Connecting up the coil

1 Sep

Steve Priston alerted me to the following details about connections to the coil.

Firstly, his experience of the running of his TC before and after the coil connections were swapped and secondly, John Saunders’ technical explanation.

Over to Steve………

“My 1948 TC did not like climbing the last part of a local, long but gradual incline, something that I had attributed to the much talked about modern fuel related problems.

It would easily maintain just over 3000 rpm, for two thirds, to three quarters of the climb, then really protest, causing me to rapidly lift off the accelerator, then after a short pause, gradually re-apply it, in doing so, losing a lot of momentum but all seemed well again.

I experienced this in a more dramatic way, when climbing another long hill but this time, requiring the use of third gear, holding 3500rpm, for the duration of the climb, knowing that the under-bonnet temperature would be high, again thinking fuel was the cause.

Then, as we have been experiencing higher ambient temperatures, my previously smooth engine, had developed a noticeable but slight harshness, felt through the accelerator pedal, when cruising on the flat, at 50mph, which earlier in the year, had certainly not been the case.

Having previously been assisted by John Saunders, drawing on his experience gained from living with a TC, I contacted him for his thoughts on my problems. I was quite taken aback by his response and the speed with which it was given and I immediately went to the car, still with him on the phone, with a torch and mirror in the other hand, to see how my coil was wired.

The phone was left on the battery box lid, for a moment, whilst I checked and sure enough, based on John’s own findings, my coil was wired incorrectly!

A couple of days later, having come off shift, I swapped the wires to the coil and out I went, heading for that damn hill again. No problems this time or traces of harshness from the engine – what a relief and thanks to John Saunders for the correct diagnosis.

I then delved through the paperwork, that had come with the car, to see when the coil was last replaced and who was responsible. It turned out to be the same company, that I discovered had claimed to have also replaced the condenser, which was definitely still the one fitted at the factory or by Lucas, by the superb standard of soldering!

The receipt was from 14 years ago, the car having covered 4000 miles like it, so I decided to treat it to a new Bosch coil, from Euro Car Parts, costing £20, just for peace of mind.”

Ed’s note: John Saunders‘ technical explanation and a diagram follows:

When the MG TC was new, if an ignition coil was to be retired for any reason, the replacement coil would be marked SW (for switch) on the low tension terminal receiving current, at 12 volts, from the battery negative terminal, through the ignition switch.

The other low tension terminal would be marked CB (contact breaker) and would be connected to the distributor low tension terminal, and thus grounded intermittently through the CB to the positive vehicle earth (see the diagram which follows).All this is well known and is not disputed or controversial.

Now step forward to modern times when all new vehicles are negative earth, and you will find, unless you buy a special coil marked SW and CB, that the coil low tension terminals are marked + and -. You will normally not receive with your purchase an illustration showing the coil internal electrical circuit arrangement as in the diagram which follows.

You will probably and understandingly connect the negative low tension coil terminal to the 12 volt negative supply terminal and the positive coil terminal to the positive distributor terminal. This is perfectly logical, but for any positive earth vehicle (e.g. MG TA, TB and TC) is exactly WRONG!

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.

So, what’s to do? Very simple; ignore the polarity markings on the new coil and install it with its “+” side toward the battery/ignition switch supply side and the “–“ terminal connecting with the distributor (CB). When the LT terminals are connected in this fashion, the current is fed to both the low tension LT coil and to the high tension HT coil, by the internal design in approximately equal proportions, thus halving the LT heating effect.

It will pay to check the resistance between the low tension terminals. For a TC in normal use, i.e. no competition, using about 5,200 RPM as a maximum, the resistance should be around 3.3 to 3.6 ohm. If it is about 2.6 ohm, as my new coil, use a ballast resistor; I have 0.8 ohm ballast fitted. An LT resistance of about 3.6 ohm should be fine for a normally used TC.

You can check coil temperature with your hand; warm to very warm is fine, too hot to touch is trouble.

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2 Responses to “Connecting up the coil”

  1. Graham Knight 08. Oct, 2019 at 8:04 pm #

    Thanks very much for an interesting article but I’m having trouble understanding the explanation. I don’t see that reversing the polarity changes the path the electrons take nor the resistance they encounter. Doesn’t it just make them flow in the reverse direction?

    I came across this,, which confirms that polarity is important but for a different reason. That is that the central electrode of the sparking plug needs to be negative since electrons prefer to jump from hot to cold.


  2. Graeme Hogg 10. Oct, 2019 at 10:54 am #

    I too have been confused as to correct coil polarity and when researching the subject came up with two articles that appear to disagree with Steve and John’s conclusions as to the correct way around to connect the coil. Both also suggest that spark plug polarity is the more important factor. Moss Motors at and MGA Guru at make for interesting reading. Perhaps Steve might check his spark plug polarity as described in the two articles and observe whether his current coil connection gives the “correct” plug polarity as defined in the articles.

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