Correct coil for my 81 XJS? Just replacing it with one

I got my ECU’s back from Roger, aj6engineering. He found no faults it either one. Which I am kinda glad but would have loved for it to be my problem. After over 7 months, I put the 6cu back in, it started right up after the fuel circulated. I drove it around a bit, nice, smooth thinking “cool”! Then I gave it some gas, lots and it immediately bogged down and backfired through the exhaust a few times. Not a steady backfiring but a few and around 4k rpms where the engine would not rev past!

I have replaced practically everything fuel or ignition related. Tank, pump, regulators, had injectors cleaned, new stuff in the dist, wires, plugs, ignition module. I have a vacuum switch and a manual switch that works together to richen the fuel. The throttle switch reads like its supposed to.

So, the one thing that is about 4 years old is the ignition coil. Really not old for a coil but its about the last thing I can think of. I forget the ohm reading I need for a single coil so was going to ask if there are any preferable ones that most have had good service with. Any suggestions?

It revs up nicely to around 4k then It just bogs, the occasional backfire, almost like a rev limiter but I am pretty sure these did not come with that feature.

Thanks

Russ

Almost everybody, seemingly, just goes with the DAC6093. All the usual Jag vendors will have it, or just Google the part number

Cheers
DD

that’s the one I bought, and have had zero issues.

I originally bought a PerTronix FlameThrower. Most people here on the forum had good results, but I must have got a bad one, as it ran terrible with it.

If you leave the tranny in park, can you rev past 4k under no load ?
Not quite the same as 4k under load, because under load much less vacuum.

Does it make a difference when driving if you slowly accelerate to 4k as opposed to planting your foot ?
That gives a clue.

I would check that dizzy again, an outside possibility the rotor has a gap to the terminal posts at 4k and you are not firing the plugs. A new coil might help too.

Changed my two oil drum coils for one rapid firing French made solid state coil. Had all sorts of problems under load. The supplier said it was common for these to fail. Sent me a couple of oil drums to replace it and all’s good. That was three years ago.

It’s well documented in The Book that coil failure will cause break down at 5000 RPM.

I had the same, and switched to the Ducellier single coil mentioned above, and the problem’s gone.

Rob

Well, you guys were right. It was the coil. Broke down at around 4k on my car in park. After putting a new one in it drives perfect! Power, idles without a miss, starts right up. Nice to finally be driving it again. It was an Accel coil, round, oil filled.
I looked for the DAC 6093 and found some but Jagbits was the only Jaguar parts store I could find one. $99.00. Not sure if I should go with the DAC 6093 now but there were no numbers as far as resistance on the Accel coil I bought. I don’t know if it will break down the spark as I drive it or if it would have not worked from the begining.
I noticed some of the dry coils, the 6093, had a 4 prong plug on it. Others had the spade plug ins. XKs unlimited showed the picture of one but said discontinued.

So, now that I know, any advice as to whether I should get the offered DAC 6093 or stay with the Accel oil filled coil?

Thanks

Russ

If the Accel coil works no reason to change it unless somebody on this list has suffered failures with them.
Oil filled coils still work. Nowadays solid epoxy insulation is probably more common in coils. That might not be because it is a better insulator or have better thermal conductivity, it could be related to environmental concerns. I guess making an oil filled coil on plug unit would be tricky.

I got my dac 6093 from SNG for $75.

Oil filled are fine, they just might not last as long in the hot Vee.

i use an ACCEL coil oil filled, been 25odd yrs, still running good , revs out fine !
also ACCEL yellow wires ,and plug ends !

.
coil mounted on fan shroud, blows on it! coil cost $29.dollars(25yrs ago).
ron

Thanks! That is a very clean engine bay. That is a good idea about the mounting for the coil too. Did you remount the ignition module too?

Russ

No, it’s because modern electronic ignitions control the current through the coil so it doesn’t need so much cooling.

The HEI ignition system calls for an ignition coil with 0.6 ohm primary resistance. When introduced, such things were difficult to come by, so Jaguar resorted to two 1.2 ohm coils with the primaries wired in parallel. With the secondary capped off on one, its energy is diverted back to the primary coil of the other, so the energy of both coils is applied to the secondary of the main coil. Later, 0.6 ohm coils became more common, so Jaguar introduced its DAC 6093. It doesn’t work a whit better, it’s just simpler and lighter. And you don’t have to use Jaguar’s version; you can use any coil with a 0.6 ohm primary resistance.

Coils have less heat dissipation for sure.
I doubt that alone explains why solid epoxy has been adopted.
Electric supply requires large power transformers which need oil for insulation and cooling. It was found a number of years ago that the insulating oils degraded over time to produce PCBs which are carcinogenic. I would expect auto coils originally used the same oil, but they would have to use better oil nowadays which might be more expensive.
Epoxy has been improved over the years so it can be a cost effective alternative.
Epoxy insulation for transformers has become more popular in smaller electric supply transformers, a matter of cost, performance and avoiding health problems.
The same situation will apply to automotive coils.