Weak Spark 76 XJS

[andrewh]

I have a 1976 XJS with Hopeless Opus but Opus 2 maybe. I have checked the spark and its pretty weak although the car runs its not really burning up the fuel/air mix fully in my view. Checked the coil , which is the original, and its showing a resistance of 1.4 OHM which I think is about correct for a ballasted system. Just wondering whether any of you had any ideas how to improve the spark which on a spark plug test lead was pretty pathetic. Its probably the coil but would be interested in anyones views as to what coil resistance I should be expecting. thanks

[abowie]

"Weak spark" can sometimes be a bit subjective, especially if the car runs. Adequate combustion is more a matter of things such as head design, mixture, compression, spark timing and engine wear than of spark itself.

I agree that the coil resistance sounds ok to me. You could try another coil and see if it looks better. Check you have the coil polarity correct. Some will tell you that you get a difference of up to 30% depending on which way it's wired.

[cactusman]

If the car runs happily then I'd say all is well. The coil produces it spark voltage when the points, or in your case the opus system, opens and disconnects the primary winding. The rapid collapse of the magnetic field induces a large voltage in the secondary. So if the disconnect is incomplete (faulty opus) the spark will be weak. Equally if when closed there is a poor connection and lower than usual magnetic field in the coil you get a poor spark. Ideally the coil produces a negative going spike so electrons are emitted by the centre electrode in the plug. Providing your coil primary is connected correctly it is wound so this will be the case. If the car runs poorly especially under load or acceleration and it is spark related I would suspect the opus unit before the coil.

[andrewh]

thanks Chaps. To expand. The car has a misfire and a lack of power. as this is a fuel injected engine I have been concentrating on Fuel pressure, air leaks, temperature sensors, ECUs, Injectors , injector wiring and all manner of other things. Nothing yet has shown up as an obvious fault. Checking the ECU requires a special machine called a Lucas Epitest or an Oscilloscope and a good working knowledge . This led me to pop a spark tester on some of the leads and hence my concern that the spark is pretty insipid tbh.
Looking at the XJS manual it suggest the resistance of the coil should be 0.8 to 1.0 ohm. I am running at 1.8 ohm on the coil. I must admit I thought Ballasted coils should run about 1.4 to 1.8 Ohm so the specs in the book seems very low. Perhaps its a special coil. Any thoughts on the effect of a coil with too high a resistance?

Reference the Opus , I shall look again in the Manual to see whether there is a specific check . I totally get your point about a potential problem with this. thanks andrew

[cactusman]

Unless you have a high quality meter, e.g. Fluke, measuring low resistance directly using a ten pound meter from a motor factor is pointless as they are remarkably inaccurate at low resistance measurements. Anyway the primary will either be good or open circuit as it has few turns of thick wire. If you doubt the coil best to replace although if indeed it is the spark that is poor I would suspect the opus or wiring

[MarekH]

Dear Andrew,
You are partly answering your own question:- a coil with a high resistance will pull less current than a lower resistance coil and so make a less powerful spark at the same voltage.

Two gotchas:- firstly, the XJS manual may not cover the car you think it does. The original XJS ignition setup was much as the Etype was, then came the Lucas CEI with twin coils in parallel, to get the coil resistance down, then it changed again. Secondly, it is very difficult to measure coil resistance. Have a readup of the Roger Bywater website on this topic.

Implicit in the first paragraph is that a lower voltage gives a smaller spark, hence the ballast bypass at cranking, so it's be good advice to check the integrity of the wiring and of the connectors. I'd check the resistance of the wiring between all of the components as you may see a significant voltage drop there which was unanticipated.

It'd also be easy to substitute in a cold or preheated hot coil or spare ballast resistor and compare.

kind regards
Marek

[andrewh]

Thanks Marek. I do have the correct manual, specifically for the Pre HE and not the HE with CEI. I have also read the Roger Bywater website and followed much of his advice. I have moved on from the coil, after removing the coil to distributor lead and testing the spark there. It's ferocious. My latest idea is to look at a faulty rotor arm. They are apparently prone to losing a lot of the energy. I shall investigate , although a preliminary inspection did not throw up any tell tale arcing marks. Thanks for the intel on the coil resistance. I do have a proper old school meter which I can use to test accurately, however, I have found information on the resistance for my coil, and its in line with what my meter is showing. Perhaps the Jaguar Operator Manual is incorrect with its quoted figure?

[Bob.]

Hi Andrew,
I appreciate that you have moved on from the coil, but just for the record a contemporary Lucas Technical Service Note (not numbered) describes the coil having been specially designed for use with the original OPUS system on EV12. It quotes Lucas Pt No 13C12 having a primary resistance of 0.8 to 1.0 ohm at
20 degC. (This coil is now sold as DLB198)
These are the same figures quoted by Jaguar in the EV12 ROM.
Bob

[andrewh]

Thanks Bob. Yes the coil is 13C12 and says clearly for OPUS. I am right in the middle of some more diagnostics at the moment, just popped in for a tea! Will report on findings, but it seems that some plugs are sparking and some are not. I need to work out whether its a problem with the distributor OPUS eye or what ever it has, or the rotor, leads or cap. More soon I hope

[andrewh]

So I think i may have got nearly, to the bottom of my misfire. 2 cylinders are not receiving any spark at all. Looking inside the brand new Distributor cap, 10 of the contacts have been working, and one looks like it is brand new, no firing. The other one has been firing a little. Both of these contacts correspond to the 2 cylinders 1A and 2A , that are not firing. I have concluded that its not the cap, not the rotor arm but possibly the timing rotor. This is the flat plastic disc with the 12 ferrite rods embedded that stimulate the action of the coil when they pass the pick up module. I had heard tales of these dropping out of the plastic underheat. Mine are all there, but one looks like its only half length and another looks pretty corroded. I have ordered a new timing rotor which I am hoping will solve the mysterious lack of a spark on those two cylinders. If its not that, it could be the air gap not being correct or the pick up module . I will report back but if anyone has any other ideas whilst the dizzy is stripped down do pass them on !!

[cactusman]

Be worth cleaning the affected plugs too....use a plastic brush....an old tooth brush is ideal and a little petrol or methylated spirit. Avoid using a wire brush as it may damage the insulator.

[andrewh]

I will clean the spark plugs but to be fair they really dont need a lot as they have not been firing. Left to atmosphere and the fuel soon dried out. I am looking forward to sorting this out now. I have owned this car for 12 months and whilst I have been concentrating on a full bare metal respray and other issues , its always run on 9 or 10 cylinders. ( one injector is known to be bad which is the 3rd misfire ) Hoping it is the timing rotor!

[Bob.]

Hi Andrew,
It may work OK but when you get the new rotor check how accurately the ferrite rods have been set in the moulding. The one I bought from one of the usuals had some rods breaking the outer rim, some set well in. Not what you want when the coil to rotor gap only has a 2thou tolerance (0.020-0.022"). I sourced a good used original rotor.
Bob

[andrewh]

Ummhm that doesn't sound good! I have opted for the Jaguar OEM part so I am hoping that they still have standards of manufacturing that will be with tolerations. Gee whizz, its getting harder to keep these cars on the road as they need to be. I shall report back anyway thx Bob