XJS Ignition
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XJS IGNITION SYSTEM TYPES and HE REPLACEMENT COILS

 

 

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 V12ís

 

 

 

 

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.

 

Original Main HT Coil 7 Auxiliary HT Coil setup on HE Engine

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