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Triumph Repair/TR7, ignition coil


another wee question.....I have a TR7 and I am tracking the cause of it misfiring, I am currently checking the ignition coil (its a lucas 15C6)the primary reads 1.6 ohms, the secondary is not giving me a reading at all. I am at a loss as I did expect something, I applied the meter probes to the central socket that takes the ht lead and the -ve terminal

ANSWER: Hi Robert,

I don't have an ohms resistance specs on the secondary either but you should show a high resistance between the center high tension coil wire socket and the case of the coil. Your primary reading of 1.6 ohms is ok. You must not have any connection between the high tension secondary and either of the primary winding terminals.

Do you still have the Opus/Lucas ignition system on the car? That would be very rare as most failed once or more times while the car was still in warranty of one year. They were so prone to failure many aftermarket companies started making replacement ignition systems. Usually the black box on the distributor (ignition module) was left on because it supported the vacuum unit. So they would just clip the wires and install the new system. Some of the new systems used  a 12v coil and no resistor wire nor bypass system like the original system used.

If you still have the original Opus/Lucas system and need the test procedure let me know and I will outline it for you.


---------- FOLLOW-UP ----------

QUESTION: Hi Howard,
I think I know what you are saying my system has the distributor, points and condenser with the Lucas ignition coil, so its original. some guys have changed to electronic, I havent at this stage, if this sounds correct then your advice on the testing would be good. To replace it the they talk about fitting a ballast resistor with it, something I dont see on my vehicle.
one thing for sure I am learning and enjoying, thats why I bought the car

Hi Robert,

No, none of the US versions of the TR-7 had the Delco pointed distributor, They were only used on the UK and Euro versions. So if you have electronic ignition, it was installed by someone along the way. I am not familiar with the UK versions enough to tell you that the factory didn't put an electronic ignition system on the cars later.

As for an resistor, if you have a 6v coil you must use a resistor. On the US versions it is a resistor wire not a normal resistor like a ballast resistor. I have a wiring diagram of a UK version and it does show that there is a resistor wire used to power the coil.

You can test with a volt meter to see if the resistor wire is in place. Do this, disconnect the (-) side of the coil then turn the key to "ON" (not start) Test the voltage on the (+) side of the coil. It should be 12v+. leave the volt meter connected to the (+) side of the coil and use a jumper wire to ground the (-) side of the coil just for a second and read the volt meter. It should read from 6v to a high of 8v. Don't leave the jumper on the coil long as you will over heat the coil. By grounding the negative side of the coil you are placing a load on the resistor wire and that drops the voltage close to 6v for coil power.

If you see a high voltage of 11 to 12 volts when you ground the negative side of the coil then the resistor wire has either been removed or bypassed and you must install a ballast resistor on the power side of the coil (+). About 1.5 ohm resistor should work.


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Howard M. Fitzcharles III


Triumph TR-4 up & Spitfire, and Engine theory


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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Import Car magazine

ASE Master Auto with L-1 certification up to 2000

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