[xj-s] Marelli question

XJ-S
161

In reply to a message from mike90 sent Tue 10 Sep 2013:

''As regards the ‘crank-to-run’ theory that is presently under

consideration:’’

‘‘theory’’ is not the proper term as the above statement is made by
both AJ6 Engineering and JDHT in their documentation. I believe
the ‘‘crank to run’’ is the transition from direct triggering of
spark by the timing fingers on the crank at 8 deg ATDC (during the
beginning of the starting) to the timing map at 350 rpm. The
timing map then generates the spark signals at the desired timing
subject to the input variables, but the timing map is still
referenced to the crank position by the timing fingers - the
fingers no longer directly triggering the spark.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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162

In reply to a message from Pele sent Tue 10 Sep 2013:

I read where the speed sensors have been looked at from the inside
out, and don’t appear to be coax. However, I fully understand the
whole concept of a signal verses a actual voltage change. I may
sound dumb here, but is it possible that the sensors weren’t made
as coax, but once they get plugged into the harness, they become
coax, as they then are running along beside wires that could
degrade the signal big time. i.e. the amp signals being confused
with the input of the crank signals, and the result is it stops
firing after a very short batch of signal(s). I feel as though I’m
grasping at straws here. I’m not desperate, as much as I am super
inquisitive. It sounds as though the real truth is that although
Kirberts question of using what I have is a great idea, but then
I’m faced with knowing which signal is more important? the center
wire of he sensor, or the braided side? There definately are two
individual inputs per sensor on the ECU. Surely that makes a
difference. Another thing that jumps out at me now, is the fact
that the previous person(s) that worked o this car, had indeed
twisted the two wires on the output wire going to the Lucas CPU,
and had used a aligator clip wire as a jumper to the center lead of
the Lucas harness, as well as a ground jumper going to the braided
part of the Lucas wire, attached to the braided part of the ‘‘new’’
wire. I am still awaiting the oscope, but from the sound of it,
even if I get a good signal from point of exit of the sensor, to
the point of entry to the Marelli ECU, things get a bit dicey once
those signal start passing each other duing input/output.

I sincerely cannot put into words how much I appreciate the input
of all of you.–
89 XJS convertible Marelli ignition
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163

In reply to a message from Kirbert sent Tue 10 Sep 2013:

It can’t possibly as I was describing the
cross-section of the original marelli sen
8s sensor as seen where the downpipe
burned through the isolation layers.–
The original message included these comments:

It did, thanks. Are we to presume, then, that a harness in which the
coax lines have been replaced with a pair of conductors surrounded by
a shield is a problem?

–
'91 xjs v12 5.3 early facelift in BRG
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164

In reply to a message from lockheed sent Tue 10 Sep 2013:

Lockheed:

I was miss-interpreted, here. Of course the ‘crank-to-run’
function of the ECU appears to be of solid provenance. I was
referring to ‘theory’ in this way: does this ‘crank-to-run’
behavior explain Gene’s situation? At present, it IS a theory
relative to explaining Gene’s car. When it is proved that Gene’s
car’s issue can be so attributed, then, I think we can take it to
the bank.

Before that, not so much.

-M–
Mike, 1990 5.3 XJS Conv., 5-speed, SE-ECU, TT Extractors
Lakewood, OH, United States
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165

In reply to a message from Gene Holtzclaw sent Tue 10 Sep 2013:

I’m with you, Gene. The actual run lengths of these wires
are short in a relative sense, so I can’t picture the
signal degrading all that much. I would have to believe
the shielding is to prevent the signal from being changed
when the signal wire passes close to other high amp wires
within the harnesses. Examples would be fuel injector
pulses, ignition amp pulses, solenoids, and/or motors.

And, for the record, I think the wrong cables may make a
car run poorly, but will not keep it from running. That
is the beauty of digital signals…they ‘‘burn’’ through
noise much more effectively than analog does.–
John. '95 XJS 6.0L convertible. Southlake, TX
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166

In reply to a message from Gene Holtzclaw sent Tue 10 Sep 2013:

‘‘as well as a ground jumper going to the braided part of the Lucas
wire, attached to the braided part of the ‘‘new’’ wire.’’

Therein lies part of the problem - if the braided part of the Lucas
coax is grounded, then one side (the braided side) of the sensor
circuit is improperly grounded.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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167

In reply to a message from lockheed sent Tue 10 Sep 2013:

That jumper wire was one of the first things I removed.
Once I determined what that jumper wire was feeding, I
figured it wasn’t needed until I determined the firing
problems. I actually had hoped my ECU wouldn’t work, and
that would have solved my problems. As is, the harness is
now laid wide open, and about the only thing I can hope to
discover is these speed and crank signals have to be in
coax cable to work.–
89 XJS convertible Marelli ignition
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168

In reply to a message from lockheed sent Tue 10 Sep 2013:

That jumper wire was one of the first things I removed.
Once I determined what that jumper wire was feeding, I
figured it wasn’t needed until I determined the firing
problems. I actually had hoped my ECU wouldn’t work, and
that would have solved my problems. As is, the harness is
now laid wide open, and about the only thing I can hope to
discover is these speed and crank signals have to be in
coax cable to work.–
89 XJS convertible Marelli ignition
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169

Hi Kirby,

Without getting too theoretical and technical, I hope this has
helped.

It did, thanks. Are we to presume, then, that a harness in which
the coax lines have been replaced with a pair of conductors
surrounded by a shield is a problem?

Not enough info to answer. The type of driving source is critical
along with the type of signal and how crucial the leading and falling
edges of the signal are to the information being transmitted.

And: If the answer is yes, what would happen if the owner in
question twisted the two inner conductors together and used them as
one conductor and used the shield as the other? Perhaps just as a
test to see if it makes any difference?

It is a certainty that coax is better but the question would remain,
is the twisted pair sufficient for the application and also an
improvement over a single wire. Shielding is a question of being
infected or infecting. Which case is it? This situation is
dependent on the environment, what’s in the environment and will the
signal be affected at all. Most signals are not affected by exterior
sources unless the signal is very small and the environment has very
large signals such as a radio receiver in a car. Unless this is the
situation, shielding will serve no purpose at all.

In the case of the sensor signal getting back to the ECU, that’s
entirely a source, transmission and termination impedance, signal
fidelity problem. Probably the only external danger to that
situation is a lighting strike hitting the Jag or an EMP going off
nearby.

– Kirbert

Ptipon
Sonora/CA, 90 XJS-V12 conv, United States

// please trim quoted text to context onlyOn Sep 10, 2013, at 1:57 AM, Kirbert wrote:

On 9 Sep 2013 at 23:06, Paul Tipon wrote:

170

In reply to a message from Paul Tipon sent Wed 11 Sep 2013:

We have our own little lightening/EMF generators right on
board…the ignition coils with 12 antenaes.

I have flown RC planes for years with an ignition coil
within inches of the receivers. When the receivers were
analog, we had to twist the wires to prevent the ignition
EMF noise from turning into unwanted noise in the
receivers. Any wire acts like an antenae. When you twist
the wires, the EMF noise on one wire is countered by the
opposing EMF noise on the second wire as it twists around.

When we changed to digital receivers, the EMF was no
longer an issue, and we stopped twisting the wires. It
was still there, but digital receivers ignore the smaller
noise and just look at the larger signal. The shape of
the pulses are not that relevant in a digital system
either. The system just looks at the signal in relation
to a control value…it can either be high, or low, in
relation to that value. That is a beautiful thing in a
high noise environment, like next to an engine sealed in a
steel wrapper.–
The original message included these comments:

fidelity problem. Probably the only external danger to that
situation is a lighting strike hitting the Jag or an EMP going off
nearby.

–
John. '95 XJS 6.0L convertible. Southlake, TX
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171

In reply to a message from CJ95 sent Wed 11 Sep 2013:

Hey guys, the new handheld oscilloscope showed up today. No
clue as to how to use it, but luckily, at least I know what
the signal should look like for the TDC sensor, as well as
the flywheel sensor. Not only did Mike test my ECU, he sent
me copies of the factory manual that shows what the signals
should look like per engine revolution. There has been so
much input on this thread, that I am looking forward to
posting the results.–
89 XJS convertible Marelli ignition
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172

In reply to a message from CJ95 sent Wed 11 Sep 2013:

‘‘but digital receivers ignore the smaller noise and just look at
the larger signal. The shape of the pulses are not that relevant in
a digital system either.’’

Apparently Jaguar and Marelli don’t buy that argument. They started
using coax on the crank and flywheel sensors to preserve the shape
of the waveform (and amplitude?) since the inception of the Marelli
ignition system in the XJS.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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173

In reply to a message from lockheed sent Fri 13 Sep 2013:

With the Marelli we have an EMF transmitter with 12
antenae in a steel can with the signal wires. The noise
may not be ‘‘smaller’’ in our case. My point is merely that
digital systems are far more tolerant of noise and pulse
shape distortion than analog systems.

Gene, can’t wait to see how everything looks on the scope.–
The original message included these comments:

Apparently Jaguar and Marelli don’t buy that argument. They started

–
John. '95 XJS 6.0L convertible. Southlake, TX
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174

In reply to a message from CJ95 sent Fri 13 Sep 2013:

Guys:

I encouraged Gene to get the scope. However, I am leaving on a
trip out of the country later today, and he’s going to need some
help on how to use the instrument to get some idea of the ignition
firing signatures at different points in the circuit.

I am hoping that those of you who are familiar with scopes may be
able to lend a hand to Gene!

I will be back in the U.S. late in September.

Thanks!

M–
The original message included these comments:

Gene, can’t wait to see how everything looks on the scope.

–
Mike, 1990 5.3 XJS Conv., 5-speed, SE-ECU, TT Extractors
Lakewood, OH, United States
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175

In reply to a message from mike90 sent Sat 14 Sep 2013:

Ok guys, its 3:00 on Saturday afternoon, and I just have got to
work with the oscilloscope on the car. I am extremely frustrated
over my lack of understanding. I simply don’t know what scale to
set this thing. It is a Velleman HPS140i. I put it on the rear
sensor (flywheel), and I can get a reading on the scope, and then I
cn move to the ECU, and get what appears to be the SAME reading at
pins 3 and 16. However, the signal I’m getting looks nothing like
what is shown on page 18.2-5 Fig 1. I move to the front sensor, and
I get a much lessor signal, and even then, only on one wire,
whereas I get a signal on BOTH wires on the rear sensor. I don’t
know if the front sensor is bad, or??? The signal is shown for the
front signal on page 18.2-4 Fig 1. Not touching the ocsilloscope, I
don’t get a signal like I do from the rear. It is very faint at
best. And this is only on one wire.

I feel that I am doing something very wrong. Both sensors appear
new, and if you don’t want to read back through, my rear sensor was
at .045 from the flywheel, but I fixed that, and got it to .024.

I am not a parts changer, but am wondering if I may have a bad
timing sensor along with the flywheel sensor being too far away.

Any help at all on what to set this oscilloscope to would be a
great help. Thanks.–
89 XJS convertible Marelli ignition
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176

Hi Gene,

You are doing Great!. Now that you can hook up the scope and see
something, we can now get down to business.

First step: Hook the scope up to the sensors open circuit and get a
reading from both. This is done by just looking at the sensors with
no leads coming off of them at all.

Note: I don’t know how you are recording the scope data but what you
want to do is note the scope setting on amplitude/voltage and in
conjunction note the actual voltage you are reading. You can write
this down or send the data from the scope to some recording device
such as a computer. I believe.

In reply to a message from mike90 sent Sat 14 Sep 2013:

… I put it on the rear sensor (flywheel), and I can get a
reading on the scope, and then I move to the ECU, and get what
appears to be the SAME reading at pins 3 and 16. However, the
signal I’m getting looks nothing like what is shown on page 18.2-5
Fig 1.

This change in the signal integrity is a factor of two things, the
load/terminus and the transmission medium. Do the open ckt test
above and we can go from there.

I move to the front sensor, and I get a much lessor signal, and
even then, only on one wire, whereas I get a signal on BOTH wires
on the rear sensor. I don’t know if the front sensor is bad, or???
The signal is shown for the front signal on page 18.2-4 Fig 1. Not
touching the ocsilloscope, I don’t get a signal like I do from the
rear. It is very faint at best. And this is only on one wire.

Once again, the signal at the ECU is a function of the load/terminus
and the mode of transmission. Two separate causes. Once again, do
the open ckt test stated above and we can go from there. Oh, whether
you get a signal across two wires or one wire is a function of the
setup of the sensor ground and the transmission medium ground.

I feel that I am doing something very wrong. Both sensors appear
new and if you don’t want to read back through, my rear sensor was
at .045 from the flywheel, but I fixed that, and got it to .024.

So far you have done nothing wrong but discovered that … Hey,
Yeah! Looks like there is a function of signal integrity/fidelity in
going from the sensor and transmitting that signal, intact back to
the ECU.

–
89 XJS convertible Marelli ignition

Ptipon
Sonora/CA 90 V12 conv, USA

// please trim quoted text to context onlyOn Sep 14, 2013, at 12:09 PM, Gene Holtzclaw wrote:

177

In reply to a message from Paul Tipon sent Sat 14 Sep 2013:

Well, tomorrow is a whole new day, and seeing as I’m not
committed to anything, I hope to see sparks fly soon. I did
get to work with the oscilloscope a bit more. The sensors
both read the same with a DVM and again with a analog volt
meter. Not the same reading of course, but the same signal.
Put the oscope on it, and I am a believer. It was right at
170.00, and I now know that to diagnose one of these
ignitions, obviously the oscope has to be used per the
factory manual. I don’t yet know exactly what scale or
setting I need to have the oscope on, but for sure Paul is
right. With the sensors unplugged, they SHOULD have the same
signal, although timed differently. Luckily, Mike sent a
sensor when he sent back the ECU, so in the morning, I hope
to change the front sensor, see that both sensors get the
same signal (timed differently), and then move on to the
shielded wire vs coax. I feel that I am going to wind up
changing the wiring regardless. If Marelli saw a reason to
keep the signal wire separate in the sensor lead, surely
they meant for it to be separate in the wiring harness, i.e.
coax.

My next question is which coax to use. There is a local
supplier of RG174, but seems I need a RG174/U, or even a
RG316/U. It appears the /U is a designation for braided
center, and the RG316 has a higher heat rating resistance.

I may be getting ahead of myself, but I see now where there
is a huge difference in shielded wire verses coaxial wire.–
89 XJS convertible Marelli ignition
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178

In reply to a message from Gene Holtzclaw sent Sat 14 Sep 2013:

How is the air gap at the front sensor?–
John. '95 XJS 6.0L convertible. Southlake, TX
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179

Hi Gene,

All sounds great. Some responses below.

In reply to a message from Paul Tipon sent Sat 14 Sep 2013:

… Put the oscope on it, and I am a believer. It was right at
170.00, and I now know that to diagnose one of these ignitions,
obviously the oscope has to be used per the factory manual. …

170.00 what? I have no idea what that 170 is.

I don’t yet know exactly what scale or setting I need to have the
oscope on but for sure Paul is right. With the sensors unplugged,
they SHOULD have the same signal, although timed differently.

With this test, you are going to set up a base reference point from
which all subsequent measurements will be compared against. It is
nice to match the factory reading but it is not an absolute must.
One of the signals from the two sensors has to be correct. They both
can’t be wrong or nothing would happen upon cranking the engine. So
when we find which is a true, good signal we can go from there and
also compare that true good signal against the manual.

Luckily, Mike sent a sensor when he sent back the ECU, so in the
morning, I hope to change the front sensor, see that both sensors
get the same signal (timed differently), and then move on to the
shielded wire vs coax. I feel that I am going to wind up changing
the wiring regardless. If Marelli saw a reason to keep the signal
wire separate in the sensor lead, surely they meant for it to be
separate in the wiring harness, i.e. coax.

I may be getting ahead of myself, but I see now where there is a
huge difference in shielded wire verses coaxial wire.

How did you come to that conclusion?

–
89 XJS convertible Marelli ignition

Ptipon
Sonora/CA 90 V12 conv, USA

// please trim quoted text to context onlyOn Sep 14, 2013, at 3:08 PM, Gene Holtzclaw wrote:

180

In reply to a message from Paul Tipon sent Sat 14 Sep 2013:

First, my front sensor was .042 from the fingers on the
front pulley. I filed the mounting boss until my fingertips
were raw, (hard to file where it is) and got it to .031-.034
depending on which finger I measured. I filed the boss on
the rear until I got it to .024, and felt this was well
within tolerance.

Second, I gave $170.00 US dollars for this Velleman
oscilloscope.

Third, I began reading up on cabling. It appears coaxial is
ideal for electromagnetic pulse signals. Shielded wire
appears best when you are trying to protect the signal from
outside interference. What I have, is two wire shielded. It
has a outer braid, that I presume could be used as a signal
conductor. However, from what I have read, and I VERY open
to correction, as I would love to use what I have, ($$$,
time). Yet, if the sensor is transmitting an electromagnetic
pulse, the two wires twisted together, could very well cause
a degraded signal once running, even if not during cranking.
Kirbert had a great question, as to using my current wire by
twisting the two inner wires together, and then using the
braid as a conductor.

Since I have the wiring harness laid wide open, seems cheap
insurance to just go ahead and replace at least the two
sensor leads, as well as the EFI signal wire. The ECU to amp
wires wires, the wires between the amps and coils, and maybe
even the signal wire to the coils could be left shielded
wire, although I’m not sure why. Evidently the 91 and up
cars had some reason to change from the 89-90 cars.

Again, I am VERY open to correction from those who know.–
89 XJS convertible Marelli ignition
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