Jaguar XJ-S. Manual - part 40

 
 

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such part is made by Standard, part number RC-4.  Joe Bialy says, “More than likely, the cap in the connector will work 
as well as the Lucas cap if it is wired identically.  Noise suppression caps are not the most critical components in a 
system.  Typically, -20/+80% tolerance items.  I'd even bet an old "points" type would work just as well too.”  Peyton 
Gill says, “I replaced the condenser (capacitor) in the ignition amp about a year back.  I used a condenser from a Chevy 
inline six distributor.  As memory serves, it required removal of the mounting bracket which was soldered to the 
condenser body.  It is a little longer but the diameter was about the same.” 

About the resistors, Walter Petermann says, “The white piece of rubber has 2 resistors in it.  One is 10k, the other 6.8k. 
 They are connected together at one end to terminal C of the GM module.  This same terminal goes to the coil minus 
and the large diode.  The other end of the resistors go to tach and fuel ECU (white/gray wires)”  The 10K resistor 
connects to the tach and the 6.8K connects to the EFI ECU.  On the ignition amp, both wires are the same color but the 
tach wire has a male terminal and the ECU wire has a female terminal; the wires they connect to are clearly 
distinguishable, since the ECU lead is a shielded cable and the tach wire is a plain W/S/U wire.  Craig Sawyers points 
out that the 6.8K resistor is actually shown on the fuel injection system wiring diagrams.  You can easily test the 
resistors without even unbolting the amp from the manifold; just disconnect the W/B wire from the ignition coil and the 
leads to the ECU and tach and measure from the W/B wire to each.  If the resistor feeding the tach fails the only 
symptom would be a nonfunctional tach, but a failure of the resistor feeding the ECU will stop the car with conviction. 

The trick to testing the zener diode is in recognizing that it only exists to protect the amp against overvoltage, which 
normally occurs only when the system is operated with a spark plug wire disconnected.  Under normal conditions, it 
does nothing.  So, to find out if it is causing trouble, merely disconnect it.  If nothing changes, the diode is probably not 
the problem.  If the problems go away, you can consider finding a replacement zener diode or simply leave it 
disconnected; if you choose the latter option, you might want to be careful not to operate the ignition with any spark 
plug wires disconnected from then on. 

 

FIDDLING WITH THE GM HEI IGNITION MODULE:  This author found a warning inside a Lucas CEI ignition 
amp that read: 

 CONTAINS 
 BERYLLIA 
 

DO NOT OPEN 

Judging from the fact that this label was attached to the surface of the GM HEI module itself, the beryllia referred to is 
probably inside that module; there is nothing else within the Lucas ignition amplifier that appears to be this type of 
substance. 

Beryllia is the oxide of Beryllium, and is used in a type of ceramic used to mount or encase electronic parts.  This 
ceramic conducts heat very well, helping keep the part cool, while being an electrical insulator -- an unusual 
combination.  Unfortunately, beryllium is really as dangerous as indicated.  If you are inside the box repairing your 
amp, just remove and replace components.  Don’t go sawing open the GM module, the dust created can 

kill

 you.  Patsy 

Lokensgard says, “Beryllium is nasty stuff.  I used to work someplace where people worked with it, and there were 
many rumors about people who had died from exposure.  For more information, check out 

 

 

http://tis-nt.eh.doe.gov/be/webdoc1.html-ssi

 

“The health hazard information is towards the bottom of the page.  I think it is a good description of the risks, neither 
overstating them nor minimizing them.” 

Mike Morrin adds, “There is actually very little of it used in anything built in the last 20 years except high power 
transmitters and some specialised power modules.  I am not sure about the GM ignition module, but if there is any, it is 
likely to be a pad under the power device about 2mm square and .5mm thick.  Beryllium Oxide is very expensive, so 
the semiconductor manufacturers avoid it where possible, the safety aspects not withstanding.” 

 

MORE TECHNICAL INFO ON THE GM HEI:  Motorola once offered a publication MC3334/D which provided 
“analog IC device data” on a chip they made, MC3334 (and a couple of variations) which was described as a “High 

 
 

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Energy Ignition Circuit” and was apparently used within the GM HEI module.  This document was very informative, 
not only on the circuit itself but on the entire ignition system it’s used in.  Unfortunately, such info is apparently NLA; 
perhaps Motorola no longer makes that chip, leaving it to the aftermarket businesses. 

 

ELECTRONIC IGNITION PICKUP:  The electronic pickup used in the Lucas CEI distributor involves a magnet that is 
mounted with two screws.  Be careful tightening these screws; the ceramic magnet is much more brittle than metal 
items, and can easily crack.  If already cracked (notably around one of the screw holes), do not be concerned, it will not 
affect operation.  However, be sure not to leave any chips in there that could come loose and move around within the 
distributor.  It is better to discard small bits of the magnet that have broken away. 

The pickup itself is apparently quite reliable, but there are a couple of indications that it may develop intermittent 
problems.  Don Lawson of Northeastern Classics had a customer with a car that would run fine and then suddenly quit, 
and after a considerable amount of frustration they finally simply replaced the distributor in entirety -- and never had 
the problem again.  The original distributor was sent to this author for analysis, but nothing could be found wrong with 
it.  The pickup was monitored while immersed in water being brought to a boil, and it showed no problems.  It’s 
possible the problem was elsewhere in the V and it was accidentally corrected while changing distributors. 

Martin Walker had similar problems.  “Mine would run fine if I kept moving, but let it idle for 30 minutes, or run it in 
slow traffic for a long time in hot weather and it would suddenly die.  It would then not start for about 2 hours, and 
would then fire up OK and run well again.  This cost me much time, money and aggravation, as it eventually did this 
some distance from home.  The AA trailered it to the nearest Jag main dealer, who spent lots of my money in man-
hours trying - and failing - to find the problem.  I paid to have it trailered back to my garage, and replaced the ignition 
amp before turning to the pickup.  I took it out and mounted it on a soldering iron - and at a consistent temperature (80-
odd degrees, I recall) it would simply go open-circuit.” 

Keith Morris concurs:  “I believe that the epoxy in the pickup heats up and breaks a close tolerance gap.  I have 
experienced this problem on an MGB and a Jaguar.  A simple replacement of the part solved the problem.” 

If you have decided that the pickup is a problem, don’t replace the entire distributor; the pickup itself is available at 
most auto parts stores, made by several different aftermarket brands.  It’s not cheap -- over a hundred bucks -- but it’s 
cheaper than a new distributor. 

Considering how many different cars use the same GM HEI ignition amp, it may be possible to find a cheap pickup that 
fits some other car and make it fit the Jag distributor.  The ones that fit Chevy V8’s of the same era look totally 
different, but there are other pickups in the catalogs that do look fairly similar. 

 

PICKUP GAP ADJUSTMENT:  David Littlefield says, “The Haynes manual calls for readjusting the gap between the 
magnetic pickup and the iron star wheel with a plastic or non-ferrous feeler gauge.  Believe it or not, these are readily 
available.  I found some at Pep Boys hanging on a card beside other feeler gauges, I think they were made of brass.”  
Note: brass corrodes easily, and polishing removes a little metal so your feeler gauges would get progressively less 
accurate.  It is recommended that you spray a little WD-40 or something similar on each blade before tossing it back in 
the tool box, or take some other measures to prevent corrosion. 

“Also, the Haynes manual says to turn the crank until one of the points on the star is next to the pickup so you can 
check the gap.  Unnecessary!!  Just remove the U-shaped thingy holding the star wheel in place and it will turn on the 
rotor carrier so you can get a point of the star in the proper position.” 

 

CONNECTORS:  Mark Whitnell reports, “For the last several months the only way I could get it started was to use 
starting fluid.  I checked out the spark since that was an area recommended by Kirby.  The spark was very small and I 
suspect that the small spark could not ignite the gas but could ignite the ether (lower vapor pressure). 

“I checked the distributor pick-up coil for the proper resistance according to Haynes (2.2K to 4.8K).  I checked at the 
connector attaching to the ig amp first...very high megohms.  Then I pulled the connector apart at the distributor.  

 
 

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Checked the continuity of the wiring to the ig amp...checked ok.  Then measured the resistance at the connector to the 
distributor...high resistance.  Looking at the two prong connector...visually it looked fine.  I went ahead an cut off the 
connector and measured resistance of the two wires to the distributor pick-up.  Measured resistance... 3000 ohms within 
the range described in Haynes.  Replaced the connector.  Voilà,  starting problem solved.. good spark, etc.” 

 

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 (“the points open” 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 won’t 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-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.  

According to Alan Jenks, “Jaguar now recommends replacing both coils with a single “solid” (not oil filled) coil 
(#DAC 6093) that fits in place of the main coil.  The aux coil and wiring is removed.”  Roger Bywater says, “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 “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) 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...” 

He adds that the original parallel coil arrangement was “conceived when coils of sufficiently low resistance were not 
available.” 

John Goodman defends the earlier parallel coil setup:  “The only difference (between the two coils) is the HT socket is 
sealed on the aux coil, and is no longer stocked by Lucas.  The primary coil is still available from a Lucas distributor 
and is cheaper than the universal coil Jaguar sells to replace the two V12 coils.  I am experiencing coil/ignition related 
problems on my car and have tried a few of these Ducelier coils (original equip on the XJR-S).  I am convinced it is the 
coil causing intermittent starting and cutting out problems, thinking of resorting to the early twin Lucas coil system 
which never gave any trouble on previous cars.” 

Jim Isbell:  “BTW, if you measure the primary, be careful because when you remove the meter the secondary sparks 
and it can really make you jump; also, when the field collapses you can feel the bite on the primary as well.” 

 

LUCAS CEI IGNITION SYSTEM REPLACEMENT:  There is rarely any good reason to replace the Lucas CEI 
ignition system.  The performance is excellent, and replacing with aftermarket rarely results in any improvement.  The 
centrifugal advance mechanism is prone to seizing, but that’s easily avoided with regular maintenance.  The vacuum 
advance module fails after a few years in the heat in the vee, but it’s not that expensive to replace; besides, most 
reasonably-priced aftermarket ignition systems continue to rely on the original centrifugal and vacuum advances 
anyway.  Repairing a failed amplifier costs $20 with parts available everywhere.  The two coils are reasonably-priced 
and easily obtainable (or you can spend bigger bucks for a modern single coil).  The only parts that are even remotely 
expensive to replace are those in the distributor, the star wheel and the magnetic pickup, but they rarely cause any 
trouble.  The pickup itself is available at regular (non-Jaguar) auto parts stores for around $100. 

 
 

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Terry Pegler:  “I ordered the Crane XR/700, part number 700-0300 conversion system; it arrived the other day.  The 
instructions on Page 1 state "If the distributor has a 3 terminal connector it is an OPUS system and you can proceed.  If 
the distributor has a 2 terminal connector, it is not an OPUS system and you must use Crane HI-6R part number 6000-
6400."”  Unfortunately, Pegler was replacing his CEI system due to a severely mangled star wheel and therefore had to 
wait until he could obtain that additional Crane part to get his car running again. 

Of course, if you’re a performance enthusiast, you might wish to replace the Lucas CEI with something more 
sophisticated or flexible.  Most owners would not be enthused about upgrading to the later Marelli system because it’s 
too expensive and failure-prone, but there are modern aftermarket systems that have all sorts of features.  See the 
discussions on upgrading the Marelli on page 171; any of those systems could replace the Lucas CEI as well. 

 

Marelli Ignition 

 

MARELLI IGNITION:  In this age of electronic wizardry, many modern cars have done away with the mechanical 
centrifugal and vacuum advance mechanisms and do the whole job with microprocessors.  In some cars, the distributor 
is done away with altogether, and separate ignition coils are used instead. 

Since Jaguar incorporated the Marelli ignition system in 1989, this is essentially the type of system used.  There is no 
centrifugal or vacuum advance mechanism; the timing is all done electronically.  However, rather than using lots of 
separate coils, the distributor was kept, using two coils and directing the spark mechanically.  This distributor is clearly 
lacking many of the internal parts of the earlier models, since it no longer handles the timing functions.  The rotor is 
actually two rotors in one, and the cap is two six-cylinder caps in one, so each coil is operated as though it is running a 
six-cylinder engine.  The computer that handles the timing is located near the passenger’s feet, and has a vacuum line to 
it. 

The source: 

 

Magneti Marelli S.p.A. 

 

Viale Aldo Borletti, 61/63 

 

20011 Corbetta (Milan) - Italy 

 +39-2/972001 

 FAX: 

+39-2/97200355 

 

http://www.marelli.it:80

 

As of this writing, this WWW site doesn’t have anything on the XJ-S ignition system; only general info on the 
company. 

 

MARELLI OR NOT?  If you don’t know if you have a Marelli ignition system or not, measure the diameter of the cap. 
 If it’s around 4-1/4”, it’s a Marelli; the Lucas cap is about 5-1/2” in diameter.  You can also check the height from the 
mounting surface of the cap to the top of any one of the 12 spark plug wire connectors around the edge; the Lucas will 
measure about 2-1/4” while the Marelli will measure about 3-1/8”. 

If these descriptions aren’t good enough, you can try looking at pictures at: 

 

http://www.jag-lovers.org/xj-s/book/LucasMarelli.html

 

 

EFI TRIGGERING:  The Digital P fuel injection system, in use on the Jaguar V12 since 1980, is triggered by the 
ignition system.  With the Lucas OPUS and later with the Lucas CEI, this signal was taken from the - terminal of the 
coil, with a resistor in the line to limit spikes to the electronics.  The Marelli has two coils, each of which only see six 
cylinders of pulses, so the ignition ECU itself includes circuitry to provide a suitable signal to the EFI.  Don Neff says, 
“This signal is carried by a small shielded wire from pin 24 of the ign ECU to pin 17 of the EFI ECU.