throw them out if either of the electrodes are too worn.

Nothing to do with the thread, but I think it's interesting.
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