Coil variations & Ballast Resistor..

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johnfrancis898
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Coil variations & Ballast Resistor..

#1 Post by johnfrancis898 » Sat Apr 30, 2022 10:47 pm

While upgrading from a TC to an ‘S’ dashboard the rev counter would not work. I was reliably informed by several of the ‘Gurus’ that it was a straight swop and a ‘plug & play job’ as far as wiring is concerned. On looking at the wiring it appeared it did not correspond with the wiring diagrams. On looking at some notes from the previous owner he mentioned about some ignition problems and modified the wiring on the coil circuit and fitted a 12v coil. For some reason he had by passed the ballast resistor wire circuit and ran a wire direct from the + on the coil to the fuse box where the white wire parks before the fuse. It all worked well. Further investigation revealed the 2 wires which I was after taped back and concealed and also I could see where the ballast resistor wire connected in. After disconnecting the modified wiring and correctly connecting everything as the wiring diagrams I thought I would check the coil. I could not find any markings on it so I swopped it for the correct Lucas 15C6 which I had and knew to be ladled and good. After a final check she started on first turn of the key and the rev counter worked fine. A short test drive proved fine and a final tidy up of the wiring loom finished the job. I have seen a few posts about ballast resistors and being curious decided to investigate what goes where and does what. The ballast resistor on older models is a visible block according to the Manual but later models have a ballast resistor wire built into the loom. This is visible behind the alternator in the loom and is coloured pink & white. It is quite obvious as it is a braided wire and not pvc. It would appear to join onto a white pvc wire just before the bulkhead and appear at the end of the loom as a white wire at the fuse box. This feeds the green permanent lives which do the wipers, instruments etc. I have noticed that now it is wired correctly starting is much better
What I am unsure about is what voltage the coil should actually be as most of the coils I have do not say wether they are 6 or 12 volts rating. There is a mention on the TR site about the ballast resistor assisting starting and only a 6v coil is required but with no markings on the coil how are you supposed to know what it’s rating is? There have been similar posts on the TR site about this. There was also a mention about a 12v coil overheating and burning out if used instead of a 6v one. I am familiar with testing coils and measuring the resistance on both sides. (Coincidently Mike Allam has given this a very useful mention in the latest Six Appeal) So what I am asking here is how has everyone got on with coils and ballast resistors and more importantly identification! One supplier mentions about a ballast coil but the part numbers are the same for a non ballast one! As always all information greatly received.
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2500TC Mk 2 saloon 1976 od/PAS BRG

Charles H
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Re: Coil variations & Ballast Resistor..

#2 Post by Charles H » Sun May 01, 2022 10:40 am

If you are using the ballast wire, (or resistor in early cars) you must use the 6V coil. If you bypass the resistor, use a 12V coil. As said before, the 12V coil in a resistor circuit will break down.
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Re: Coil variations & Ballast Resistor..

#3 Post by Alec » Sun May 01, 2022 5:29 pm

John,

the primary resistance of a ballast voils is about 1.5 ohms, i.e. measured from negative terminal to the positive terminal, a twelve volt coils is about 3.0 ohms. I have seen ballast coils marked as 12v, which is very confusing.

The theory of the ballast is simply that it takes a feed from a starter motor auxiliary terminal when cranking the engine, which is at battery voltage and bypasses the ballast. This is to counter the drop in battery voltage when starting and so there is no corresponding drop in coil voltage that occurs as on a non ballast system, i.e. it gives a higher spark voltage with the ballast system.

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johnfrancis898
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Re: Coil variations & Ballast Resistor..

#4 Post by johnfrancis898 » Mon May 02, 2022 9:30 am

Thank you Alec for that comprehensive reply. That explains it all perfectly and I have now worked out what goes where and does what.
Charles, thats also useful information. My query is that coils sometimes seem to be unmarked and resistance readings can vary. There doesn’t seem to be any clear information on this. As I understand it a ballast coil is only 6v?? Luckily in my spares box I had a good original Lucas and also another good one acquired on an engine I bought both of 1976 vintage and both marked 15C6 on the base so a good clean up after a resistance check and fitted to the car they both worked fine. There are also posts about this on other sites the TR site being one. Mike Allam also provided some useful info on coils in the latest Six Appeal. As for now since restoring everything to the book the car has never started or ran better. Thanks again guys for your, as always, useful info and tips.
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