Coil polarity question..

[SteveT]

My TC coil seems to run hotter than normal. Reading an article on TTypes.. it says that modern replacement coils no longer marked CB and SW but + and - are really made for negative earth vehicles. So when fitting one to a positive earth car , like my TC, it is incorrect to connect the distributor wire to + as you would logically do. The article states to wire it the other way round if still on positive earth, I.e. - to CB and + to SW, the opposite of what you think.
Wiring it + to earth CB is wrong and can cause the coil to overheat. Is this right and should I go - to CB then?

[Steve Simmons]

Wiring the coil backwards will slightly reduce its efficiency, but I've never head of it causing the coil to heat up more. If you want to experiment, you can reverse the leads and see how it goes. The coil won't care. But it's true that standard coils these days are made for negative ground. There's an old test you can do with a pencil to see if it's the right way around. Put the pencil between the wire and plug, and see which side has the bigger spark. It should look like this, and if it's backwards then so is the coil polarity.

[SteveT]

Thanks Steve, here is a snip from the article..

The reason is that when the coil terminal marked “–“ is fed directly from the battery, via the ignition switch, the coil primary winding carries the full battery output to earth, through the contact breaker, with a higher current load the faster the car is run. This quickly leads to significant overheating of the coil and missing ignition pulses, and ultimately to an internal short circuit in the high tension coil and complete coil failure. This has happened to me twice, necessitating a tow home by a friendly neighbor each time.

[timberstone]

There may be some heat that can be derived from the coil post leads being reversed. However, the usual reason the coil is overheating may be due to a short in the circuit somewhere else than inside the coil. Examples: the wire from the coil to the distributor can come into contact with loose parts such as the tachometer drive gear box that can droop, or due to defective insulation of the connection post on the side of the distributor body. Both of these are common occurrences that may cause the coil to heat up and short out, usually more so than any defect inside the coil. Also, the engine will have difficulty in starting and running.

Please keep an open mind about what is written concerning coil polarity and wiring to the distributor. With a positive ground car, using a generic replacement coil, the positive (+) post of the coil should be wired to the CB post of the distributor. This is because most generic modern replacement coils are usually intended for negative grounded cars; therefore mandating the reversal of posts.
If however, you have the ORIGINAL Lucas coil, that is intended for a positive grounded car, then connect the wire from the CB post (or actually the positive + post) to the side of the distributor where the contact breaker points are located and connect the white wire from the ignition leads to the SW (or negative - post).
Coils labeled with just "+" or "-" should be connected appropriately for the polarity of the system circuitry. "-" to the CB post of the distributor on a negative grounded car or "+" to the CB post of the distributor on a positive grounded car.

Note, that the spark derived from the primary lead of the coil is good regardless of which terminal lead from the coil is connected to the distributor. There is only a fractional decrease in spark output if the wires are reversed, and the spark at the plug gaps hardly notices the difference. This point is verified by recent studies noted by the University of Manchester.

[SteveT]

Thanks, I apologise for being rather dense on this but Timberstone you say..

With a positive ground car, using a generic replacement coil, the positive (+) post of the coil should be wired to the CB post of the distributor. This is because most generic modern replacement coils are usually intended for negative grounded cars; therefore mandating the reversal of posts.

Surely on a positive earth car and using a modern replacement coil, reversing the posts as you suggest would mean the (-) negative wired to CB?

I have what I assume is a modern coil fitted so i'm still unclear to reverse or not?

[Charles]

Timberstone is exactly right. Unfortunately on my 1936 T.A. (TIZI) not only the wire from the coil to the distributor can come into contact with loose parts such as the tachometer drive gear box - so can the HT feed to the distributor. It has burnt out, rather completely fried, the entire Distributor ( Bakelite base and cap) as well as both connections to the LT side of the coil. It smelt terrible. I hate to think what the ignition coil looks like inside ! After surviving 85 years it took ME to screw it up. I am not sure if any additional in line fuse would help, or, if so, where it should be. Fortunately only one small wire in the loom was affected. The one from the coil to the fusebox. Of course the fuses were 35A when it says they should be 25A. Now all corrected. Now plan to heat shrink insulation around the Rev counter cable near the HT leads. Charles

[Steve Simmons]

Fear not, Charles. You aren't the first to have the tach reduction box contact the distributor lead. You can rotate the distributor 90-degrees (or any other amount) and re-orient the plug wires to suit. Or install an o-ring under the threads of the tach box to prevent it from loosening. Or cut a piece of old inner tube and stretch it around the box. But I'm curious how the coil HT lead fried the distributor? The tach box making contact with the side terminal will only cut the ignition due to the short.

[Eric Worpe]

There's an article on coil polarity in Totally T Type 2 Issue 57 December 2019
https://ttypes.org/a-clarification-of-t ... on-system/ that provides some answers.

[John Cockrem]

Charles, for what its worth some years ago my TC2268 was running rough and upon inspection of the dizzy cap, I saw to my horror that the area around the carbon contact was seriously burnt away and I couldn't believe that the car ran at all.
I think what happened, and I could be wrong, is that the knurled acorn nut attaching the HT lead had come loose and

the spark was sort of jumping straight through to the rotor button resulting in the burn. And it was pretty bad as I needed a new cap.
Anyway today I checked the acorn nuts on the distributor and coil and they were all pretty loose, including the affore mentioned HT one. I was surprised at the amount of turning most of the nuts needed to be secure again. So the moral of this story is make sure these little sods are tight without busting the things. Any thoughts on this gents?
John Cockrem TC2268