After months of trying to determine why 1986 XJS would run poorly for 1 to 2 minutes, a quick check of the voltage at the coil indicated that voltage was flickering between 8 and 12 volts. I tried wiring directly to the battery in the trunk (circumventing the ignition switch etc.) and it provided no difference. Then I took a second battery out of my 66 MGB and connected it directly the coil and the engine runs beautifully!!! Any thoughts here?

Submitted by kevin@evepartn… on Tue, 02/28/2012 - 09:59

That would certainly explain a lot "the secondary is not a back and is wired in parallel". I probably interpreted any potential performance issues with just the one.

From Kirby Palm

COILS: The Lucas CEI system uses two conventional ignition coils wired in parallel. The high-tension lead of the
secondary coil is sealed off, and only the lead from the main coil is connected to the distributor. Between firings, energy
is built up in both coils. When the 12V supply is broken (ÔÇ£the points openÔÇØ in the lingo of the pre-electronic age), the
energy stored in the secondary coil cannot escape through the high tension lead because it is sealed off, so the energy
comes back through the 12V leads instead. The primary coil then not only has to release the energy it has stored itself,
but also the energy coming back from the secondary coil. These two energies add to produce a powerful output at the
high tension lead on the primary coil.
The secondary coil, located in front of the radiator, is not a spare or a backup; it is designed into the system for
producing a good spark. If the secondary coil goes bad or gets disconnected, the performance will suffer; typically, the
engine will top out at around 4500 RPM and wonÔÇÖt go any faster. The secondary coil is not special, however, and can
be replaced with a conventional coil provided the high tension connection is covered so that it cannot arc to ground.
Since mid-1989, XJ-SÔÇÖs have gone to a Marelli ignition system that also uses two coils. However, the Marelli coils are
not wired together; each one fires only six cylinders.
According to Alan Jenks, ÔÇ£Jaguar now recommends replacing both coils with a single ÔÇ£solidÔÇØ (not oil filled) coil (#DAC
6093) that fits in place of the main coil. The aux coil and wiring is removed.ÔÇØ Roger Bywater says, ÔÇ£The best coil to use
on Lucas HE V12s is DAC 6093 (Ducelier coil - 0.62 ohms primary); only one needed and works well in place of earlier
twin coil set up. If you can find another coil with primary winding resistance of 0.5 - 0.6 ohms maybe it is worth a try.
If the resistance is any more than that it will not be able to build up enough coil energy to fire a spark at the higher end of
the rev range when the coil ÔÇ£on timeÔÇØ is very short (about 1.4 milliseconds at 6000 revs). It might also struggle around
the peak torque point. Although the V12 constant energy ignition module is fairly tolerant (it runs OK with the blanked
second coil removed - albeit with a loss of spark energy) I have encountered some that behave very oddly if the coil is
not the correct load match. The DAC 6093 might be a bit expensive but it does the job...ÔÇØ

Submitted by kevin@evepartn… on Mon, 02/27/2012 - 18:33

The primary coil is a brand new 1.2ohm coil and the secondary has been disconnected. Just focusing on steady rpms right now (cleary I will need to reconnect for higher revving. The voltage has a solid 12 with the ignition on when then engine is running the voltage flickers between 12 volts 8 or 7

Submitted by bonnettoboot@e… on Sat, 02/25/2012 - 00:23

There are 2 issues here, First the secondary coil, (the front one) make sure it is wired properly and that the high tension connector is sealed. Secondly, from your description there is a draw/high resistance in the ignition feed to the coils, were both coils connected when you did the test &, did you try connecting the remote battery to the secondary coil? It looks like you will need to trace the wiring back until you find the reason for the volt drop.

Submitted by rcmaury@bellso… on Fri, 02/24/2012 - 13:54

Most likely the insulation has broken down and has shorted out causing a high demand resulting in low voltage. Overheating is usually caused from the oil inside the coil leaking out. There are two coils on your car and as they are wired in parallel, one or the other could be shorting. Best to disconnect and check resistance. It is important that the proper resistance coil is used or you could have problems with the ignition amp in the future.

Submitted by rgrisar@hotmail.com on Fri, 02/24/2012 - 12:10

I am a new JCNA member this month, and have been watching your thread (I own a 95 XJS). Your diagnosis with the second battery connected to the coil is brilliant. No where in any literature was this suggested. Now, you have it running. So the problem has to be wiring between the coil and the point where the wire gets its source. Absolutely amazing. Congratulations to getting this far.