Ignition Coil polarity

Discussion in 'Automotive Electronics' started by MaxHeadRoom, Aug 22, 2016.

Jul 18, 2013
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Has anyone actually confirmed the way the pri and sec of the old style of ignition coil is connected internally?
IOW which are the outer pair, HV and +ve terminal or HV to -ve terminal as below, the many examples out there contradict each other by about a ratio of 1:1.
Below is one of them.
My meter does not register exact enough to confirm the difference.
Max.

2. tcmtech Well-Known Member

Nov 4, 2013
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1,447
Both are correct.

On the old engines that used positive ground electrical systems they most often had the secondary connected in series with positive post and on the engines that ran negative ground systems they had it connected to the negative post.

I've dissected many coils over the years and saw both layouts so I'm assuming that was the reasoning based on what they came off of.

Jul 18, 2013
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Thanks, I guess I will have to do a pulse/polarity test to confirm which one I have.
Although this will depend on the direction of the primary coil to the secondary!
Max.

4. crutschow Expert

Mar 14, 2008
12,538
3,072
Apply a small voltage across the primary terminals from a supply.
Measure the voltage from the secondary to each of the terminals.
The one showing the lowest voltage is the terminal the secondary is connected to.

5. ian field Distinguished Member

Oct 27, 2012
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At the end of the day - all you need to know is that the HT voltage has negative polarity.

Conventional wisdom has it that the centre electrode gets hotter than the side electrode, so electrons jump off it with much more enthusiasm. The lower voltage requirement to give a fat blue spark makes a *BIG* difference.

If you look in the right place online - there's a trick using 2 pencils to find out the polarity.

Strip the blunt ends of both pencils so you can attach the HT lead to one and ground the other. Make the spark jump between the 2 pointy ends - one end of the spark will have a tiny shower of sparks at the point. I can't remember which way round, so you'll have to look it up.

Jul 18, 2013
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Here's the puzzler:
A common (old style) Delco-Remy coil, resistance measurement between T+ & T- = 2Ω, resistance between T+ and HV ~12k, same from T- and HV (or Very close).
Apply a LiPo 12v battery to T+&T-. voltage from T+ to HV = 12v, from T- to HV = 0v??
Obviously there is no transformer effect at play as the applied is DC.
Did the test a few times just to confirm.
Max.

7. crutschow Expert

Mar 14, 2008
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Max, what's the puzzler about that?

Jul 18, 2013
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Mental block for a moment, obviously T- is the common!
Max.

9. ian field Distinguished Member

Oct 27, 2012
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Depending on the design of the coil - the HT could be anywhere between 25 - 40kV.

The back emf on the LT winding is somewhere between 200 - 600V - it isn't going to make much difference to the HT potential, which way round the LT coil is.

The only thing you need to worry about is; making sure the HT potential is negative. The electrons jump much more eagerly from the hotter centre electrode.

The sharper pointier centre electrode also means It starts a *LOT* easier in winter if you get it right.

Jul 18, 2013
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In what I am using I just need to know the common, I am not using it in the conventional automotive way.
Max.

11. ian field Distinguished Member

Oct 27, 2012
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Buy 2 identical units and "reverse engineer" one of them.

Jul 18, 2013
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It's OK I think I have it now, if right, it is post#8.
Max.

13. ian field Distinguished Member

Oct 27, 2012
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I'm kindof thinking; if it matters that much - you can find out by trial and error.

You have a 50% chance of getting it right first time.......................

14. Tonyr1084 Active Member

Sep 24, 2015
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Make two small coils of magnet wire. Place them one on each side of a compass. Power them with a small voltage (say - a AA cell battery). Note the polarity and which way the compass moves. Next, hook up the high voltage output to the coils. Tap the ignition coil with the same small voltage. Obviously you'll get a much higher voltage out of the coil when you release the connection, but in that instant the compass will either swing or spin toward the positive (or negative) coil. Compare the results. If the first test revealed that a positive charge across the coil moved the compass counter clockwise and the second test with the high voltage pulse also moved the compass counter clockwise then you've determined polarity for your ignition coil.

I think anyway. I've never done this. But it seems logical to me. At least I think this is what you're trying to determine.