[v12-engine] Dumb Question on Marelli Fires

V12-Engine
1

Ahoy !

Caveat: Electronics are beyond me. My question is rooted in
naivete.

What would the procedure be to install a warning device or
even a cut-off switch w/ manual over-ride when ‘‘that’’ coil
or either coil fails ?

Thanks.
ps. Hmmmm ? I wonder if the large XJS boot is a design
feature to hold all the spares and tools ? - RC–
R.Cielec 1961 MkII 3.8 Auto 1992 XJ-S Convertible
Chicago/Illinois, United States
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2

In reply to a message from RCielec sent Sun 31 Jan 2016:

‘‘What would the procedure be to install a warning device or
even a cut-off switch w/ manual over-ride when ‘‘that’’ coil
or either coil fails ?’’
kirbert has in his book a couple of failure detection
methods.a safety item to consider other than coil
elimination via a switch would also involve turning off the
injectors on the failed bank at the same time.
this would prevent fuel contacting the hot catylists.
however…if the marelli dizzy is checked for carbon
tracking problems when you make oil changes then the risk is
drastically reduced.
‘‘an ounce of prevention’’ :slight_smile:
=dok=–
thewytchdoktor=v12 fun!/94 xjs 6 litre/ s=k.log w
Winchester Virginia, United States
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3

In reply to a message from RCielec sent Sun 31 Jan 2016:

More importantly might be an ounce of prevention, make sure
your cat gets fresh plugs and wires replaced every year or
two (at the max)and you will probably never experience an A
side failure.
Also the dizzy mod would add more protection for you. The
owners who carry out the yearly maintaince are the ones
who never have this problem!
Also take a good look at the fuel injection hoses, they
should be changed every ten years.

Dan–
The original message included these comments:

What would the procedure be to install a warning device or
even a cut-off switch w/ manual over-ride when ‘‘that’’ coil
or either coil fails ?

–
DanS
columbus ga. usa, United States
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4

In reply to a message from DanS sent Tue 2 Feb 2016:

Amen to that.

I have a car that I bought as a Marelli victim, and the
cause was poor maintenance/cheap maintenance of the
ignition: a very poor quality aftermarket cap and rotor had
failed.

The PO had not been all that conscientious as regards plug
changes, either. I found two different brands (!) in the
heads.

Keep that ignition up to snuff, and keep the fuel system up
to snuff (filters, pressure regulators, all important hoses
at the injectors), and you really don’t have much to worry
about.

My car’s history: 1990, by 2002 failed by Marelli. Since
when I got it back on the road (2003), it has not missed a
beat- had to be towed twice: once due to a failed starter
(consequence of the fire, I suspect) and once the very
first time I took it out after fitting the manual box: I
had set the Marelli flywheel sensor clearance incorrectly,
and after the car warmed up, it shut down. After I
adjusted the clearance, car has been fine ever since.

My own opinion is that, sure, the Marelli cars will bite,
but not unless wantonly neglected.

Pay attention, and you will be fine.

-M–
The original message included these comments:

Also the dizzy mod would add more protection for you. The
owners who carry out the yearly maintaince are the ones
who never have this problem!
Also take a good look at the fuel injection hoses, they
should be changed every ten years.

–
Mike, '90 5.3 XJS Conv., 5-spd+3.54, SE-ECU+TT F/R bars
Lakewood, OH, United States
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5

In reply to a message from wytchdoktor1 sent Mon 1 Feb 2016:

What would oil changes have to in relation with the Marelli
dizzy for carbon tracking. I think I’m missing something on
that part.
Larry
91 XJ-S 12cyl 5sp
95 VDP 6cyl–
The original message included these comments:

however…if the marelli dizzy is checked for carbon
tracking problems when you make oil changes then the risk is
drastically reduced.

–
Larry Hartman
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6

If you ignore the distributor you could eventually have problems. If you
check it and clean it periodically, such as every time you change your oil,
then the risk is drastically reduced.

Mike Eck
New Jersey, USA

'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2

What would oil changes have to in relation with the Marelli
dizzy for carbon tracking. I think I’m missing something on
that part.
Larry
91 XJ-S 12cyl 5sp
95 VDP 6cyl

The original message included these comments:

however…if the marelli dizzy is checked for carbon
tracking problems when you make oil changes then the risk is
drastically reduced.

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7

In reply to a message from LarryHartman sent Thu 11 Feb 2016:

hi larry…the reference to checking the cap for carbon
tracking is in regards to time intervals for
inspection.nothing to do with the actual oil change.
it’s a good thing/practice to get into as carbon tracking
tends to either follow small(micro) fractures in the cap or
buildup of dirt on the inside of the cap(insufficient
filtratio9n of the incoming vent air).
sorry for not getting back sooner.i only check this every
couple of weeks when I am doing a build.
been very busy as of late:-)
=dok=–
thewytchdoktor=v12 fun!/94 xjs 6 litre/ s=k.log w
Winchester Virginia, United States
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8

In reply to a message from RCielec sent Sun 31 Jan 2016:

If you want a warning device that is cheap and easy then why not
cobble together two pickups from a pair of timing lights and have
them drive a pair of LEDs on the dashboard instead of the strobes
to check timing.

If one of the LEDs stops flashing, pull over and investigate.
Either one cylinder has no spark any more or if it persists when
you swap the pickup from 1A to 2A (etc) then you have lost ignition
on that whole bank.

kind regards
Marek–
The original message included these comments:

What would the procedure be to install a warning device or
even a cut-off switch w/ manual over-ride when ‘‘that’’ coil
or either coil fails ?

–
v12 E-type running MS3/3X sequential lpg and petrol
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9

If you want a warning device that is cheap and easy then why not
cobble together two pickups from a pair of timing lights and have them
drive a pair of LEDs on the dashboard instead of the strobes to check
timing.

If one of the LEDs stops flashing, pull over and investigate.

It’s not as though you can’t TELL there’s a problem. The car dramatically
loses power. Those of us on this forum know enough to pull over
immediately and investigate. The problem is, what happens when it’s the
wife or some friend driving the car? A pair of LED’s on the dash won’t mean
any more than the sudden loss of power will. The car continues to run
smoothly, so they’ll decide to just continue on home and tell you the car
doesn’t have the ooomph that it should when they get there. You need a
device that will either A) positively TELL them to shut it off NOW; or B) shut it
off for them; or C) make it safe to continue driving on one bank. And none of
that is simple.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 14 Feb 2016 at 6:11, MarekH wrote:

10

You need a> device that will either A) positively TELL them to shut it off NOW; or B) shut it

off for them; or C) make it safe to continue driving on one bank. And none of
that is simple.

– Kirbert

I do not have a Marelli car.
You could wire the injectors through relays, and since there are 2
groups on every bank you need a total of 4 standard single pole
relays, operating 2 will disable one bank.

If there are two separate coils, one per bank, you need monitor the
low voltage switched output to each coil.
When there are no pulses on a coil you operate the relay.
That still lets you drive on 6 cylinders, much safer than cutting all power.

For cranking you have to inhibit this system.

All up it would be a fairly simple safety system to get working.

Richard Dowling, Melbourne, Australia. 1979 coupe + HE V12 + manual;
1989 convertible; 2003 XJ350.

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11

You could wire the injectors through relays…

That’s easy.

If there are two separate coils, one per bank, you need monitor the
low voltage switched output to each coil.

The problems with the Marelli happen after this point: Either the coil fails or
the spark shorts to ground. So monitoring the low voltage accomplishes
nothing.

For such a scheme to be truly helpful, it needs to monitor the spark in the HT
wires to the plugs. The pickups from inductive timing lights would work, as
suggested earlier, and you really only need monitor ONE plug wire on each
bank. But then you have to have a circuit that will watch those sparks as
they vary from idle speed to redline and decide when they have quit
altogether, which means some sort of timer that waits until it hasn’t seen a
spark for a half second before it decides to take action.

For cranking you have to inhibit this system.

Yeah, and that’s another level of complexity.

All up it would be a fairly simple safety system to get working.

What you consider simple, many owners would find complex. This whole
idea has been discussed before, and someone even provided a schematic for
the circuitry. But I’d be willing to bet that NO ONE here has actually installed
it in his car.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 15 Feb 2016 at 19:26, Richard Dowling wrote:

12

What you consider simple, many owners would find complex. This whole
idea has been discussed before, and someone even provided a schematic for
the circuitry. But I’d be willing to bet that NO ONE here has actually installed
it in his car.

– Kirbert------------------------------------------------------------------------------

You can detect the HV pulses in a plug lead without major cost or difficulty.

If I did have a Marelli car I would have done it long ago.

A rough idea for making a control unit plus 2 HV plug lead pickups
would be $200 per set.
That would need a minimum of 10 units to be viable.

Trouble is, I would only do these things out of interest and I can use
the facilities of my business to subsidise it.

Right now not motivated to do this for many reasons.
Seeing the risk factor it is surprising nobody else tried it.
If nobody has done it perhaps the risk is underestimated.

Richard Dowling, Melbourne, Australia. 1979 coupe + HE V12 + manual;
1989 convertible; 2003 XJ350.

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13

In reply to a message from Richard Dowling sent Mon 15 Feb 2016:

Overestimated…–
The original message included these comments:

If nobody has done it perhaps the risk is underestimated.

–
66 ‘UberLynx’ D, 70 FHC, 79 S2 XJ12L, 97 XJ6L
Gaithersburg, Maryland, United States
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14

In reply to a message from Richard Dowling sent Mon 15 Feb 2016:

I do have a Marelli car and my solution is very simple. A
spare coil, rotor, and an amp in the boot.
Plus a small tool box.
Well under $200.

I have experimented with cutting off one bank and must tell
you that if you cannot detect the loss of power and keep
driving, you should NOT be in an XJS. Or perhaps part of the
human race (just kidding).
The car BARELY moves, even if you press the pedal to the
metal. That simple!

The danger of engine bay fires due to cracked injection
hoses is FAR greater.

Also, I could argue that clogged/faulty injectors, faulty
FPRs and CTS, as well as vacuum leaks that contribute to
making the fueling ECU think that the engine needs more fuel
to run are probably the major reason the catalytic
converters on these 20+ years old cars melt every so often.

Best regards,
Steve

PS FYI, I did remove, in 2013, my ORIGINAL Morelli cap and
rotor. The car had over 80,000 miles. 30,000 miles on the
spark plugs…
Put new everything, but keep the originals, as I said, in
the boot as spares.–
The original message included these comments:

You can detect the HV pulses in a plug lead without major cost or difficulty.
If I did have a Marelli car I would have done it long ago.
A rough idea for making a control unit plus 2 HV plug lead pickups
would be $200 per set.

–
'95 XJS convertible - V12 6.0L
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15

In reply to a message from sbobev sent Mon 15 Feb 2016:

Correction, in 2011. Car was 16 years old then…

http://www.jag-lovers.org/v.htm?id=1329074918

Picture #2 and #3. The careful eye will recognize the brass
terminals of the real Marelli caps. Too bad the resolution
is not sufficient to let you see the Magneti Marelli and the
date stamp – June 1994!

Steve

PS IIRC, the Japanese market solution to careless owners is
quite elegant – thermocouples in each catalytic converters.
Once fuel starts burning in the cats and they get hotter
than normal, a warning light comes out (the circuit can be
easily modified to cut the respective bank completely)–
The original message included these comments:

PS FYI, I did remove, in 2013, my ORIGINAL Morelli cap and
rotor. The car had over 80,000 miles. 30,000 miles on the
spark plugs…

–
'95 XJS convertible - V12 6.0L
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16

Actually the Japanese (and I assume others now) have had misfire detection
on their ignition circuits since the early 80’s. I think this was Nippon
Denso gear. This detects absence of HT spark by monitoring the LOW tension
switching. More than a certain count of misfires triggers a warning light
and fault code (and alternative fueling?). Emissions safety?

The low tension reflects the high tension spark quite elegantly so you can
“see” the break down voltage and spark duration, thus monitoring the health
of your plugs and coils from the “safety” of the ignition driver itself.

I have used this technique to identify coil on plug problems where the coils
slowly fail causing misfires, which is a very common problem on modern
engines (not helped by poor quality after market replacements). A weak coil
can be detected by lack of spark duration with a scope on the low tension
side. It is quite instructive to see the differences from cylinder to
cylinder(!)

Having chased a number of these faults now, it is my opinion that the most
dangerous thing for the ignition system is a misfire. This results in the
spark energy finding the next best path to ground. This causes an
unintentional breakdown in insulation somewhere in the system, which finally
becomes THE preferred path to ground.

With my MKII, there was never any need to celebrate Guy Fawkes. You simply
needed to open the bonnet at night to observe the spectacular light show
from the ignition system!!! Still don’t know how those plugs ever managed to
fire.

Rgds
Mark

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