Coil Polarity

I was converting my older Euro car over from positive to negative ground when I came across the question of coil polarity. I discovered coil polarity is very much misunderstood. In researching it, I was very confused until I found out there are two definitions of coil polarity. I talked to three or four knowledgeable people on the subject and read several technical books and articles. Everything made sense in itself but didn't jive together until I found out they were talking apples and oranges.

Definition #l Coil Polarity (In relation to battery)

The polarity of the coil should match that of the battery by connecting it so (+) goes to (+) and (-) connects to (-). But don't worry about which way you install the battery (positive or negative ground) or which way you install the coil (regardless of coil markings) it will automatically adjust itself. The coil will work efficiently and put out the same voltage either way it is hooked up, but the spark plugs are more sensitive when it comes to polarity, hence our second and more important definition.

Definition #2 Coil Polarity (In relation to spark plugs)

Coil polarity should be such so as to provide negative polarity to the spark plugs center electrode.

It has been found that it takes approximately 15% less voltage to form an arc at the plugs, if the hotter center electrode is negative, and the cooler (by comparison) ground electrode is positive. The center electrode is hotter since heat transfer from the tip must make its way through the porcelain insulator past the sealing gaskets to the shell block and then to the water jackets.

If your center electrode is positive, your car will probably still run fine until, with its 15% handicap, it exceeds the coil output. If you live where temperatures dip down to 0° you may not get your car started. Driving with a full load and accelerating hard up a hill may cause an ignition miss. If your ignition system is well worn to where you have various voltage losses, you could get a miss.

Correct coil polarity won't eliminate these problems, just put them off by 15%.

If your coil has - & + markings by the primary terminals, you will be pretty safe by hooking it up by those marks, but test it for correct polarity anyway, using one of the tests listed further on. If your coil has CB & SW or BAT & DIST markings, there is no way of telling if the coil was marked in relation to a positive or negative ground car, and the only sure way to tell if the coil is installed right is to test it out.


You test for correct polarity by hooking up a voltmeter with the negative lead to the plug terminal (which should be of negative polarity) and the positive lead to the block (which should be of positive polarity). Set the meter on the highest volt range. These connections remain the same whether you have a positive ground or negative ground electrical system. The secondary winding's polarity which we are testing is determined by the combined hookup of the battery and primary windings, so it may or may not match the battery's ground.

Cranking the engine over (you don't have to start it) should show an upward swing of the voltmeter needle (don't be concerned with taking a reading). If the needle swings down off the scale, your coil is hooked up wrong. To correct, reverse coil primary leads. Do not worry about the coil markings (refer to definition #1).

If you don't have a voltmeter, test by removing a plug wire from a plug and hold a plain lead pencil point in the path of the arc. A flair (hard to see) towards the plug shows correct polarity while a flair towards the coil shows reversed polarity. 

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Ford Barn Thread:

Original Post:  

In the Model A Ford Mechanics Handbook by Les Andrews, shows the  Black wire from the coil connecting with the Yellow/Black Tracer wires on the driver's side terminal of the box. This would then be routed by the Yellow/Black Tracer to the Ampmeter charge side and to the generator cut out. The instructions here seem to  be helpful because they identify "drivers" or "passengers" side.

*In the original Ford service bulletins, the Black wire goes to the terminal with the Yellow wire connections ( Fig 444). This would be routed to the Ampmeter discharge post and the Starter Motor

terminal. No mention of drivers or passengers side.

Also, my coil is a slant pole and does not indicate which side is positive and which is negative (drivers or passengers side would be really helpful here). Les Andrews says the red wire from the ignition switch should to the positive coil post. Therefore, the black wire should go from the negative coil post to the Ampmeter / Generator terminal post which would be Yellow / Black Tracer??

Can anyone shed some light on this wiring contradiction? I have thoroughly confused myself. But the two diagrams are clearly different.

Response #1:

Either wiring will work. The only difference is that one will make the ammeter needle twitch a little bit when you turn the key on and crank the engine (because the power going to the coil from the battery has to go through the ammeter). The other will not make the needle move because the current is taken from the battery before the ammeter.

I like having the ammeter needle move a little bit when I turn the key on and crank. It confirms that the points are working. To do that, take the power feed for the coil on the generator side of the ammeter, not the battery side.


Response #2: 
If you look closely at the Service Bulletin on the wiring change you will see that with the slant poll coil you need to crisscross the black wire from one side of the coil to the opposite side of the terminal box. This changes it for the ignition to read through the ammeter while retaining proper polarity of the slant pole coil.

Response #3:
If you read the service bulletins you will see that a change was made to the ignition circuit to pick up it's power after the amp meter. I have no idea why the original design had the ignition circuit not flow through the amp meter.


Response #4:
About the same time Ford switched sides for the feed wire, they switched coils from slant pole to straight pole. To add to the confusion Ford switched polarity on the coil posts when they switched the wiring to run the coil through the ammeter. However, a few slant pole coils were made with the polarity the same as the new straight pole coils. So, on ALMOST all slant pole coils, the driver's side is the red wire, which leads to the switch, then points, and is the POSITIVE side.

See page 390 in the Service Bulletins. If you have any questions about coil polarity, check out what is posted by the Chicagoland MG club:  http://www.chicagolandmgclub.com/tec...neral/574.html

I did the same as Jim and moved my slant pole feed wire to the other terminal, so the drain would show on the ammeter. Now while the generator is charging, the ammeter shows ONLY the amps going into the battery, where the original way it showed the amps going into the battery, plus the amp or so to run the slant pole coil.

Response #5:
Also, after the latest specified Model A Ford wiring changes, it was recommended to connect all additional Model A electrical items to the Driver's Side Terminal of the fire wall terminal box; however, "if" one obtains & installs "Logo Lite" turn signals, the Logo Lite manufacturer recommends connecting their Logo Lites to the Passenger Side Terminal of the fire wall terminal box.


Source:  http://www.chicagolandmgclub.com/techtips/general/574.html
Source: http://www.fordbarn.com/forum/showthread.php?t=89736