Polarity of Ignition Coil

[timberstone]

Purchased from ( ) a few months ago part number EL214 "New Ignition Coil model Lucas Q12"
See attached photo
Ready to install in TC0820 (Boxley) using an RF91 voltage regulator as similar to wiring diagram Number 50 of a technical bulletin -- but for NEGATIVE ground battery but not the Positive ground system that is shown,
Wonder if anyone can please confirm what polarity or terminal orientation is correct for a NEGATIVE Grounded electrical system as opposed to positive ground??
For example diagram #50 (Positive Grounded) shows the CB terminals going to the Switch and SW terminals going to the Contact Breakers, however later wiring diagrams for TD and TF show the terminals reversed for the same Positive grounded systems.
Were the coil polarities changed over the model years?
Because according to issue of "The Sacred Octagon" (December 2018 page 30 Technical Topics "Wiring the Coil") in Negative Ground systems, using the coil pictured in that article -- looks like the Lucas model Q12 -- the "CB" terminal should go to the Switch and the "SW" terminal should go to the Contact Breakers
.(From the TSO article) "If you happen to hook up the coil the wrong way, there will be a difference in the voltage delivered to the spark plugs of perhaps 15% -- 30% and could be noticeable in a car needing a good tune up."
Octagonally yours

[Duncan M]

TC's came from factory with positive grounding. The switch was connected to SW and the contact breaker was connected to CB on coil. If switching to negative ground, then those can be reversed.

[Steve Simmons]

I run the same way, although I've always wondered what they did for negative ground cars back in the days when those markings were standard. Did coils come with a label saying which way they were connected? Much easier to figure out now with + and - markings instead.

[timberstone]

According to the original owners manual, which came with TC0820 in May 1946, on page 87 the wiring diagram (Number 50 - for a positive grounded battery) shows the white wires from the A3 post of the RF91 connected to the CB terminal of the coil and shows a white and brown wire from the SW terminal of the coil connected to the distributor.

Either the diagram is wrong
OR
the polarity of the coil at the time was reversed
OR ??

What is needed is to convert this RF91 and coil to Negative grounded battery -- which means that we would need to know the polarity of the Lucas model Q10 coil reliably.
Octagonally yours, Robert Griffey

[Steve Simmons]

You can test the polarity of the coil with the old pencil trick. Then again, a coil connected backwards is only supposed to be reduced in efficiency by a few percent as I recall. I doubt that would affect a road-going TC.

To change polarity, one needs only swap the battery cables around and polarize the dynamo. Then flip the coil connections if desired. Of course if there are any electronic devices in the car then they either need to be isolated from ground, or swapped for negative ground models.

[timberstone]

The pencil trick has been used with doubtful results.
I think there must be a better way.
At any rate the efficiency of the coil is at issue due to intention to use TC0820 at speed (i.e. for hard driving such as fast racing)!
Would like to keep the Lucas Q10 coil for originality but if not able to reliably determine positive and negative for the terminals on it, then may have to find a "Sport Coil" (with + and - marked on the terminals) if I can find one, or use a coil from an old Ford tractor that has + or - marked on the terminals.
Octagonally yours,
Robert Griffey

[Steve Simmons]

I haven't had great luck with the pencil trick either, but some swear by it. You could always try this method, written by David Edgar of TC Motoring Guild (and a member here):
http://www.mossmotoring.com/coil-polarity/
(testing method down near bottom)

I was converting my older British 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.
David Edgar
La Mesa, California

[Richard Michell]

Perhaps this explains and gives an alternative test.
https://mgaguru.com/mgtech/ignition/ig104.htm

[Steve Simmons]

Looks like not only the same test, but some of the same text and graphics. It appears several sites have copied the article, but none as old as David's. I wonder if he's famous and doesn't even realize it.

[Richard Michell]

You could well be right Steve. However the article does make a definitive statement about CB and SW on a positive earth car. Whether this is an overconfident statement is a moot point.

I thought the addendum test with the LED involved may have been an addition.

[timberstone]

Wiring Diagram May 1946 Positive Ground & RF91 voltage reg.pdf

Wiring Diagram number 50 from the May 1946 MG Owners manual showing battery grounded Positive and the "CB" and "SW" terminals switched

(327.44 KiB) Downloaded 258 times

Hope the photo of Coil and PDF of the wiring diagram are attached

Thank Dave Edgar and all your posts for the pointers
Robert Griffey, Kansas City