Experience
in a Book
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 conventionally. 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.
CATALYTIC CONVERTORS --
MELTDOWN/FIRE (ONE-BANK FAILURE): Greg Maddison says,
"My car has Marelli ignition and several mechanics have told
me how faults in the system cause the car to run rich and
ruin the cats. If you suspect a spark coil is weak or you
might have a bad rotor, change them now is the advice I've
been getting. Dealer told me they junked two cars last month
because of fire from overheated cats."
While any misfire is a cause for concern with catalytic
converters, 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 engines).
Hence, the V12 is actually driveable on six cylinders;
rather than a "misfire", it runs quite smoothly, and drivers
who have not read this section may decide it's not serious
and press on. 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. What was
interesting was that it happened all of a sudden. I would
expect a bit of intermittent running on 12, 6, 12, 6, etc.
before finally shutting down one bank permanently."
Considering that people have been known to drive with
seized centrifugal advances, inop vacuum advances, kickdown
switches that don't work, one of the two intake butterflies
disconnected, or various other problems, perhaps it
shouldn't come as a surprise that some people would continue
to drive with the engine only running on half its cylinders.
There are lots of people driving around every day with their
dashboards lit up like Christmas trees, and many drivers
seem to feel that such things as idiot lights or drastic
reductions in power output are cause to visit a mechanic
within a month or two, whenever they get some time or money
to spare. Unfortunately, the Marelli-equipped Jag V12
appears well-designed to remove such drivers from the gene
pool!
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 hot before the failure occurred, he will not be
lucky; the cats will burn the fuel as it arrives, not only
keeping them at operating temperature but 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 and fire within seconds if the car is not shut
off immediately. You don't want to be driving on six
cylinders, not even for a minute.
AFTERMARKET DISTRIBUTOR ROTOR PROBLEMS:
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: arcing through the distributor rotor to the shaft.
This has been blamed on the aftermarket rotor since it has
been known to arc through within 5000 miles of installation,
while the genuine Marelli rotor apparently causes fewer
problems.
The Jaguar-issue Marelli cap and rotor are inordinately
expensive, even for Jaguar parts. This provides a powerful
incentive for owners and aftermarket shops to look to
cheaper aftermarket products -- Bosch, Beck-Arnley, World
Parts, Lucas, etc. Unfortunately, the aftermarket items are
apparently all made by the same company (InterMotor) and are
highly susceptible to defects or failures that result
in the engine running on six cylinders. This
characteristically results in a fire or meltdown in the
right side cats only, since it is always the "A" bank
ignition system affected.
Boyce, who was lucky enough to suffer only a coupla
thousand dollars worth of damage (cats melted down, cooked
several items nearby), had problems with both Bosch and
Beck-Arnley parts: "The aftermarket rotor had etching from
the spark and you could see where the spark had jumped all
over the rotor and cap. I am convinced that the aftermarkets
are junk and both failed and caused problems. I will bite
the bullet and put in the Jag cap and rotor when I next tune
it up."
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." Michael Aines had one
such fault, but he also had a center coil feed on the cap
burned away; perhaps one caused the other.
Boyce sent a set of defective aftermarket parts to this
author for analysis. The biggest and most obvious problem
was that the A bank conductor on the rotor, which contacts
the center electrode, had burned right through the plastic
rotor and was arcing to the distributor shaft, exactly as
Wilson had described. The plastic of the aftermarket rotor
is .150" thick at this point, which one would think would be
more than enough to insulate against arcing over of this
kind. It makes me wonder if the type of plastic itself isn't
ideal, or whether there may have been flaws or voids in the
molding. There are voids in it now, but it's only too
likely those are secondary -- the area around the arcing is
burned to a crisp. I don't see any obvious voids anywhere
else in the part, but of course we could be talking about
microscopic voids and I'm only using a 10x magnifier to
inspect it.
AFTERMARKET DISTRIBUTOR CAP PROBLEMS: The problem
with the aftermarket cap inspected by this author was less
obvious: the spring behind the carbon brush on the center
electrode wasn't long enough. If this contact is pushed all
the way up into the cap, it barely comes back down to flush
with the surrounding plastic. Since it is clear at what
level the rotor was riding from the wear on the electrodes,
I can establish that a carbon brush held at this position
will not contact the rotor. Instead, the ignition
pulses will be arcing from the carbon to the rotor, perhaps
aggravating the already weak rotor.
Furthermore, upon pulling this spring out, I found that
it hadn't been installed all the way -- perhaps
because it is too short. As a result, it had been
arcing inside between the top end of the spring and the
aluminum cup inside the cap rather than being in direct
contact with it. Of course, the spring was getting all
charred and probably wouldn't last long in this
condition.
Offhand, it's not known for sure if this spring problem
actually contributes to the one-bank ignition failures. Mike
Morrin says, "From my knowledge of high voltage dielectric
behaviour in ignition systems, I would hazard a guess that
the short spring may well cause the failure of the rotor and
cap. I can easily imagine the effect being something like an
accelerated life test for the rotor, causing micro-arcing,
heating, cavitation and failure."
DISTINGUISHING MARELLI CAPS AND ROTORS: Since it
is apparently of some importance to know whether you are
buying the genuine Jaguar/Marelli caps and rotors or the
aftermarket substitutes, some guidelines on how to tell the
difference are in order. Randy Wilson provides the most
basic indication: the genuine items will say "Marelli" right
on them.
Regarding the rotor itself, the clearly-defective
aftermarket one this author cut up is off-white and says
"MADE IN ITALY" and "F 258 6E" on it. The metal parts are
brass. Greg Maddison says, "I have a genuine Marelli rotor,
it's orange and has the Marelli logo clearly embossed on the
top of it. I think the contacts are brass."
Regarding the caps, Boyce describes the Bosch and
Beck-Arnley items he had trouble with: "The aftermarket caps
were identical (maybe just different boxes) and much lighter
than the original. They appear to have aluminum as terminals
while the Jag cap has copper (?). If you compare the OEM and
the aftermarket side by side, there is no comparison!" This
author inspected one of these aftermarket caps as well; it's
black, and other than the plug numbers and the "A" and "B"
coil connection labels, it doesn't say anything at all on
it.
Just because the part says "Made in Italy" doesn't mean
it's made by Marelli. Randy Wilson points out that "The
cheap-junk cap and rotor is made by InterMotor, also an
Italian firm. Note the weight of the new cap. If it is as
you would expect for a piece of plastic that size, it's the
imposter. The OEM cap will feel (weight-wise) more like cast
iron."
GENUINE MARELLI ROTOR FAILURES: Julian Mullaney
reports: "I recently had a very close call with my '91 V12.
It quit firing on the A bank and the cat got red hot. Today
I diagnosed the cause as a fault to ground through the
distributor shaft via the rotor. This is precisely the way
that Kirby diagnosed the fault with an aftermarket cap and
rotor. There was a black pitted hole going from the center
contact down throught the bottom shorting on the top of the
dist. shaft (less than 50K miles on car).
"So why am I telling you? Because this was not an
aftermarket rotor or cap. It was the original Marelli cap
and rotor. Both have the Marelli name molded on them. So
this means that these failures occur the same way with OEM
equipment."
So, it appears that the aftermarket parts may cause
problems sooner, but eventually the genuine Marelli
items will too.
AVOIDING THE PROBLEM: There are already rumors
that Marelli has ceased making these caps and rotors; even
if false, it's likely they will cease making them sooner or
later -- the V12 is no longer in production. And as we have
learned, even the genuine Marelli parts may cause trouble.
So, unless and until Jaguar issues a recall and installs
redesigned caps and rotors (would be easy to do; keep your
fingers crossed!) it will be necessary to make do with the
parts currently available. Note that, as of this writing,
the following ideas have not been tested, so if you put them
to use be very sure to keep an eye on things and be certain
to pull over immediately if symptoms of six-cylinder
operation appear.
AVOIDING THE PROBLEM -- POTTING THE ROTOR: Perhaps
the most obvious approach is to "pot" the hole in the bottom
of the rotor where it sits on the shaft, in hopes of
increasing the amount of dielectric material the arc would
have to break through to get to ground. When the rotor is
installed, there is a convoluted air space surrounding the
top of the shaft within it, and the arc seems to break out
of the plastic and run along the surface until it gets to
the shaft. filling this space with a dielectric material
should make it harder for electricity to break through,
hopefully making the rotor perfectly reliable. Of course, I
would only suggest trying this at all with a brand new
rotor, rather than trying to refurbish one that has already
burned through and is all charred and dirty.
The simplest idea is probably to set the rotor down on a
table and pour enough potting material in to fill the bottom
1/8" of this hole. Care should be taken to make sure it
doesn't get in the way of the shaft, possibly by carefully
cutting or drilling some of the potting material back out
after it hardens. The hole is shaped like an asterisk (*)
and it's gotta be an airtight potting job to do any good, so
either a very fluid potting substance must be used or a very
careful job must be done filling all the little corners.
Mike Morrin suggests, "If you need a 'runny' epoxy, use HV
cable jointing epoxy (can't remember the brand name), but
you need a mould, or it goes everywhere." No mold needed for
this job, just set the rotor on the table hole side up. "The
job we used it on also required it to run into a mould and
be bubble free, so we mixed it, poured it in, and then
heated it to about 100 degrees C to reduce the
viscosity."
Pouring potting in the hole will still leave a lot of air
space, of course, and Julian Mullaney points out that you
might accidentally cause an interference with the fit on the
shaft. "Yes, it is very difficult to tell. The plastic ears
would flex down when you tighten the screws and you'd never
know it." This may result in a stress failure of the
plastic, and at speed this could in turn cause all sorts of
damage. To avoid both problems, the second idea was
proposed: make a dummy plug to simulate the shape of the
shaft, and use it to completely fill the space while leaving
a formed hole to fit the shaft.
A dummy plug can be made from a metric bolt or cap nut
with a suitably-shaped 10mm hex and a flat washer. 6mm cap
nuts with 10mm hexes are commonly used on bicycles and come
in a wide variety of configurations, so a visit to a bike
shop should provide a selection from which one could be
found that closely resembles the shape of the stub on the
shaft. Obviously, having a shape that makes a hole a little
too big on one side or the other is no problem, but you
don't want the hole any smaller; either make sure the plug
is properly shaped, or make sure you cut any interferences
out before installing. Spray a little release agent (Armor
All, Pam, whatever) on the plug before using to make sure it
comes out easily. Once the plug is created, it can easily be
saved for use on replacement rotors in the future. Perhaps
local Jaguar clubs could provide such a device, or even pot
rotors for their members.
When preparing the plug, you might choose to forget the
washer entirely and simply carefully insert the plug into
the rotor by hand as far as you think it needs to go; you
could scratch a mark on the side to indicate the proper
depth. This would allow free access to the six chambers
surrounding the center hole so you could see what's going
on, air could come out, whatever. You could also opt to use
a flat washer that's only slightly bigger OD than 10mm,
thereby leaving most of these passages uncovered.
Mullaney suggests an alternate way to form a plug: "Take
old rotor and pack it full of plastic molding compound.
Press onto the dist shaft. Remove (now you have the
negative). Pour potting compound into the rotor with the
molding compound still in there. Remove potting compound
(this is the replica of the shaft). Spray replica with Armor
All (release compound). Install replica of shaft in new
rotor. Pour final insulating potting compound into the void
between new rotor and replica. Remove replica after potting
compound is cured. Install the new "potted" rotor. Preserve
replica in glass case for other jag-lovers to use."
Mullaney still ain't happy. "If we pot the rotor, it
becomes essentially solid instead of a webbed plastic molded
part. I'm concerned that our potting could expand when hot,
and cause the rotor to undergo stress during operation which
could cause it to crack over time. It's made of pretty
strong yet brittle plastic. After all, the reason they don't
just mold the thing solid is because it is less
strong that way (thermoset vs. thermoplastic resins). If the
rotor breaks then it would shatter the distributor cap ($$)
and generally wreak havoc in the distributor."
Of course, the expansion issue might be addressed by
selecting a flexible potting material. Andrew Corkan says,
"I use urethane and silicone potting compounds for various
products I manufacture. Generally your local supply house
will carry this stuff in small quantities. The 90 durometer
hardness stuff is the most common and cheapest, just a tad
harder than your average rollerblade wheels. Try that stuff,
it should be the combination of features you want. Also, it
is best to pull a vacuum on the stuff to get the air
out."
Mullaney is also concerned about adhesion, and recommends
an RTV compound instead of potting compound. "Believe it or
not, we happen to do a lot of testing of potting compounds
where I work. We don't do anything with dielectric testing,
but we analyze bonding of potting compounds to plastic
surfaces extensively. It's very difficult to get a rigid
potting compound to stick to a plastic surface because of
thermal expansion effects. They tend to disbond with time
and temperature cycling. Soft, compliant compounds are much
better stickers. Hence the idea of RTV silicone. Silicones
are designed to bond well to smooth surfaces such as
ceramic, glass and smooth plastics."
Of course, the potting won't do any good if RTV isn't any
good as a dielectric. So Mullaney "consulted with one of our
resident Ph.D. material scientists to find the best
dielectric barrier material for the interior of the Marelli
distributor rotor. He (Trey Simendinger) prefers a silcone
RTV material because it bonds well to the substrate, and
will not delaminate due to temperature fluctuations. He
recommended a GE brand silicone RTV standard window caulking
material from hardware store. Get a clear one because this
will have no filler in it (better dielectric properties).
Typical dielectric breakdown is 400 v /.001 inch for
silicones versus ~200 v/.001 inch for air. He says to wash
the part with soap and water first to ensure a good bond of
RTV to the plastic rotor material. He did not recommend
potting compounds because they consist largely of filler
materials like calcium which reduce the dielectric breakdown
voltage and they do not bond well." A clear material has
another advantage: you can see any air bubbles.
One idea may be Permatex Flowable Silicone Windshield
Glass Seal. It's clear, and it flows -- unlike most sealants
(it's about the consistency of honey). The flowing is good
because it will help get down into the nooks and crannies
inside this rotor.
Mullaney: "How about an alternative and even easier
solution: Why not just put a nice big squirt of RTV silicone
in the new rotor just before installing it? Of course you
would need to take care that air bubbles were not trapped in
the cavity prior to installation. I think an excess amount
would encourage air voids to be expelled (of course clean up
excess ooze). It would form nicely around the complex rotor
shaft, and cure in place. Most importantly, it would remain
compliant, therefore not exerting any stress on the rotor. I
would wipe some oil on the distributor post first so the RTV
will not stick to the metal, thus allowing easier removal of
the rotor at a later date."
Of course, a combination of ideas may work; the nooks and
crannies may be filled using a liquid with the plug, and
then a small amount of a thicker RTV could be added to the
finished rotor prior to installation as well.
The squirt-and-jam method may be easy, but you will need
to wait for the RTV to fully set up before closing up the
distributor and driving the car. Hence, the use of a dummy
plug has the advantage that you can have the rotor prepared
in advance, ready to install and run.
AVOIDING THE PROBLEM -- SHORTENING THE SHAFT:
Since it is really the flange and two screws that hold the
rotor in place, it might be a workable plan to cut part of
the top of the shaft off. Enough should be left to center
the rotor before the screws are put in, but it shouldn't
need 1/4" to do that. If the 1/4" were cut down to 1/8" it
should still work right, and that makes 1/8" farther the arc
has to jump. And it allows more potting material to be
stuffed into the hole; since the potting material has a
particular dielectric rating per thousandths of an inch,
increasing the number of thousandths the arc must break
through will increase the number of volts it takes to short
out. Of course, the dummy plug will need to be redesigned to
match.
AVOIDING THE PROBLEM -- CLOSING THE PLUG GAPS: The
collapse of the magnetic field in the coil causes a rapidly
rising voltage. When the voltage is sufficient to jump the
gap at the plug (and the gap in the distributor as well), it
does so, and the voltage never gets any higher. This is why
it's bad to have the thing firing without a spark plug
connected; the voltage continues to rise and has noplace to
go, so it may reach high enough levels to damage electronics
(the Lucas CEI has a zener diode to protect it from such
abuse). If the spark plug gaps are reduced, the spark should
occur while the voltage is still at a lower value and the
dielectric strength of the rotor doesn't need to be as high
to prevent arcing. The V12's with the Lucas CEI ran just
fine with .025" gaps, so the Marelli-equipped cars should as
well.
AVOIDING THE PROBLEM -- STRETCHING THE SPRING: I
can see no inherent problems with the aftermarket cap; the
electrodes are aluminum, but I've had no problems with
aluminum electrodes in caps for other cars. Offhand, I would
suggest that the aftermarket caps would be OK to use with
one proviso: prior to installation, the carbon brush and
spring should be pulled out of the cap, separated, and the
spring stretched. You will find that twisting the spring
counterclockwise makes it easier to remove and reinstall; a
counterclockwise twist pulls the coils a little smaller.
After some fiddling with the one I had to play with, I
suggest the following procedure: Once the spring is out, you
will note that it has a large diameter coil in the middle
(to hold the spring in the cap), widely-spaced coils
tapering gradually smaller in diameter toward the carbon
brush, and a smaller constant-diameter set of coils stacked
on top of each other (no compressibility at all) toward the
cap electrode. I suggest turning this spring upside down and
using the constant-diameter coils to push the carbon brush.
It is a simple matter to stretch the coils out so they
provide some springage. It is a little trickier but still
not too difficult to compress the diameter of a couple coils
at the end so they will grip down on the carbon brush.
Gripping the carbon brush is only necessary to get the cap
on without the brush falling down into the works.
Once you have that spring stretched out to a total length
of about 1-1/2", install it tapered-coil-end first into the
cap, making sure the upper end of the spring is firmly
positioned against the electrode at the bottom of the hole.
Again, twisting counterclockwise will help, possibly holding
the end of the spring in a pair of hemostats or small
needlenose pliers while twisting and pushing. Then the
carbon brush should be pushed back into the center of the
spring. To get the carbon brush to snap in, it helps to 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 1 ohm, but it
won't be zero because carbon isn't really that good a
conductor. If it's an open circuit (infinite ohms), you
didn't get that spring seated against the electrode; pull it
apart and try again.
ELIMINATING THE PROBLEM: Bill Freeman points out
that one sure way of avoiding the Marelli ignition system
problems would be to toss it and install an aftermarket
ignition system that doesn't rely on the Marelli rotor and
cap. Any of the systems that use six double-ended coils
would do, but note that some aftermarket systems use only
one or two coils along with the stock cap and rotor and
therefore won't help.
Another idea might be to retrofit the earlier Lucas
system into the later cars. It would be kind of a shame to
replace the high-tech electronically-controlled Marelli with
the comparatively archaic Lucas with its mechanical advance
controls, but it would definitely eliminate the possibility
of catalytic convertor fires from that particular cause.
Note that the plugs would have to be gapped to the earlier
specs (0.025", 0.64mm) and that the centrifugal
advance mechanism should be attended to prior to
installation.
ELIMINATING THE PROBLEM #2: Hey, the problem isn't
the entire Marelli system, it's just the cap and rotor. One
positive fix would be to eliminate the suspect cap and rotor
altogether by replacing the Marelli distributor with two
generic 6-cylinder distributors; connect each coil to one
distributor cap and connect the plug leads accordingly, and
you'll be good to go. Since the timing is actually handled
by the crank and flywheel pickups, coordination of two
separate distributors is not a problem; they merely need to
be close enough for the spark to jump to the correct
terminal. One other plus: if you choose the right 6-cylinder
distributors, caps and rotors will be a lot cheaper
than the Marelli parts -- even buying two of each!
Really silly idea, right? Well, before you dismiss it out
of hand, note that Mike Morrin points out that the Jaguar
V12 was designed for dual distributors! If you pull off the
valley cover, you will find an extra bore for the jackshaft.
The valley cover itself has a boss for a second distributor
underneath the bellcrank pedestal, although it's not
drilled. So maybe it's not such a silly idea after all.
Not easy, though. If you make this modification, you're
gonna face two challenges: 1) coming up with a longer
jackshaft with two distributor drive gears on it; and 2)
devising a throttle linkage when you have a distributor
where the bellcrank pedestal once stood.
DETECTING IGNITION FAILURES: As soon as the
Marelli failure mode was discussed and understood on the
online xj-s discussion list, several members proposed
methods to detect such a failure and thereby avoid disaster.
Andrew Corkan promptly designed a dirt-cheap circuit for
detecting whether or not there are sparks going to each bank
and providing a warning indication if they are not -- see
Figure
7.
Corkan describes his creation: "The circuit detects a
spark in either bank by means of an induction pickup placed
on a plug wire, similar to the ones used by tune-up timing
lights. When a spark pulse is detected it shorts out an RC
circuit that would otherwise cause an LED to light up."
"You have to make your own inductive pickup using a
so-called RF choke (really just a two-part inductor core)
and magnet wire. Open the core and remove half of it from
the plastic holder. Wrap 50 loops of magnet wire around the
half of the core still in the plastic holder. You do not
want the magnet wire exposed in the engine so solder regular
wire to it and pot that half of the pickup sensor, magnet
wire and solder joint in RTV. The wires you solder on should
be long enough to reach into the car where you will place
the circuit. Replace the rest of the choke when you install
it around the plug wire. The choke will fit around the plug
wire, no need to remove the plug wire.
"There are two options for hooking up indicator lights
(see dotted lines in circuit schematic). The first option is
to use LED's. Just wire them as D2 and D4 and bring the
wires into the interior. Kirbert suggested a second option
of using the 'trailer' bulb in the dash as an indicator
light. For this option you can wire up D5 and D6 to the base
of Q3, which will in turn power up a 12 volt lamp." Note
that in this latter case, a spark failure in either bank
will light the same light; arguably, the driver doesn't
really care which bank just failed, as long as the
light tells him a bank just failed (with the Marelli rotor
failures, it's always the A bank that fails). The owner may
choose to use the trailer warning light, the seat belt
warning light, or any other 12V light he wishes, and he may
want to relabel the indicator accordingly, color it red,
etc.
"All references in the schematic to "+12V" are to a
switched (accessory) power source. When you hook things up
you should see the lights come on when you move the key to
'accessories' but they should turn off as soon as you start
the engine." Hence, no need to provide a separate bulb check
circuit.
"All the parts for this circuit are available from Radio
Shack (but our overseas friends should have no trouble
finding these parts). In addition to what is listed here you
will need wire to run from the sensors to the box you put
the stuff in, and wires to run the LED's or the signal light
into the dash (if you use that option). You will also want
to get a small circuit board or breadboard and a little
plastic box to put the circuit into; a 3 in. x 4 in. box is
just fine."
Part #
|
Value/type
|
Radio Shack #
|
R1
|
9.1K
|
|
R2
|
2.2K
|
|
R3, R5
|
22K
|
|
R4, R6
|
40K
|
|
C1, C2
|
470uF, 16v
|
272-957
|
Q1, Q2
|
IRF510, N-Channel MOSFET
|
276-2072
|
Q3
|
TIP31 NPN Transistor (option B)
|
276-2017
|
D1, D3, D5, D6
|
Germanium Diodes
|
276-1123
|
D2, D4
|
LED any kind will do (option A)
|
|
L1, L2
|
RF choke (two part inductor cores)
|
273-104
|
LAMP1
|
Lamp on the dash (option B)
|
|
Magnet wire
|
any kind will do
|
278-1345
|
Wire
|
any kind will do
|
|
Box and board
|
any kind will do
|
|
"It is easy to build, all you need is a soldering iron.
It will monitor both banks and turn on a light if there is a
Marelli boo-boo."
Different colored D2 and D4 LED's may be used for the
indicator lights to indicate which bank, or red could be
used for both to indicate trouble. As shown, the circuit
monitors one plug wire (chosen at random) on each bank. One
plug wire on each bank would be enough to detect the common
Marelli failure (really, you might as well save your money
on circuit components and just build one circuit to monitor
the A bank), although you could get carried away and build
12 circuits instead of the 2 shown and install a complete
set of 12 LED's if you wanna really be sure you detect any
possible problem.
Corkan goes into more detail for those interested: "The
circuit uses two identical channels for each bank. Pulses
from the inductive pickups trigger the MOSFET transistors
(Q1, Q2), which are sensitive to voltage. They are pretty
standard N-channel MOSFETS that will trigger at about 3.0 -
3.5 volts. To make sure they work reliably with the faint
pulses coming from the sensors, R1 and R2 provide an offset
voltage of about 2.5 V. (If you can not locate the right
MOSFET you may have to change the values of R1 and R2, they
should be selected so they bias the sensor pickups to 1.0 V
less that the MOSFET needs to trigger.)
The diodes (D1, D3) are very important. The diodes keep
the voltage pulses from going back down the sensor wire,
forcing it to drain more slowly through R3 or R5. Germanium
diodes have a small voltage drop, unlike standard diodes. If
you do not use Germanium then you will need to increase the
bias voltage on the sensors by about 0.5 V to make up for
the additional voltage loss.
Each time the MOSFET's fire they short out the RC circuit
formed by R4 and C1 (also R6 and C2). This RC circuit takes
about two seconds to charge, and when it does voltage then
flows into D2, lighting it up and warning the driver (or
flowing through D5 to the NPN transistor which lights the
lamp, etc...). When a pulse hits Q1 (or Q2) it drains C1 (or
C2) preventing the indicator circuit from ever getting
enough voltage to light. As long as there is a steady supply
of pulses the RC circuit never charges and the light never
lights."
The polarity of the pickups on the plug wires makes no
difference, since the pickup will put out a voltage pulse in
one direction when the spark starts and a very similar pulse
in the opposite direction when the spark stops. Corkan:
"...and thus the reason for the first diode. In fact the
circuit is sensitive enough that polarity will not make a
difference, even though you can see a small difference in
the signal on an oscilloscope."
An alternative is to detect
the overheating of the cat itself. This has the clear
advantage of indicating a problem no matter what the
cause.
AUTOMATIC SHUTDOWN: Once you have some sort of
failure detection system that will tell you that the Marelli
has quit firing one bank, there is still the question of
what to do with the indication. You could provide an idiot
light, but anyone that drives despite a sudden lack of
horsepower may likewise drive despite a warning light; some
XJ-S owners allow others to drive their cars and don't want
to have to lecture them on the importance of a particular
idiot light each time. Another possibility is to have the
sensor shut down the car; the power to the entire ignition
system could be shut off. If Corkan's ignition monitoring
circuit can light one of the dash indicators, it could
probably be used to operate a relay to shut off power to the
ignition -- but you'd have to install the relay so that it
doesn't cut off the ignition when the ignition switch is in
the "start" position or you'll never get the car
started.
LIMP-HOME MODE: Automatically shutting off the
engine in the event of six-cylinder operation protects the
car but aggravates the driver, especially if it happens in
traffic or in a bad neighborhood; false failure indications
would suddenly become very distressing. A very popular idea
was to shut off fuel to the bank that has lost spark,
thereby making it safe to drive the car on six
cylinders. It's not known if this is practical; simply
disconnecting the injectors might cause an EFI ECU failure,
for example. Just to be safe, maybe it would be a better
idea to switch the injector power from the injectors to a
resistor pack, so the ECU doesn't see any difference in
load. That same relay idea might work for this, using
Corkan's circuit with two separate Q3's (you can omit diodes
D5 and D6 if each subcircuit controls its own Q3) to control
separate fuel injector shutoff relays for each bank. Again,
provision will need to be made to get the car started.
The idea of shutting off fuel to one bank and continuing
to run does have one important benefit: it is likely to
provide a better cooldown of the overheated parts. Several
owners, having already been made aware of the Marelli
problems, nevertheless reported that the right side cats
were cherry red hot by the time they figured out what was
happening, shut it off, and got the hood open. When the
engine is simply shut off under such conditions, the
residual heat within the cat may nevertheless fry nearby
rubber parts, sensors, etc. If the fuel is shut off and the
engine continues to run, cooling air continues to flow
through the engine compartment, and air continues to flow
through the cylinders of the inop bank and out through the
exhaust system. A running cooldown may be better on the
engine and ancillaries than a sudden shutdown. Perhaps even
if a sensor provided only a warning light indication, a
switch could be provided to manually shut off fuel to the
inop bank to allow the car to continue moving.
CATALYTIC CONVERTORS --
MELTDOWN/FIRE (SENSOR-CAUSED): Greg Maddison lamented a
large loss to his wallet: "The front two had melted into a
solid mass and the debris ruined the back two." Obviously,
this symptom differs from the typical Marelli six-cylinder
operation described above, since that will toast the cats on
one side only.
"It seems there is yet another reason the Marelli system
can cause a misfire that brings the cats up to kiln
temperatures. This would be problems with the TDC
sensor/magnets. There are 3 equally-spaced bar magnets
around the crank damper and a sensor that "looks" at the
magnets and sends a pulse directly to the Marelli computer.
There is a specific air gap between the magnets and sensor
that must be maintained for the system to work properly.
The Jag dealer replaced the damper/magnet assembly on my
car saying the rubber had split. This caused the air gap to
be too wide between one of the magnets and the sensor. They
first tried to grind down the other two magnets and move the
sensor closer (sounds like the damper deformed and became
eccentric), only lasted 600miles so I had to have damper
replaced. The dealer's mechanic explained to me that each
magnet produces a timing pulse for 4 cylinders. If one of
the pulses are missing, 4 cylinders don't get spark and send
unburned fuel through the cats. The mechanic said that a
different damper is fitted to cars with the 100-amp
alternator (like mine) and fails more often than the older
cars." Note: the 100-amp alternator requires a grooved belt
instead of a V-belt, and the alternator belt is driven by
the damper itself rather than the bolt-on pulley, so the
100-amp alternator does require a different damper. Perhaps
it fails more often because of the increased torque load of
the alternator being applied to the rubber.
"After removing the crank pulley they found the woodruff
key sheared and cone split, pulley had rotated 6 degrees on
the shaft. Mechanic says the Marelli computer can very the
timing up to +/- 28 degrees. So, at this point I don't know
if they misdiagnosed the sheared key problem as a damper
problem (they are claiming both are toast) all along. I can
certainly understand how having the pulley rotate on the
shaft can introduce timing errors on a Marelli car."
CATALYTIC CONVERTORS -- MELTDOWN
(SECONDARY DAMAGE):
Greg Maddison says, "Also check the engine speed sensor
located on the flywheel housing. It is a $40 part located
very near one of the catalysts. Mine melted when the
catalyst overheated. After that the speed sensor became
temperature sensitive, car ran well when it was cold. After
it warmed up the car wouldn't develop any power and couldn't
go over 40 MPH."
INTERMITTENT FAILURES: Greg Maddison suffered from
electrical connections: "Occasionally when warm/hot the
engine just stops. It will usually restart after a few tries
or if left for a couple of minutes. It turned out to be the
crankshaft sensor connector. I could jiggle the crank sensor
and car would start again. If you have the Marelli digital
ignition system you will have a crankshaft sensor and a
flywheel sensor; engine will not run if the signal is
interrupted from either. Look for a two-wire connector on
the left front top of the engine; wires should be in a hard
plastic wire protector running down the front of the engine
to the back side of the crankshaft pulley. I found the
contacts in this connector to be green and nasty. Cleaning
with spray-on relay and contact cleaner cured the problem.
Run the engine and wiggle this connector and see if it
stops. Also check the flywheel sensor connector, it's on the
rear top left of the engine, looks like the crank sensor
connector and hard to get to."
John J. Lynch had a similar case: "The problem went
something like this:
1) Car started fine
2) Ran normally for 15 minutes
3) Died completely due to spark failure
4) Engine cooled down for 1-4 hours
5) Repeat step 1
"Turns out the Engine Speed Sensor was the real
culprit. The specs say that the sensor should read about 700
ohms but when I measured it after the engine died I noticed
it was only reading about 260 ohms. Since I had previously
tested the sensor with an O-Scope and saw that it was
producing the square wave I was expecting, I thought
everything was fine. I was wrong. For some reason when the
engine warms up the sensors resistance is dropping below 700
ohms causing the voltages that are delivered to the ignition
computer to be way out of spec. Consequently the computer
was shutting down the ignition amps. For kicks I decided to
put a 500 ohm resistor in series with the sensor to validate
my new conclusion and sure enough the car ran fine. As soon
as I removed the resistor, the car died again. Problem
solved!!!"
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.
"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, ëtis 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."
1992-ON: Richard Mansell quotes from a Jaguar
Publication on the changes for the 1992 model year:
The Marelli
digital ignition ECU has been software upgraded.
CAP VENTING: The Marelli distributor is vented in
the same manner as the Lucas
distributor, 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.
On to the
XJR-S
Ignition
|