Electrical: Coil Polarity in January Turning Wheels

[Frank DuVal]

Is it just my reading, or is there some disconnect on thoughts from the first part of Randy Rundle's column to the second part?

In the first part he describes how to test for proper coil polarity by reading the secondary voltage on an analog meter. If this shows wrong polarity, the cure is to swap the primary terminal wiring.

Then in the second part he says if you change battery polarity, you DO NOT need to change the polarity of the coil, as the current still gets there the same way. Hunh?

Changing the primary terminal wiring is the SAME as changing the battery polarity. So if it is good for the - terminal of the coil to go to the - terminal of the battery (whether through the points on negative ground systems, or through the ignition switch in a positive ground system), the when you change the polarity of the battery, you DO need to change the coil terminals.

Did something get changed in editing?

[TWChamp]

You're right that the coil polarity should match the battery polarity, so if the battery is positive ground, then the + of the coil should go to the points. The coil output voltage is the same either way, but the plugs fire easier with the correct polarity.

The Chicagoland MG Club has the best article I've seen about coil polarity. Just Google "coil polarity" and they will be the first or second listing. It's a short but very informative read.

[Jeff_H]

Have not read the article yet....

That said, the high voltage coming out of the coil tower should be negative with respect to the engine block for best performance. Whatever connections on the low voltage (points) side it takes to make that happen is what you want. Are there pos vs neg wound coils?

There are physics reasons for wanting negative high voltage at the spark plug. Did some R&D on this about 25yrs ago at my job and forgot most of what I knew then.

[roadster dave]

Guess I'm just a doofus, but on my 27 commander it of course is a pos. ground. But the coil has a marking of Bat. and something else I cant read on the other side.
a wire coming from the add on ammeter is going to the Bat. side of the coil , then a wire from the other side of coil to the dist. Is this right? Would it mess something up to switch?

Thank you

[TWChamp]

Guess I'm just a doofus, but on my 27 commander it of course is a pos. ground. But the coil has a marking of Bat. and something else I cant read on the other side.
a wire coming from the add on ammeter is going to the Bat. side of the coil , then a wire from the other side of coil to the dist. Is this right? Would it mess something up to switch?

Thank you

You have it right. The other marking may be PTS. for points. Or, it may be DIST. for distributor.

[Frank DuVal]

I agree with TW and Jeff on coil polarity. That's why I find it hard to believe the wrong statement in the Turning Wheels column. There is no need to spread more misinformation to add to the "black magic" of electricity.

Dave, your hook up is right. It would mess up the polarity of the secondary (spark plug voltage) to switch your wires. The effect of switched polarity is poor performance. Spark does not jump the gap as easily when the polarity is switched.

[TWChamp]

I never cared for coils marked "BAT and "DIST", because unless you know what car the coil is for, you don't know which terminal is +. They should all be marked + and - IMHO.

[Dwain G.]

This whole thing involves a couple electrical theories. Not a lot of people are big on theory, including me. Some coils are wound differently internally, and this is where the test comes in. We accept that electricity flows from positive to negative, but it actually flows the other way.
A 'hotter' spark is produced if the charge sent through the wire to the spark plug terminal is negative and travels to the ground electrode. I'm not so sure about the voltmeter connections as printed in TW. The simplest common test uses the tip of a lead pencil to check the flow of electricity as shown in the attachment. The spark flare should be on the spark plug side. If it faces the wire side, then you want to reverse the wires at the coil.

[TWChamp]

If you do use a meter to test coil polarity, you might want to put a resistor between the two meter leads before touching the spark plug. Something like a 10K ohm would be fine. It will keep the high voltage from pegging the needle, but it will still move enough to detect the flow polarity. The first time I used my meter I just switched to the highest DC voltage and didn't think about using a resistor. The needle pegged hard to the right, but luckily didn't get damaged.