XJ12L S1 hot coil & balance resistor


I’m new to the forum and would like some advice please.

I have recently become the proud custodian of a 1973 s1 XJ12L. The car always run on the hot side, but after a relatively short time both the coil and ballast resistor are too hot to go near, let alone touch!

Using the Haynes manual the various components were checked for voltage, and the balance resistor is drawing 3 time the voltage it should. Haynes says that this means the amplifier has failed.

I have decided to bite the billet and change the entire system of coil, ballast resistor, amplifier and distributor and there are 2 obvious alternatives, both costing about the same £650.

Do I go for a Lumenition system or a replacement for the OEM Opus kit sold by SNG Barrett/Holden? Any thoughts would be appreciated.

1 Like

Hello Nigel,
if your OPUS amplifier has lasted 50 years then it’s done very well !

When faced with a similar problem I chose the CRANE XR700 system which is a kit to modify your existing distributor with an optical trigger and slit wheel. You get a replacement amplifier and keep your existing ballast resistor and coil (assuming they are AOK). Significantly cheaper than your proposed alternatives and mine has been problem free for past 15 years. Search the JL archives and Kirby Palms book for more info.

Best regards,
Mike Badger
1973 DDS S1

Good evening Nigel,

congratulations to such a nice car! Being a temporary custodian is a very comforting notion as well:-)

Though I know even less about 12 cyl engines than about XK engines I’ve heard very often that the 12 cylinder engine, while very robust as of itself, reacts sensitively against overheating. So, when you say “run on the hot side”, what are you really talking about: is it engine water or oil temperature, engine bay air temperature or just the ignition elements?

If the former you better address this question: 105°C water temp is no problem, but overheating is very bad.

If the latter I’d first try and check, clean, file and lube all the ground connections and other connectors. Poor connections give high resistance give high temperatures.

Other than that - if the engine runs well, I find it hard to believe that any central part of the ignition sytem has failed. Usually, things are the other way round, as broken ignition parts typically lead to immediate problems …

Even if the coil heats up I’d start with the coil and the ballast resistor. Then comes the amplifier. The distributor was good when it was new and so it should still be good if it is in good shape. The only terminal failure would be a wobbling shaft - but that has nothing to do with your heat problem. With new contacts and cap and a bit of oil all should be well.

Cars age and parts fail, but in my experience way too many parts get exchanged because it is assumed that - due to their age - they must have failed, even though the issue is either very minor - mostly lost contacts due to corrosion - or elsewhere.

Good luck and keep us posted


75 XJ6L 4.2 auto (UK spec)

Welcome, Nigel!

As Jochen says; it’s more to this than meets the eye; the sole purpose of the ignition system is to deliver spark at the right time - and if the engine performs as it should (does it?); the present system works as it should. Have you checked the coil resistance?

Voltage drops relates to current flow and resistance, have you checked the resistance in the resistor unit. Coil and resistors normally do get hot, but will burn out if excessive…

But if you have decided to change the ignition system - fair enough…:slight_smile:

You also said the engine runs ‘on the hot side’ - this is, as Jochen says, another matter, not related to the ignition system fitted. Have you checked ignition timing? Wrong timing may cause hot running and should be checked/adjusted.

Prime suspects in hot running is the cooling system itself - get an infrared thermometer for verifications; the dash gauge is good enough. The radiator is a prime suspect, of course but difficult to assess; the IR will enable inlet and outlet temps to be measured. It’s not conclusive, but an indicator. The V12 has ‘separate’ circulations for the two banks with separate thermostats; a thermostat failure may lead to hot running - the IR will provide clues.

In any case; both ignition and hot running should be pursued as separate issues…

Good luck with your acquisition…

xj6 85 Sov Europe (UK/NZ)