RobH Posted May 18, 2021 Report Share Posted May 18, 2021 (edited) On a recent thread about coils, people have mentioned that a note on the topic might be useful. Here is my attempt which covers all the ignition system and which I hope will fit the bill. Please let me know if you see any glaring errors. This has been updated from the original to include a bit more detail including coil polarity, and to correct a few typos. Coils 2.pdf Edited May 21, 2021 by RobH Updated to issue 2 Quote Link to post Share on other sites
Mk2 Chopper Posted May 18, 2021 Report Share Posted May 18, 2021 Just read it through, very informative read. Gareth Quote Link to post Share on other sites
John Morrison Posted May 18, 2021 Report Share Posted May 18, 2021 Magnificent work Rob, This will help many folk, yours truly included, understand just what does happen, when you flick the starter. Many thanks. John. Quote Link to post Share on other sites
John L Posted May 18, 2021 Report Share Posted May 18, 2021 Thats was just what was needed, thank you, is there a way to keep that as a locked document on the forum please? Quote Link to post Share on other sites
Charlie D Posted May 18, 2021 Report Share Posted May 18, 2021 Rob, Unless I missed it you seem to have not made any mention of changing a car from positive earth to negative earth and the need to change the connections to the coil terminals. I know it’s drawn correctly on the drawing, but some people may still be confused as to why it has to be done (I am one of those people …). I have a collection of old coils and used one for my TR3A. It is an old Lucas “Red Top” sports coil with just “CB” and “SW” marked on the connectors. (From, I guess, a positive earth car, with the "CB" terminal actually ending up connected to the positive side of the battery when the points were closed.) In my hurry to get things going I connected my negative earth car up to the terminals as they are marked, not thingking any more about it. The car has gone for almost 1000 miles like that without much problem. It’s only last week when I decided to connect up my timing light to see what was actually going on that I discovered my error. Oddly, the timing light did not flash properly. Eventually I changed the coil terminal around and the strobe gun worked as it should (I had tried turning the ferrite pickup around on the lead, to the opposite of what was expected but to no avail.) I didn’t actually notice any difference in performance when I made the change, just the correct functioning of the timing light. I’d be grateful on your thoughts about connecting the coil the right way, or does it not really matter. Excellent article, by the way. Charlie. Quote Link to post Share on other sites
RobH Posted May 18, 2021 Author Report Share Posted May 18, 2021 Ah yes that was an oversight - thanks for that Charlie. I will update the screed in due course to include it. Coil polarity does make a bit of difference but as you have found - not a lot on a well adjusted car. It's something of a bonus that your timing light showed that - I wonder if it is intentional by design? As a rule of thumb, it is usually reckoned that coils marked 'CB' are for positive-earth systems while those marked '-' are for negative earth but that may not be true in all cases. Having the 'wrong' polarity coil fitted will invert the coil output voltage. By design the voltage applied to the spark plug centre electrode should be negative and the reason for this is to ease electrical breakdown of the plug gap. Hot surfaces produce a cloud of electrons and the hottest part of a spark plug is the centre electrode. Breakdown is easiest if these electrons can be repelled from the centre and attracted across to the earth electrode and this requires a negative-going pulse with respect to earth. (Do not confuse this with the car battery polarity. The HT needs to be negative going for both positive and negative-earthed cars) A positive going pulse from the wrong sort of coil - or from the right type connected the wrong way round - will find it harder going and a higher voltage will be required to start the spark. However, most coils used with a well-adjusted distributor have sufficient voltage capability in hand except perhaps at very high revs, so this may not be apparent during normal driving. If you are in doubt about the coil polarity the 'pencil lead' test is a good way of checking. An orange flare should be seen to go from the pencil towards the spark plug. Quote Link to post Share on other sites
michaelfinnis Posted May 18, 2021 Report Share Posted May 18, 2021 Very good info Rob. Although it is based on the MGA, I found this link gave some useful tips as well; https://mgaguru.com/mgtech/ignition/ignition.htm Mike. Quote Link to post Share on other sites
c.hydes Posted May 18, 2021 Report Share Posted May 18, 2021 3 hours ago, RobH said: On a recent thread about coils, people have mentioned that a note on the topic might be useful. Here is my attempt which covers all the ignition system and which I hope will fit the bill. Please let me know if you see any glaring errors: Coils.pdf 3.6 MB · 40 downloads A very useful, and well written document Rob, so thanks. Agree it should be locked to the top of the forum menu. Colin. Quote Link to post Share on other sites
PodOne Posted May 18, 2021 Report Share Posted May 18, 2021 Another excellent bit of work which I've downloaded read and filed away! Thanks Bob Quote Link to post Share on other sites
Bleednipple Posted May 18, 2021 Report Share Posted May 18, 2021 Many thanks Rob, that's gone straight into my Useful Stuff file. Now I just need to remember I have it! Thanks again, Nigel Quote Link to post Share on other sites
Lebro Posted May 18, 2021 Report Share Posted May 18, 2021 Excellent work Rob, May I suggest you offer it for inclusion in TRaction. Bob. Quote Link to post Share on other sites
Ian Vincent Posted May 18, 2021 Report Share Posted May 18, 2021 I always thought that Bosch Red Coils were epoxy filled. Otherwise, a good read. Thks Rob. Rgds Ian Quote Link to post Share on other sites
RobH Posted May 18, 2021 Author Report Share Posted May 18, 2021 As I said in the screed Ian, that data came from a web search so maybe I hit on the wrong site and that is old gen - I know manufacture of those has been moved around a bit over the years so the spec may have changed at some time. Thanks for the comment - I will check again and correct if necessary. Quote Link to post Share on other sites
Peter Cobbold Posted May 19, 2021 Report Share Posted May 19, 2021 Many thanks Rob, downloaded for future reference. Peter Quote Link to post Share on other sites
MilesA Posted May 21, 2021 Report Share Posted May 21, 2021 Thanks Rob. Downloaded. Read. Learnt a lot. And filed. Miles Quote Link to post Share on other sites
RobH Posted May 21, 2021 Author Report Share Posted May 21, 2021 Apologies to all who have recently downloaded this - I have made a few additions, tried to improve the clarity here and there and corrected some typos (and probably made some new ones in the process!). It is now at issue 2 replacing the original attachment in the first post. Quote Link to post Share on other sites
Steve-B Posted July 6, 2023 Report Share Posted July 6, 2023 Excellent paper Rob, I've added it to my growing Dropbox TR 6 folder! Quote Link to post Share on other sites
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