Jaguar XJ-S. Manual - part 41

 
 

159

“The power amps above the radiator do not have any feedback, but the two ign coils each have one.  The feedback from 
the A-bank coil goes to the tach and the feedback from the B-bank coil goes to the diagnostic socket.  The engine 
continues to run if either ign amp is unplugged.”  As a result of the tach getting a signal from only one coil, a six-
cylinder tach is used. 

 

SIII XJ12:  For some reason, the Marelli ignition system was not introduced in the XJ12 until the XJ40 version model 
came out in 1993.  Roger Bywater adds, “What may not be so obvious is that the saloon standardised on the low 
compression engine at the same time that Marelli was introduced on the XJ-S.  It is interesting that while the non-cat 
Marelli XJ-S used a completely different fuel map (DAC 6338) from other engines the non-cat saloon stayed with the 
same mapping (DAC 6336) that was used for the previous high compression engine.  The cat saloon used the same 
ECU as had been used on the pre-Marelli / Lucas / cat low compression XJ-S (DAC 6335).” 

 

CAP VENTING:  The Marelli distributor is vented in the same manner as the Lucas distributor (see page 145), except 
that the vent connections are on the base rather than on the cap.  Since there are no centrifugal or vacuum advance 
mechanisms, there are fewer reasons that Jaguar would have decided to provide this system.  Either the cap needs 
cooling to prevent cracking, or explosions can be the result of fumes building up. 

 

TUNE-UP PARTS:  There is supposed to be a gasket under the Marelli cap, but the new cap doesn’t come with one.  
Part # JLM-1910.  Reportedly atrociously expensive, just like the cap itself. 

 

IGNITION AMPLIFIERS:  Everything about the Marelli ignition system is expensive, including the two amplifiers, 
DAC11520.  However, reasonably-priced substitutes are available.  Pete Bainbridge-Clayton says, “I was surprised to 
find that both my amps were different, and neither seemed to be the 'correct' one.  One is a Unipart GIM5002, the other 
one is a Bosch 0 227 100 139.  Discovered that there was an Intermotor 15000-c which was an equivalent.  The Bosch 
was running fine, and I managed to find a stockist of the Intermotor part - in fact they are available off EBay, the seller 
is cashfromthechaos and they charge a whopping GBP10!!  Yes, ten pounds plus postage - quite a difference from the 
Jaguar part!  All I can say is that it is running fine, the amp doesn't get hot, and I am very happy with it!” 

 

TIMING:  You don’t really wanna adjust the timing on this thing, do you?  Randy Wilson describes the XJ40, which is 
similar:  “Spark timing is not easily adjustable.  This car has a crank triggered digital ignition system.  Timing changes 
would require moving the pickup and/or modifying the ECU.” 

On the other hand, there are two different timing maps built into the ignition ECU.  Michael Neal says:  “There is a 
nifty little jumper on the harness at the back side of the left intake manifold.  Pull this sucker and your timing retards, 
perfect for those trips to Mexico.  Usually the connector is red.” 

Mike Wilson quotes from a Jaguar manual:  “Map Link Selector- This link permits the selection of either of (2) ignition 
maps contained in memory.  If the link is in place the ignition map suitable for 95 Octane fuel is selected, and if the link 
is removed the ignition map suitable for 91 Octane fuel is selected.” 

Mike Wilson continues, “To find this "link" look behind the lefthand intake manifold.  This link (barrel shaped and 
with two wires coming out of it.  One is yellow, the other black) is usually wire tied to the Diagnostic Socket.” 

 

DISTRIBUTOR POSITION ADJUSTMENT:  Randy Wilson, still talking about the AJ6 engine:  “Jaguar has a special 
jig for doing this, but it’s not really needed.  The whole purpose is to make sure the rotor is pointing to a wire lug no 
matter what the current computer-chosen timing is.  On normal dizzies, the centrifugal advance moves the timing up... 
but it also advances the rotor by the same amount, as they are attached.  No such luxury with the crankfire system. 

 
 

160

“First mark the dizzy body directly in line with the #1 and #6 plug wires (note: 1A and 6A on the V12).  Then remove 
the cap.  Bring the engine up to TDC, noting which way the rotor turns.  When you reach TDC, the rotor should be 
pointing at one of the two marks.  It doesn’t matter which one (why we marked both).  Now turn the dizzy body until 
the mark is just past the trailing edge of the rotor contact.  Just past it, not more than a degree or two. In this position, 
the rotor contact will line up with the wire lug in cap in the timing range of about 5 to 40 degrees before TDC; the 
operative timing range of the engine. 

“If the adjustment is off very far, the results will be subtle.  The most likely thing is the rotor and cap will tend to burn a 
little uneven...biased against the direction of misadjustment.  The cap and rotor will “wear out” a little faster.  If the 
misadjustment is extreme... and I’m not sure it’s possible on the AJ6, there will be a misfire at the extreme end of the 
timing curve as the spark has to jump a pretty large gap inside. 

“Next time, mark the position before you remove the distributor.  This is one of those rare cases where “close” is good 
enough.” 

Regarding the Marelli on the XJ-S:  “Yep, ‘tis the same ideas and principles.  I’ve never actually had to pull a Marelli 
distributor, so I can not comment on the exact position.  You would mark 1A and 6A... and set things so the rotor 
sweep-by happened in the 10 to 30+ before range.” 

 

MARELLI-CAUSED DISASTERS:  The combination of Marelli ignition and catalytic convertors can, and often will, 
result in the destruction of the catalytic convertors and possibly the whole car.  This is not a section of this book to be 
glossed over lightly!  If you have a car with Marelli ignition and catalytic convertors, you need to take a proactive role 
in making sure your car doesn’t become a burned-out hulk in a junkyard somewhere. 

 

CATALYTIC CONVERTOR MELTDOWN/FIRE (ONE-BANK FAILURE):  While any misfire is a cause for 
concern with catalytic convertors (see page 316), the Marelli ignition system provides a unique and especially 
threatening possibility: running on six cylinders.  While the various Lucas ignition systems were single 12-cylinder 
ignition systems, the Marelli is actually two separate 6-cylinder ignition systems.  Hence, it is possible -- probable, even 
-- that sooner or later one of these ignition systems will fail while the other continues working.  Since each half of the 
ignition system operates one bank of the engine, one entire half of the engine, along with its dedicated catalytic 
convertors and oxygen sensor, may see no spark all of a sudden while the other bank continues to run normally. 

The V12 can be accurately described as two six-cylinder engines; each bank has the same firing order and the same 
natural balance as an inline six-cylinder engine (note: conversely, a V8 is not two four-cylinder inline engines). Hence, 
the V12 actually runs smoothly on six cylinders, and drivers who have not read this section may make the fatal mistake 
of trying to press on to the next exit or the next gas station.  According to LaRue Boyce, symptoms to look for include:  
“Loss of power, more gas smell but no noise, just the no power feeling.  Oh, the loss of vacuum also causes the 
transmission not to want to shift.  You will know when it happens, there is no power, just enough to get you to 45mph 
on flat ground.”  Julian Mullaney says, “When my car started running on 6 it was very noticeable.  I thought that the 
tranny was slipping at first because I had to give it so much throttle to get it moving.”  Note that, with the typical failure 
mode described below, this happens all at once; there is no intermittent operation where it runs well most of the time 
and drops to low power on occasion.  Once a bank shuts off, it’s gone for good. 

Running a non-cat V12 with no spark on one bank fills the inop side of the exhaust system with fuel -- a disaster 
looking for a place to happen.  Since the XJ-S exhaust system loops up and over the rear suspension, it should be able 
to hold several gallons of fuel before it starts pouring out the tailpipe!  Since there is no spark in that bank at all, the 
owner might just get away with it -- but he’d better hope his ignition system doesn’t mysteriously start working again!  
The same goes for a cat-equipped car that had an ignition failure when started from cold; the cats won’t work when 
cold, so the fuel in the exhaust system on that side may never be ignited. 

If the car is equipped with catalytic convertors and they were at operating temperature when the ignition failure 
occurred, the cats will burn the fuel as it arrives, and things will getting considerably hotter in a big hurry.  What’s 
worse, if the car continues to be driven, more throttle will be applied to get any speed out of it, so even more fuel and 
air will be dumped into these cats.  The inevitable result will be a cat meltdown within seconds.  What’s worse, often 

 
 

161

the cherry red-hot catalytic convertors will ignite something, perhaps by melting through the fuel hoses that are above 
and not too far away.  The car is immolated so fast you’ll barely have time to pull over and get out. 

You don’t want to be driving on six cylinders, not even for a minute. 

If you were dozing when you read those last few paragraphs, pinch yourself, clear your head, and go back and read 
them again. 

 

DISTRIBUTOR ROTOR PROBLEM:  Unfortunately, a one-bank ignition failure turns out to be a common occurrence 
on Marelli-equipped V12 Jags.  Although anything that kills one side of the Marelli ignition system can cause the car to 
run on six cylinders and threaten a cat fire, there is one possible cause that clearly outweighs all others in terms of 
frequency of occurrence.  Randy Wilson: “The most common failure, the one that kills the A bank, is the center post of 
the rotor burns through, allowing a ground path from the rotor contact straight to the distributor shaft.”  This 
characteristically results in a fire or meltdown in the right side cats only, since it is always the A bank ignition system 
affected.  For a photo of a cut-away rotor that suffered this failure, see 

 

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

 

 

WHAT TO DO:  If you have a Jaguar V12 with Marelli ignition, take the following action immediately:  Remove the 
distributor cap.  Remove the distributor rotor, which is held in place with two screws.  Looking at the top of the 
distributor shaft where the rotor attaches, you will see the flat plate that the two screws thread into, and you’ll see the 
hexagonal stub of the shaft itself sticking up above the center of this plate.  Cut or grind that stub off, down to nearly 
flush with the plate.  Obviously, you will want to make sure to collect all metal particles generated.  This hexagonal 
stub helps center the rotor while you install the screws, but that’s not really necessary.  You can, if you wish, leave just 
a bit of it above the plate to help center the rotor, but really it’s better if you just hack it completely off and let the 
screws locate the rotor. 

That was Step 1.  For Step 2, hold the rotor upside down and notice the opening in the bottom; it’s shaped something 
like an asterisk.  Fill this opening completely with clear silicone sealant, the stuff used to seal windows.  Try to make 
sure there are no air bubbles down in the recesses.  Then, while the sealant is still wet, install the rotor and secure it with 
the two screws.  If done properly, you will have a bit of sealant squeeze out the edges as you’re tightening the screws 
down.  Simply wipe away the excess.  Of course, allow the sealant to set up before driving the car. 

As of this writing, I have received no reports of a Marelli ignition system suffering a one-bank failure due to shorting 
through the rotor after having done this “silicone squirt”.  It appears to be a 100% effective fix.  It was even tried once 
on a rotor that had already failed just to demonstrate how effective it is, and it did in fact render the rotor functional 
again -- but obviously using the method to resurrect failed rotors is not recommended, it should only be used to protect 
against failures in the first place. 

Just do it.  And remember to do it again each time you install a new rotor. 

The “silicone squirt” was originally suggested by Julian Mullaney.  Marelli owners everywhere will forever be indebted 
to him. 

 

CHEAP AFTERMARKET PARTS?  The frequency of the rotor failure has been blamed on aftermarket rotors.  It is 
true that some of the aftermarket rotors are highly susceptible to failure, and have been known to fail within minutes of 
installation.  Note, however, that similar rotor failures have been reported in genuine Marelli parts as well -- it just takes 
longer.  No matter which rotor you obtain, you need to perform the silicone squirt. 

It is a simple matter to tell if you have genuine Marelli parts: they say “Marelli” right on the part.  Being marked “Made 
in Italy”, coming from a Jaguar dealer in a Jaguar box with a Jaguar part number, or costing a king’s ransom are not 
indications of the genuine article, because many of the knockoffs are made in Italy (the genuine Marelli items may 
actually be made in the UK!), Jaguar apparently offers any and all of them under their part number, and they are all 
expensive. 

 
 

162

Rumors have been circulating for years that Marelli has ceased making the genuine articles and once the stock is used 
up only the aftermarkets will be available.  These are apparently false rumors as of 2002.  It’s bound to happen 
eventually, though; even if Marelli is still making caps and rotors now, they will cease making them sooner or later -- 
the V12 has been out of production for years. 

 

SPARK PLUG GAPS:  The correct spark plug gap on a Jaguar V12 with Marelli ignition is 0.025”.  Period.  Use of 
larger gaps causes higher electrical stresses on the rotor and everything else in the ignition system.  And remember that 
the gaps tend to grow with wear. 

There are two problems here.  First, there are spark plug catalogs that claim that the gap should be 0.035”. Donald 
Anthony says, “I've been checking around for parts for the last few days and all (100%) of the spark plug sellers (napa, 
oriellys, pep boys, and all the local foreign parts guys) told me the proper gap is .035".  I even saw it on their computer 
screens for all brands of spark plugs.”  Ignore them, they are wrong, and they are wrong in a very big way. 

The other problem is the fact that new spark plugs for the Jaguar V12, just like most other new spark plugs, come out of 
the box with a 0.035” gaps.  They must be regapped prior to installation.  Interrogate your mechanic, making sure to use 
the word “lawsuit” when asking if he made sure the gaps were 0.025” before he installed them. 

See page 120. 

 

AFTERMARKET DISTRIBUTOR CAP PROBLEM:  Besides the problems with the rotor, there are also problems 
with Marelli distributor caps.  However, these problems appear to only occur with aftermarket caps, and only certain 
ones; some care when purchasing should avoid trouble, as discussed below. 

The problems with the cap soon become obvious: the cap chars and melts from the inside out around the base of the 
center post where the A bank coil wire plugs in.  To see some photos of such problems, see 

 

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

 

On the cheaper aftermarket cap, the carbon brush fits into an opening in the plastic of the cap at the top on the inside.  
There is a spring behind this carbon brush to hold it against the top center of the rotor.  Inside the hole, there is an 
aluminum terminal built into the plastic that the spring pushes against and the wire snaps into on the topside.  So, the 
spark from the coil comes through the wire, into the aluminum, through the spring, into the carbon, and on into the 
rotor, all by direct contact.  The only arcing going on in the vicinity is out at the tip of the rotor.   

The primary problem with the aftermarket cap is that the spring behind the carbon brush on the center electrode isn’t 
long enough.  If this brush is pushed all the way up into the cap, it barely comes back down to flush with the 
surrounding plastic.  As a result, the carbon won’t actually contact the top of the rotor, and the ignition pulses will be 
arcing from the carbon to the rotor -- perhaps aggravating the already weak rotor. 

But the worse problem results if the carbon brush is not pushed up into the cap.  Since the spring is too short, it doesn’t 
make it all the way to the aluminum terminal.  Instead, a wide spot in the spring fits snugly against the ID of the hole in 
the plastic, and acts like it is securely installed.  Although it looks good to the mechanic, the ignition pulses will now 
have to arc inside the hole between the aluminum terminal and the upper end of the spring.  This will chew up the 
spring in a hurry, making the gap get larger and larger -- and the arcs will heat things up in that little chamber, 
eventually melting the plastic and burning through the cap. 

Obviously, as a minimum fix, the carbon brush and spring should be pulled out of the cap, separated, and the spring 
stretched.  The spring is a loose fit in the hole at both ends, but there is a wide portion in the middle that fits snugly in 
the hole.  You will find that twisting the spring counterclockwise makes it easier to remove and reinstall in the hole; a 
counterclockwise twist pulls the coils a little smaller.  To get the carbon brush to snap in and out of the spring, turn it 
clockwise; a clockwise twist makes the spring coils larger.  When done, the carbon should protrude at least 1/4” from 
the plastic each time it is pushed back and released.  If you have a VOM, check the ohmage between the carbon itself 
(without compressing it back into the cap) and the electrode up on top.  It should be well under 1Ω, but it won’t be zero