Jaguar XJ-S. Service manual - part 32

 
 

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getting “energy” to the plug gaps (energy is voltage x current x time), but that’s usually a sign of a company trying to 
peddle snake oil; the only place in an ignition system to be concerned about energy is in building up the field within the 
coil (the energy the Lucas “Constant Energy Ignition” and the GM “High Energy Ignition” are referring to). 

It’s easy to limit the current flow once the spark occurs, though:  Put a resistor in the circuit.  A resistor won’t affect the 
onset of spark at all, because before the spark occurs there is no current and therefore the resistance is of no 
consequence.  But once the spark begins and current begins flowing, the resistor comes into play and limits the current 
flow. 

There are several places to put resistors in this circuit.  Using “resistor” type spark plugs is common.  Also common is 
the use of spark plug wires with a carbon-impregnated core, which offers some amount of resistance per inch of lead.  
Perhaps not so common, it is possible to purchase resistors that fit into the wire between the coil and the distributor. 

Plug wires are all different lengths within the same car, so use of common plug wires will result in a different amount of 
resistance between one plug and another.  This doesn’t matter, as long as there is some resistance. 

Some people think spark plug wires with copper conductors are a good idea.  Clearly they fail to notice that such wires 
are generally the cheapest available -- yet do not come on any cars as original equipment.  If used with non-resistor 
plugs, there will be no resistance in the circuit at all, and the plugs will be eroded quickly -- if the coil doesn’t burn up 
first.  Copper core plug wires should be avoided for all applications, except perhaps fitting a new lead to your timing 
light; the copper is easy to solder. 

A more recent development is the spiral core plug wire.  The core of these wires has a very fine stainless steel wire 
coiled into what looks like a long, skinny spring.  Stainless steel isn’t an excellent conductor as metals go, but it 
nevertheless would provide an essentially zero-resistance current path if it were straight; the current limiting factor here 
is evidently the coiling.  Magnecor (page 704) offers such wires but is mum about the theories on which they work, 
claiming that too many competitors want their information.  Judging from their descriptions, they appear they work like 
this:  The spiral core behaves as a long inductance coil.  Before the spark occurs, there is no current flow, so the 
inductance is of no concern -- same as the resistance in the standard setup.  Once the spark begins, the high inductance 
of the leads prevents the current flow from rising instantly but permits it to rise gradually instead -- and the ignition coil 
runs out of energy before the current flow can rise to a dangerous level. 

Magnecor claims several benefits to this design.  One claim is that they will outlast the standard wires, because stainless 
steel is more durable than the carbon-impregnated silicone core.  Another benefit is that these wires are more flexible, 
and several XJ-S owners have reported this is true and a blessing in the top of the V12.  Perhaps the flexibility is a large 
part of their longevity as well, since neither stainless steel or carbon-impregnated silicone should deteriorate before the 
insulation layers do on either type wire, but bending the carbon-impregnated silicone wire too tightly will damage it for 
sure. 

Magnecor also claims reduced radio and electronics interference.  This is important because modern cars with EFI can 
get all screwed up if the spark plug wires emit enough EMI to cause spurious signals in pickup leads.  The spiral core 
wires have the theoretical benefit that the magnetic field generated is aligned with the lead rather than radiating away 
from it in all directions.  Franck Guilloteau says, “On their claims; the idle stumble that I had was reduced noticeably, 
but my stereo has developed an annoying noise....so much for RFI shielding!!”  Perhaps a good idea would be to use 
spiral-core wires in conjunction with resistor plugs, just to be sure. 

 

CAP AND ROTOR RENOVATION:  Most of us just buy new parts periodically, but Danny Rearden says, “High 
tension ignition components such as distributor caps and rotors are generally only faulty if they are cracked, or have 
carbon tracks on the surface.  Even both of these conditions are usually repairable if you are prepared to invest a few 
hours.  My dad specialized in repairing obsolete vintage and racing ignition systems and magnetos where parts were 
completely unobtainable. 

“Clean the part, first with solvent cleaner, then with strong hot detergent solution and dry thoroughly.  Inspect very 
carefully, with a magnifying glass if your vision is not 20/20, looking for any surface marks which were not intended to 
be there. 

 
 

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“If there is no visible sign of high voltage tracking, go to polishing.  Otherwise use a hobby knife and scrape the surface 
until you get to totally clean material, even if this means making a hole in the component. 

“Grooves and holes can filled with epoxy filler.  We always used Epiglass Low Density Filler, designed for boat 
repairs.  It seems to have very good thermal and electrical properties, and it does not run while curing.  File and sand 
the repair back to original profile. 

“Polish out any marks (both in the original part and any repair), using abrasive paste, such as perspex polish or cutting 
compound.  These surface marks, if not removed, will be future failure points. 

“If the part has a dull, porous look to the surface, a light spray of a suitable clear paint may be in order, but don't overdo 
it.  If the repairs were to a visible part, then coloured paint can be used. 

“In the 25 years my dad was doing these repairs, I can only recall 1 or 2 items failing again, out of several hundred, and 
these were subject to 'owner abuse'. 

“The important things are: 

• remove any trace of previous tracking 

• polish or fill any holes, cracks or scratches which could accumulate dust and moisture 

• stop moisture getting into the ignition in the future” 

 

KEEPING THE IGNITION SYSTEM COOL:  Possibly the worst area for heat problems is within the “V” on top of 
the engine.  Early XJ-S’s had so much trouble with cooking the ignition amp that Jaguar created a relocation kit to 
move it out of this area.  Cracked distributor caps have been a problem.  Seized centrifugal advance mechanisms are a 
problem.  The wiring harnesses within the V always seem brittle.  All of these are symptoms of excessive heat. 

Maintaining a good airflow through the engine compartment does wonders for minimizing such heat-related problems 
on components.  However, airflow to the V is largely blocked -- not by the A/C compressor so much as by the plate 
supporting the front of the compressor.  See page 503 for notes on correct installation of this plate. 

One simple way to improve things would be to cut a big hole in this plate.  Be careful to leave enough metal to properly 
support the compressor, but this will still allow a substantial opening.  Since this area is directly behind the main fan, 
the hole should allow some airflow under the compressor and throughout the V area. 

 

IGNITION SYSTEM TYPES:  There have been four distinct types of ignition system fitted to the XJ-S.  The first two 
were Lucas systems, so you can’t simply refer to the “Lucas ignition system” without causing confusion. 

Up to 1982, the XJ-S 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 (Lucas CEI for short) was used; this system uses a 12-pointed iron star wheel 
inside the distributor.  These two systems can be distiguished by the amplifier; the OPUS amp is a finned aluminum 
block that may be located between the banks, on top of the radiator top support, or any of several other places; the 
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 EFI system was used, and it merely picked up 
the ignition pulses -- no trigger board required.  So, the same four screw holes in the distributor housing were used to 
mount a clear plastic anti-flash shield, and a new rotor with no magnet was used.  A different cap was introduced to go 
with the new rotor.  The OPUS ignition system, with the plastic wheel, continued in use for two more years. 

The XJR-S was fitted with a Zytek ignition system. 

In mid-1989, the Lucas Constant Energy Ignition system in non-XJR-S cars 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 

 
 

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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 center and one off-center, and has no vacuum advance 
module. 

There is a fifth type of ignition system fitted to the Jaguar V12, and that is a Nippondenso distributorless system used 
the last year the engine was made.  However, reportedly this system didn’t make it into the XJ-S; it was used 
exclusively in the last year of the XJ12. 

 

IGNITION SYSTEM REPLACEMENT:  Replacing an ignition system isn’t automatically an improvement; exactly 
what an ignition system replacement accomplishes depends on what you are replacing and the sophistication of the 
system you’re installing.  The pros and cons of ignition system replacement are therefore covered within the discussions 
of each type of ignition system found on the Jaguar V12. 

 

Lucas Ignition (OPUS and CEI - up to 1989) 

 

IGNITION SYSTEM DESIGN:  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. 

 

ROTOR REPLACEMENT:  Replacing the cap is straightforward enough, but getting the rotor off is likely to be 
somewhat difficult since it tends to jam.  All you can do is twist, rock, and pull, and hope you get lucky and don’t break 
it.  Or just have a spare on hand.  On the Digital P cars, there’s not enough room between the rotor and the anti-flash 
shield to get a good grip on it, so Ned Wesley says, “if you need to remove the rotor, the method I use is as follows: 
make two loops of string or wire about equal in length to the rotor.  Place one loop over the front of the rotor and the 
other over the back.  Bring the loops together so that pressure is applied equally to both sides of the rotor.  Give the 
loops a slight tug and the rotor will come off.” 

If the rotor carrier shaft seems to want to come upward with the rotor, the rotor carrier shaft retainer is broken.  You 
need to try to hold the rotor carrier shaft down while pulling on the rotor by inserting a screwdriver through an opening 
in the anti-flash shield or some such.  This is a good idea anyway to avoid breaking the rotor carrier shaft retainer.  
Once the retainer lets loose, pulling upward on the rotor carrier shaft will stretch the centrifugal advance springs far 
below, and you will be in for a distributor recalibration. 

 

IGNITION TIMING:  The proper advance setting is indicated on a decal in the engine compartment.  If it differs from 
the book, believe the decal.  The Haynes manual on page 329 seems to indicate that a N America 1981-on Digital P car 
should be timed at 25 to 27° BTDC at 3000, which doesn’t seem to have any basis in reality; the underhood decals 
always indicate 18° BTDC at 3000. 

 
 

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The next thing to confirm is that the woodruff keys that align the front pulley with the crankshaft are in good condition. 
 They are a known problem, and clearly if the pulley is allowed to reposition itself on the crank, use of the timing marks 
will be a disaster.  The woodruff key problem is discussed further on page 89. 

Before you get under the car with the engine at 3000 RPM, you might want to note what the timing mark actually looks 
like on the damper.  There is a simple line with the letter A next to it.  Unfortunately, the timing indicator plate covers 
most of this up, so when you’re trying to set the timing all you see is the tip of each so it looks like VI.  Trying to figure 
out whether you’re supposed to be using the I or the point of the V to indicate the timing can be confusing.  An 
illustration in the Haynes manual makes it clearer:  You must use the I mark. 

On the Jaguar V12, the timing indicator itself is adjustable.  If there is any chance it has been tampered with (the oil pan 
and sandwich plate have been removed), then the position of the indicator must be calibrated before checking the 
timing. 

The official method for setting this indicator is to do it when the right side (A bank) head is off.  A dial position 
indicator can be set up to determine when the 1A or 6A piston is at TDC.  If a position indicator that will fit through a 
spark plug hole is available, this same method can be used with the head in place by removing the spark plug from 
either cylinder 1A or 6A.  Once TDC is determined, loosen the two sandwich plate bolts that hold the timing indicator 
plate in place, and slide the plate on its slotted holes until 0° lines up with the mark on the pulley. 

If you happen to have the 1A head off and are going through this setting procedure, Craig Sawyers has an idea to make 
the setting more accurate -- or, conversely, to make it accurate enough using non-precision measuring tools:  “The 
manual says to set piston 1A at TDC by using a dial guage, but even this is highly inaccurate.  If the dial guage has an 
accuracy of 1 thou, this corresponds to a setting accuracy of 2 degrees for a 70mm stroke engine.  If, with a 
screwdriver, you can guesstimate to say 0.5mm, the angle error will be nearly 10 degrees, all of which makes the slotted 
holes on the guage plate a bit of a joke. 

“What I did when rebuilding my engine was this.  When piston 1A is at TDC firing stroke, piston 6A is at TDC 
exhaust, and pistons 2A through 5A are half way.  I forget which stroke they are on, but two of them are on their way 
up, and two are on their way down.  So I set the crank up so that all four of these pistons were exactly the same distance 
down from the top of the cylinder liners.  If this can be measured to 1 thou, the crank angle error will be 0.04 degrees.  
The trick is to carry out the measurement at the point of maximum sensitivity (half way down a stroke) rather than the 
point of zero sensitivity (top of a stroke). 

“I'm not sure how you could achieve this with the engine in and the heads on, but if you could fashion some feeler and 
probe the piston positions in 1A to 5A with an accuracy of 0.5 mm, you could set the position of the gauge to an 
accuracy of 0.8 degrees, which is more than adequate.” 

There is an alternate method to set the timing indicator that doesn’t require the position indicator or removing the head. 
 All that is required is a device that will obstruct the motion of the piston near the top of its stroke.  Such a device can be 
made from an old spark plug by breaking the ceramic out of it and installing a bolt through the middle.  Ideally, the 
length of the bolt into the combustion chamber should be just enough for the piston to hit it only a few degrees from 
TDC.  If you make this device strong enough, it might also come in handy for removing the crankshaft pulley someday 
-- see page 88 -- although for that purpose it would be better if it hit the piston farther away from TDC. 

Turn the engine a ways past TDC, and then screw this obstructing device into the spark plug hole of either 1A or 6A 
cylinder.  Then turn the engine backwards until the piston hits the device and you can’t go any farther.  Note the 
reading from the timing marks.  Then turn the engine forward through one complete revolution until the piston hits the 
device again, and note the reading of the timing marks.  The two readings should be exactly the same amount before 
and after TDC.  If they are different, loosen the sandwich plate bolts holding the indicator plate and move it an amount 
corresponding to one half the difference between the two readings. 

Bob Egerton provides another method of finding the true TDC:  “Get an old plug and beat out the ceramic centre.  Then 
braze in a length of copper or other fairly small bore tube (you could probably use a really good-fitting bit of polythene 
tube if you cannot get access to brazing kit) long enough to see from where you are when turning the engine over by 
hand with your extra long wrench.  Apply a small amount of soap or detergent solution to the end of the tube and slowly 
turn the motor forwards.  When the bubble is largest you are at TDC.”  Note again that it may be easier to use the 6A