Jaguar XJ-S. Service manual — part 42

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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.

While the spring stretch might render the cheap aftermarket cap workable, there is a far better fix: making sure the hole
for the brush features a metal sleeve rather than simply being a hole in the plastic. Thus, the spring will make a good
contact with the terminal whether it reaches the bottom of the hole or not; the wide portion contacting the sides of the
hole serve as electrical contact. It also protects the plastic from heat, wear, etc. And the spring seems to be easier to
install properly, since the wide portion is sliding on smooth machined metal rather than molded plastic.

This is an easy fix on the part of the manufacturers; they merely have to redesign the aluminum terminal to include a
sleeve that extends all the way to the inside surface of the cap. And, believe it or not, they have made this fix. Andy
Klopfenstein reports that “Apparently the manufacturer discovered the apparent flaw we have seen in the cap. This
new cap has a different inner aluminum cross section. The new inner aluminum shank now comes all the way to the
bottom of the center post (inside cap). You can see it when you look at the contact button. The contact button now
slides up and down in the aluminum shank.” Hey, even the aftermarket caps aren’t cheap; since you’re spending good
money, make sure you can see aluminum around the carbon brush within the cap as Klopfenstein describes before
paying for a new cap.

If you are unable to find an upgraded cap or have a relatively new pre-upgrade cap and don’t wanna throw it away just
yet, the following fix to the pre-upgrade aftermarket cap is easy: Remove the carbon brush and spring. Fit a 9/32” bit
into the drill, preferably a variable-speed critter so it can be run slowly. A drill press might also be helpful here. Drill
right down the center hole inside the cap until you get to aluminum. Keep drilling until you are drilling the bottom of
the hole, not just the sides. Then take a piece of 1/4” ID brass tubing (available at better hobby shops) and bevel one
end’s OD edge with a file. Poke this end into the hole, set the cap down on a firm surface, and tap the other end of the
tube with a hammer until it seats securely in the bottom of the hole. If it fits tight, great; if it fits too loosely, pull it back
out and apply a little JB Weld around the outside of the tube about 1/2” from the end (not at the very end!) so that it is
smeared into the edge of the plastic as the tube is driven home. Let it dry. Then, using a Dremel with a cutoff wheel in
it, cut the tubing off flush with the surface of the plastic. Clean up the edges a bit, then reinstall the spring and brush.

Regardless of what type cap you end up with -- even a genuine Marelli -- it wouldn’t hurt to stretch the spring as
described above, just in case.

THAT’S NOT ALL: George Schulte says, “I came out of a store after 15 min of shopping to see smoke billowing out
of my hood. When I opened it the flames were pretty well involved. Luckily a fire extinguisher was close at hand.
The fire began from the center of the distributor cap. The lighter cap besides being cheaper and lighter is also
flammable. I noticed some arcing around the center hi voltage lead but didn't think too much at the time because the car
was running fine. The plastic is not hi temp and the entire cap caught fire which spread to the rest of the wires in the
area which spread out and melted the fuel hoses.”

REPLACING THE DISTRIBUTOR: Perhaps the easiest and cheapest way to avoid the problems with the Marelli cap
and rotor is to replace the entire Marelli distributor while keeping the rest of the Marelli ignition system, which has no
significant problems. Any 12-cylinder distributor (or even two 6-cylinder distributors, if you can figure out how to
mount them) will take the spark from a coil and distribute it to 12 cylinders; you’re not worried about timing advance
curves because the Marelli electronics will still be handing that.

The distributor of choice is obvious: the Lucas distributor! It’ll bolt right in.

There is one problem, however: connecting the outputs of two separate ignition coils to the single center post on the
Lucas distributor cap. You can’t just splice them together; a spark generated in one coil would backfeed into the other
coil rather than jumping the gap at a spark plug. Charlie Welkie points out that there’s a product on the market for this
job: an MSD Automatic Coil Selector, part number 8210. It’s a diode pack with two inlets and one outlet for
connecting two ignition systems to one distributor cap. Its original intention is to allow racers to install main and
backup ignition systems in their race cars, and when the main one fails simply switch on the backup system and keep

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going. Jeff Strom of MSD reports: “There are 8 diodes in each unit, 4 per side.” Each diode is rated at 12KV, so it
will block a 48KV spark -- which is plenty for ensuring the spark goes to the plug instead of to the other coil.

Even though it’s not involved in timing, you should maintain the centrifugal advance mechanism in the Lucas
distributor. It’s helpful to have the centrifugal mechanism keeping the rotor tip aligned with the terminals in the cap
throughout the advance range. You can discard the vacuum advance hardware and plug the opening. You can discard
the Lucas CEI pickup and star wheel. If you happen to use an early model distributor, you can discard the OPUS
pickup and plastic rotor. You can discard the trigger board if there is one. Of course, rather than simply discarding all
this stuff, you can sell it on EBay.

Before removing the Marelli distributor, position the crank at TDC on 1A, which is indicated by a mark on the front
damper lining up with the sensor at the bottom. Pull the cap off and make sure the lower tip on the rotor points towards
the notch in the housing; if it points the opposite way, turn the crank one full turn.

You need to position the Lucas distributor correctly, and you won’t be able to use a timing light to do it. With the
crank at TDC on 1A, position the distributor so that the trailing corner of the rotor tip lines up with the 1A terminal in
the cap. It doesn’t require great precision since it’s not determining actual ignition timing; it’s just making sure the
rotor is positioned close enough that the spark can jump the gap.

Note that the spark plug wires do not go in the same places on the Lucas cap as on the Marelli cap. On the Lucas
cap, the 12 wires go in firing order. They do not on the Marelli; they are switching back and forth across the cap.
#1A is marked on the Lucas cap, and the firing order is counterclockwise looking down on it:

1A-6B-5A-2B-3A-4B-6A-1B-2A-5B-4A-3B

The distributor ventilation scheme connects to the base of the Marelli distributor, but connects to the cap on a
Lucas. Reconnect accordingly.

After you’ve driven the car a while, pop the distributor cap back off and check where the arcing is etching the tip of
the rotor. If it’s distributed along the tip of the rotor, just put the cap back on. If the arcing is concentrated on one
corner, turn the distributor a bit to try to get it towards the center of the tip. Clean the tip up a bit so you can check
it again later. It may take a couple of tries to get the position right, but once you do you should be able to forget it
for the life of the car.

“Slover Jr” performed this mod on his car with great success: “Yes, I'm using the exact twin coil separator
mentioned in your book which btw has been so helpful in my years of ownership! I purchased one off E-Bay from
a racer and it's been working flawlessly. The swapping to a Lucas dist only required a small modification to the
coil(s) mounting to make room for the larger cap and new wires as the original marked ones did not reach their
respective plugs (you could maybe make them work but I needed new ones anyway) and you need to find/make
some short ones with 90° boots on one side, straight boots on the other for the extra coil/cap connections to the
separator but other than that it was no problem.

“Also using the method you described in lining up the Lucas unit after replacing the dist and running the car for a
bit I looked inside the cap and the rotor was making contact right on spot, not bad for a w/e hack like me. It's such
an easy mod I don't know why I didn't do it sooner as I drove with fears of pyromania for too long!”

Not only will this “retro retrofit” avoid the Marelli rotor and cap problems, but it’s also likely to save the owner
some money! A used Lucas distributor plus the MSD Automatic Coil Selector probably won’t cost much more than
a new Marelli cap and rotor, so it can be done instead of the next scheduled tune-up. And the next tune-up will be a
bunch cheaper -- and you can even postpone it a long time, since the Lucas cap and rotor lasts nearly forever.

Slover Jr says, “...my shopping list was as follows:

ƒ Used Lucas dist with cap & rotor (very nice condition ready to run) $75

ƒ Used coil separator (could have went new for $60) $25

ƒ New V12 wires set (used some of my old ones to make the short leads for the coil separator) $70

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“Once I had it all done and verified it was running perfectly I sold the Marelli unit for $50 (It had 70K on it, was in
real nice shape, and I had no plans to ever go back) so my total cost was $120.

“Even if I want to sell the car I will not worry about being able to put it back to original; most people would not
even be able to notice and someone who knows these cars that well would have to agree it was a clever thing to do!”

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 11.

Figure 11 - Ignition Failure Sensor

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.

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“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

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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

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“Kirbert” is Kirby Palm’s online pseudonym.