There
have been three distinct types of ignition system fitted to the XJ-S.
Up to 1982, the car was fitted with the same Lucas Opus system that was
used in the Series III E-Type. This system uses a plastic disk with
12 ferrite inserts within the distributor to trigger the ignition.
From 1982 to mid-1989, the Lucas Constant Energy ignition system was used;
this system uses a 12-pointed iron star wheel inside the distributor.
These
two systems can be distinguished by the amplifier, the Opus amp is a
finned aluminium block that may be located between the banks, on top of
the radiator top support, or several other places.
The
HE Constant Energy amp is a black, flat rectangular item, bolted to the
top of the left side intake manifold.
It
must be clarified that the most obvious distinction within the Lucas
distributors has nothing to do with ignition types. Up until 1980, the XJ-S
had a Bosch D-Jetronic EFI system that required a trigger board within the
distributor and a rotor with a magnet in the counterweight.
From
1980 on, the Digital P EFT system was used, and it merely picked up a
signal from a coil wire - no trigger board required. The same four screw
holes in the distributor housing were used to mount a clear plastic
anti-flash shield, and new rotor with no magnet was used, with a different
cap. The Opus ignition system with the plastic wheel, continued in
use for two more years.
In
mid-1989, the Lucas Constant Energy ignition system was replaced with the
Marelli, which is an all electronic system - there are no mechanical or
vacuum advance mechanisms, the timing is handled by an electronic control
unit based on crank sensors.
The
Marelli system still uses a distributor, but it only serves to allow two
coils to fire twelve cylinders; it does not include any timing or
triggering functions. This distributor is very distinctive, in that the
cap has connections for two separate ignition coils, one at the centre and
one off-centre, and has no vacuum advance module.
I
have updated my XJS with a Wolf computer and full electronic fuel &
ignition control, removing the distributor altogether with the two Jaguar
late model V12 coils directly connected to the spark plugs, controlled by
pick up sensor on front pulley.
Twin
6 port coils as fitted to late model Jaguar V12’s
HE IGNITION SYSTEM DESIGN 1982 - 1989
An
ignition coil requires a certain amount of time to build up enough energy
to produce a spark. The faster an engine is turning, the less time there
is between sparks, so the output of an ignition coil starts to drop off.
It is also apparent that the more cylinders there are, the less time there
is between sparks, and the output of the ignition coil drops off even
faster.
Another
lesson in physics is that the higher the compression, the more
resistance
there is for electricity to jump a spark gap, so higher voltage is
required.
The
Jaguar V12 H.E. has 12 cylinders, turns at 6500 RPM, and has 11.5:1
compression, making it one of the biggest challenges for an ignition
system in production automobiles. To cope with this, Jaguar has
incorporated some sophisticated ignition technology. Also, Jaguar uses a
spark plug gap of only .025" to make it easier for the electricity to
jump the gap.
1 Distributor,
2 Main HT Coil, 7 Auxiliary HT Coil
The
Lucas Constant Energy ignition uses two conventional ignition coils wired in
parallel. The only difference (between the two coils), the high-tension lead
socket of the auxiliary 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
("the points open" in the lingo of the pre-electronic age), the
energy stored in the auxiliary 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 auxiliary coil. These two energies
add to produce a powerful output at the high-tension lead on the primary coil.
The
auxiliary 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 either coil goes
bad, the performance will suffer.
Since
mid-1989, XJ-S's have gone to a Marelli ignition system that also uses two
coils. However, these two coils are totally separate; each one fires
only six cylinders.
HE MAIN COIL REPLACEMENT
Initially
I had a problem with the main oil filled coil located adjacent to the
distributor, attached to the throttle pedestal. I originally replaced this
coil with a non oil filled Bosch MEC 723, which coupled with the auxiliary
front coil worked well without trouble for over 5 years and 70,000 kms.
The
original parallel coil arrangement was "conceived when coils of
sufficiently low resistance were not available."
Bosch
MEC 723 solid coil replacement for original main HT oil filled coil, coupled
to auxiliary HT coil, located in front of the radiator.
HE SINGLE COIL REPLACEMENT
Jaguar
now recommends replacing both coils with a single "solid" (not oil
filled) coil (#DAC 6093, Ducelier coil - 0.62 ohms primary) that fits in place
of the main coil. 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 "on time" 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) the
ignition system will behave oddly if the coil is not the correct load match.
The
DAC 6093 is a little bit more expensive at $200 than 2 separate coils, but it
does the job and works well in place of the earlier twin coil set up. The aux
coil and wiring is removed, which eliminates the problem of the auxiliary coil
failing from water damage through constant exposure being located behind the
grille, and tidies up the top of the engine from the wiring running exposed.
Single
Ducelier solid coil replacement for both original coils
