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MG MG Y Type - The question about an ignition coil

Please let me know the meaning of CB/SW which stamp and is in the terminal of an ignition coil.

According to the electric drawing, as for SW, an ignition key and CB are wired in the distributor terminal.

Furthermore (+-), which terminal corresponds to CB/SW, as for a display ignition coil terminal?

Masaaki
XPAG Masaaki Sakaguchi

Hi Masaaki

The markings CB and SW on an ignition coil are:
CB = the terminal on the coil to which the low tension wire from the Contact Breaker is attached to.
SW = the terminal on the coil to which the wire from the ignition key SWitch is attached to.

Which corresponds to which if you have a +/- marked coil depends on the polarity of your vehicle's grounding.

This is covered on the Ignition Coil Connections item on the Technical Advice page at www.mgytypes.org. The good news is though that connecting the coil up wrongly will not be really bad, as it will still work, but at reduced power output. So, if your car is POSITIVE earth, the + goes to the points, and if your car is NEGATIVE earth, the - goes to the points. Or put another way, SW for the feed or SWitch and CB for the wire going to the Contact Breakers.

I usually have to refer to this note too when I am fitting a new coil!

Paul
Paul Barrow

Hi Pual

Thank you very much for the advice.

When a CB/SW display ignition coil is used by NEGATIVE

earth, are they CB>>> distributor and SW>>> ignition

key?

Masaaki
XPAG Masaaki Sakaguchi

Good question.

As NEGATIVE is opposite to POSITIVE to get the best from your coil I would say wire

CB to the ignition and
SW to the Contact Breaker points.

As before it will only result in a loss of power if you go the other way on a NEGATIVE EARTHed car.

Paul
Paul Barrow

Hello Masaaki,
With a coil marked CB & SW the CB terminal must always be connected to the Contact Breaker in the distributor and the SW terminal to the ingnition switch. This is regardless of the vehicle battery polarity. The coil is not polarity sensitive but it must be correctly connected.
The Kettering Ignition system used since the early days has two windings within the "coil". The high voltage winding (HT) is wound with many many turns of fine wire and connects the HT terminal to the spark plugs, The low tension (LT) winding has a few turns of heavy gauge wire, both wound concentrically around a soft iron core. The "cold" end of both windings are joined together at the CB terminal.
(see any wiring diagram in "Blower" for clarification particularly P514 the "Y" is quite clear).

The principle of operation is based on electro-magnetic induction (self & mutual induction). When the ignition is turned on and the points are closed, a current flows through the low tension winding creating a powerful magnetic field in the soft iron core. When the points open, the current in the LT winding ceases, the magnetic field in the core collapses, inducing a very high voltage in the HT winding. The magnitude of the voltage induced (generated) is directly proportional to (a) the initial strength of the magnetic field in the core, (b) the number of turns on the HT winding, (c) the speed with which the magnetic field collapses, so a high quality soft iron with little magnetic retention is used. You can see an illustration of this principle in the dynamo, the faster the dynamo turns, the greater is its output.

Also, the LT winding has a high voltage induced, and this can create a spark at the points, this energy dissipated at the points does not contribute to the spark, so, the spark is "snubbed" by the "condensor" in the distributor body, if this component fails, then there may be insufficient spark to fire the engine and certainly will burn the points badly.

You can prove this for yourself (if you feel brave!)in the following way:

Take any coil, connect the CB terminal to one side of a battery, connect a length of wire to the CB terminal, Bare about 50mm (2 inch) of the free end of this wire. Connect another length of wire to the battery terminal bare about 50mm of this wire. Hold the bared wire in each hand (you'll not feel a thing as you are connected across 12v or less) now touch the two bared ends together and then break the connection - Note the effect!

This is why we time our engines when the points just open.

With the coils marked "+" "-" the "-" terminal is the equivalent to the CB terminal.

Hope that helps
Regards
David
D. P. Jones

Wow - thanks for that David. Excellent and clear logic. I stand corrected and I appreciate the update to my knowledge!

Kind regards and thanks

Paul
Paul Barrow

Thank you very much for the advice in all.

I attached the same ignition coil (LUCAS Q12 L-O 12V Sport coil )as MG-YB to MG-PB this time.

MG-PB is NEGATIVE earth.

I connected with the SW>> ignition key SW side CB>> distribution terminal.

When PB ran for about 1 hour, engine condition has sometimes become out of condition.

I investigated the bad condition.

However, it is cured automatically.

There was a time of the bad condition continuing at a certain time.

I changed CB and SW of IG coil then.

As a result, the engine became normal.
I cannot be convinced to this result.
I wrote in under those circumstances this time.

This advice of me was consulted very much.

Thank you very much for these all advice.

Masaaki
XPAG Masaaki Sakaguchi

This thread was discussed between 04/06/2010 and 07/06/2010

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