Checking Coil Connections are correct

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geoberni
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Checking Coil Connections are correct

Postby geoberni » Fri May 24, 2019 3:33 pm

We've owned Basil for almost 2 years, and aside from a couple of breakdowns, which were eventually traced to an intermittent break in the Coil Primary winding, i.e. the one with the 12v going through it, all has been pretty good so far as engine operation.

So when replacing the Coil, I just swapped like for like with the connections without any concerns.
But having read recently that a car swapped from + Earth to - Earth would still run without changing the coil connections, I thought I'd check... yep, they were incorrect, perhaps they had been since the new loom was installed when it was rebuilt 20 years ago.

I came across a quick check on a Lotus Cortina website, which I thought I'd try, and it worked; so I thought I'd share it here.
Testing Ignition Coil Wiring Reversal.

You REALLY need to test this. With the ignition off, engine off, pull a spark plug wire loose from a spark plug. Place it loosely back in place.

Now start the engine. Hold a no.2 lead pencil carefully in one hand. With the other hand, pull the loosened spark plug wire slowly away from the plug. You shoud see a good size spark jumping from the wire to the plug. Now... CAREFULLY place the sharpened lead point of the pencil into the path of the spark... you should see a "flare" of particles flying from the pencil lead TOWARDS the spark plug. If you see a "flare" of particles flying from the pencil lead TOWARDS the wire... you have the coil hooked up backwards. Remove the pencil lead from the spark path. Replace the wire. Shut the engine off, and re-reverse the coil low-tension wires and re-test. Get it right!

Why does this matter? If you got this wrong, you will be losing about 50% of your voltage at the spark plugs! Weak spark! Essentially, if the ignition low tension coil wiring is reversed, the coil will... "pull" spark, rather than... "push" the spark. Electrons prefer to leave a high-temperature (high energy) surface (like the centre electrode of the spark plug) rather than a low-temperature (low energy) surface (like the arm of the spark plug). More voltage (more spark) is available if the electrons move in the correct direction, in the direction they prefer, from the high-temperature centre electrode of the spark plug to the (relatively) low-temperature arm of the spark plug. Test it! Get it right!
Now what I will say, is that this text is actually repeated on several websites dealing with vintage cars, and is clearly a copy n'paste from somewhere as every example I found used the same American spelling for 'Center' .
Irrespective of the stuff about 'pushing v pulling' the spark (i gave up trying to think about how it would work on a +ve Earth car), if the White and White/Black are not changed over when reversing the system polarity, the spark is jumping in reverse, because that pencil lead in the spark path trick actually works and gives a little orange flare in the direction of the spark jump.

Apologies if this is old news to everyone, it was new to me and I found it a nice little thing to try. :)
Basil the 1955 series II

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King Kenny
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Re: Checking Coil Connections are correct

Postby King Kenny » Fri May 24, 2019 7:51 pm

That's clever. My Traveller is ok but I will try that method on my old motorbike.

myoldjalopy
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Re: Checking Coil Connections are correct

Postby myoldjalopy » Sat May 25, 2019 10:48 am

That is a very interesting test. Of course, seeing which battery pole is earthed will indicate the way the coil should be wired up. If the car is positive earth (as originally) then the feed from the ignition switch goes to the -ive terminal on the coil (sometimes marked SW) and the +ive terminal takes the wire that goes to the contact breaker in the distributor (sometimes marked CB). If the car is negative earth, then the connections will be the other way around (i.e. ignition feed goes to +ive coil terminal and the -ive terminal takes the wire to the contact breaker).

geoberni
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Re: Checking Coil Connections are correct

Postby geoberni » Sat May 25, 2019 11:17 am

myoldjalopy wrote:
Sat May 25, 2019 10:48 am
That is a very interesting test. Of course, seeing which battery pole is earthed will indicate the way the coil should be wired up. If the car is positive earth (as originally) then the feed from the ignition switch goes to the -ive terminal on the coil (sometimes marked SW) and the +ive terminal takes the wire that goes to the contact breaker in the distributor (sometimes marked CB). If the car is negative earth, then the connections will be the other way around (i.e. ignition feed goes to +ive coil terminal and the -ive terminal takes the wire to the contact breaker).
Very true, but Basil was seemingly running fine and until I started to trace wiring against the diagram, and noticed the White/Black&White discrepancy, I had no reasons for my suspicions to be aroused.
I have every reason to suspect that he had been like it for almost 20 years, since the recorded rebuild.
Trying the little trick with the pencil just confirmed that it was wrong, in addition to the evidence of the wiring colours.
However, I'm hoping he'll be running a bit better now his sparks are jumping the right direction and potentially some 20-30% more powerful (I personally think the 50% quoted was a bit excessive).

I'm now going to recheck his mixture with the Colourtune in case it is any different.
Basil the 1955 series II

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myoldjalopy
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Re: Checking Coil Connections are correct

Postby myoldjalopy » Sat May 25, 2019 11:51 am

You may find your spark plugs last longer as well. I think the life of the spark plugs is shortened when the coil is wired up the wrong way as well.

geoberni
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Re: Checking Coil Connections are correct

Postby geoberni » Sat May 25, 2019 2:33 pm

myoldjalopy wrote:
Sat May 25, 2019 11:51 am
You may find your spark plugs last longer as well. I think the life of the spark plugs is shortened when the coil is wired up the wrong way as well.
Yes, it might explain why I had to change all 4 plugs a couple of months ago as he wouldn't start and I found all 4 plugs had suddenly developed a high resistance so a very weak spark.
Just so happens I found this MGA page this morning https://mgaguru.com/mgtech/ignition/ig104.htm which also mentions the pencil flare check and comments:
Why then do we worry about coil polarity? Because the spark plugs do care which way the electrons are flowing in the high tension circuit. The spark plug has a thermally insulated center electrode (surrounded by ceramic). With engine running the center electrode runs substantially hotter than the exposed end electrode. Design of the ceramic insulator determines how hot the center electrode will run, leading to the designation of hotter or colder spark plugs.
Again, I think they might have lifted it from a US website due to the Americanised spelling of 'center'. :wink:
Basil the 1955 series II

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JOWETTJAVELIN
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Re: Checking Coil Connections are correct

Postby JOWETTJAVELIN » Wed Jun 05, 2019 8:07 pm

Interesting, certainly an important point if the vehicle's polarity has been changed for some reason. Though you are better off keeping things standard. The CB and SW markings are much simpler to understand, I can also highly recommend a Runbaken Oilcoil if ever the standard coil needs replacing.
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