[xj-s] Marelli question

XJ-S
141

In reply to a message from Gene Holtzclaw sent Sun 8 Sep 2013:

And Kirbert, yes, I do have a solid 12V ignition going to the ECU,
as well as all of these redundant grounds that appear to tie pins
12 and 4 together. Pin 4 just goes straight to pin 12, but pin 12
gets all sorts of grounds after that.–
89 XJS convertible Marelli ignition
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142

In reply to a message from Gene Holtzclaw sent Sun 8 Sep 2013:

I know I mentioned this, but just to reiterate, because I really
WANT to find the problem, the wires coming from the two signal
sensors, are shielded wire. But it would appear, that the shielding
itself, is the conductor per the schematic, whereas the wiring in
this car, the shielded wiring, has a two wire conductor surrounded
by the shielding, and that shielding is what has been grounded in
several places. Thanks guys!!–
89 XJS convertible Marelli ignition
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143

In reply to a message from Gene Holtzclaw sent Sun 8 Sep 2013:

OK Gene,

As I look at the wiring diagram, the shielding on the crank and
flywheel sensors are not true grounds. The shield is connected to
pins 2 and 3 on the ECU. This is important, as actually connecting
them to a chasis ground may cause a problem in the signals. I
don’t think this is your problem, though.

The 5 sheilded cables are 1) transmission control module 2+3)
amps 4+5) Crank sensors. 5 sheilds are normal for a later style
harness. Your car may not use the Tranny Control…just make sure
it was not grounded out (pin 24, R)

NOTE: You state that pin 24 is grounded…UNGROUND IT! It is the
signal to the tranny ECU, and grounding it may short other circuits
within the ECU, like the B amp signal.

Now, the shielding between the Power Modules and the ECU are true
chasis grounds. They should be grounded at BOTH the amp and the
ECU. This is also important, as it stabilizes the electronics in
the ECU with the Amps so they have a stable, common ground value.
A problem with any of these grounds between the ECU and amps could
nulify the spark signal to the amps.

Pins 12 and 4 are grounds. They should not just be jumped
together, but also have a chasis ground. This is where some of the
internal electronics are grounded, so lack of a chasis connection
may be your problem.

I think you are getting close…–
John. '95 XJS 6.0L convertible. Southlake, TX
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144

In reply to a message from Gene Holtzclaw sent Sun 8 Sep 2013:

Gene,

I think you have put your finger on one of the first faults of the
harness - you will probably find more to come - maybe more with the
same time of shielding error.

As I look at Fig 29.1 for MY 89, I see that the blue wire (U) from
the crank sensor goes to pin 1 of the ECU, and the shield serves as
the conductor to pin 2 of the ECU. Similarly, the red wire ®
from the flywheel sensor goes to pin 16 of the ECU, and the shield
serves as a conductor to pin 3 of the ECU. Neither of these
shields should be grounded according the schematic, they are part
of the sensor’s circuitry. There is nothing in the schematic that
shows a pair or twisted pair with grounded shielding. I’m not a
double EE or a RF guy, but my guess is that this deviation from the
schematic is affecting the circuit and causing much of the RF
energy to be sucked to ground.

The only shield I see connected to ground (in this schematic) is
the shield of the White wire from pin 24 to the Lucas EFI ECU.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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145

Hi Lockheed,

I am of the same conclusion. In fact the wires from both sensors
turn into a coax cable about a foot and a half from the sensor. On
my 90, those leads are coax not shielded. From there back to the
ECU, those are not shielded but coax cable. The fact that the setup
had to go to coax cable is a sure sign that that the coax is also
performing the insure that the integrity of the signals from the
sensors remains intact all the way back to the ECU. There is no
other reason for going coax.On Sep 8, 2013, at 6:02 PM, lockheed wrote:

In reply to a message from Gene Holtzclaw sent Sun 8 Sep 2013:

Gene,

As I look at Fig 29.1 for MY 89, I see that the blue wire (U) from
the crank sensor goes to pin 1 of the ECU, and the shield serves as
the conductor to pin 2 of the ECU. Similarly, the red wire (R)
from the flywheel sensor goes to pin 16 of the ECU, and the shield
serves as a conductor to pin 3 of the ECU. Neither of these
shields should be grounded according the schematic, they are part
of the sensor’s circuitry. There is nothing in the schematic that
shows a pair or twisted pair with grounded shielding. I’m not a
double EE or a RF guy, but my guess is that this deviation from the
schematic is affecting the circuit and causing much of the RF
energy to be sucked to ground.

–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States

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146

In reply to a message from lockheed sent Sun 8 Sep 2013:

Hopefully I am getting close, that’s for sure. I had a
great phone conversation with Paul, and in best layman’s
terms he could, explained to me how a shielded wire, verses
a coax wire could put off an entirely different pulse, that
would certainly cause the ECU to do things it shouldn’t.
He, like Mike, feels that the oscilloscope will tell me the
whole story. Not that I have to compare signals to anything
else, but examine the signal as close to the sensor in
question, and then again at the ECU, and this would tell me
where I am losing value of the intended signal.

Lockheed, that information gives me a lot to go on. As it
stands, the shielded wire that my harness is built of, is
all the same. A pair of wires, white, and white with a blue
stripe, inside a shield, with a small black wire running
along beside it. The small black wire is soldered onto the
shield, and grounded very well in several places and
ways.Now, the urge is to run out and buy some coax and
replace the wires from the speed sensors at least, but Paul
felt I should wait on the oscilloscope.–
89 XJS convertible Marelli ignition
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147

In reply to a message from Gene Holtzclaw sent Sun 8 Sep 2013:

Gene,

I looked at the additional shielding for the signal wires from the
ECU to the power amplifiers in the wife’s 92 - all the same, with a
single wire and the shielding used as a conductor for the other
side of the circuit, and the shielding is not grounded. In other
words, the same scheme as fig 29.1 for the crank and flywheel
sensors.

You might ask Paul what ohm value single wire coax you should use
for the replacements. I’m not sure that an OD measurement of the
wife’s coax would give a definitive answer. Nor can I tell you just
how to replace and/or splice in the affected sections.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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148

In reply to a message from Gene Holtzclaw sent Sun 8 Sep 2013:

My sensor wiring (from the cps or speed
sensor to the jack on the loom) is
exactly the same, i.e. two conductors
(black and light blue) surrounded by
braided shield and it also bears no
resemblance to what’s on the diagram
UNLESS the second conductor is unused.
That’s something I didn’t check when the
wiring was cracked open.–
'91 xjs v12 5.3 early facelift in BRG
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149

Is is possible that I have too many grounds???

Possible, yes. Here’s what happens: When the shield of a wire is
grounded at both ends, it forms a loop. That loop can pick up stray
EMI, and then turn around and retransmit it to the core. This is an
excellent way to pick up noise in a stereo system.

For shielded cables, the safest bet is to ground each one at ONE end,
and no more.

However, I kinda doubt this is your problem.

– Kirbert

// please trim quoted text to context onlyOn 8 Sep 2013 at 12:21, Gene Holtzclaw wrote:

150

In reply to a message from Kirbert sent Mon 9 Sep 2013:

Wow, you guys, in addition to Gene, have been busy! I could not
have a look at the progress on this thread on Sunday until this
Monday morning.

I might mention that one of the problems Gene is facing, too, with
his harness, is that the color codes on the wires may, or may not,
match up with the schematics (that is, there are times when the
color codes match, and there are times when they do not). That is
why I asked him to buzz out each pin on the Marelli ECU connector
to where it lands on the engine.

One other thing: people have been mentioning, on and off, that
evidently there was a problem with this car before all of this
harness stuff was done, likely in an attempt to address whatever
this problem was. I agree on this. What, exactly, the problem
was, is as yet, unknown, but we clearly have an ignition problem
now. Gene will see what else lurks once he can get fire
consistently.

But, what I am trying to get to is this: the harness Gene has
consists of what appears to be a later Marelli harness, grafted
into the original '89 harness. So, what does this mean, exactly?
It means, among other things, that the connectors to key components
(ECU, amps, coils, temperature sensor, specifically) are probably
the ones that were on the original harness…see where I am going?
I am wondering about the integrity of the connectors as regards the
B bank circuit.

However, Gene’s use of an analog VOM to try to ascertain where the
pulse train is being lost appears to show that the signal is being
lost at the ECU…this makes me come back to a ground problem. Or,
a pin/wire problem AT the ECU connector itself.

Gene: can you post the serial number or the numbers you find on
that ECU? As I recall, you will have a series that are
stamped/etched into the diecasting, and there will also be a series
in paint. I have two ECUs, one in the car, and another I have from
an '89 parts car I bought when I was returning my car to operating
condition over ten years ago, now, and I will get those numbers,
and maybe we can hazard a guess as to when/what series your ECU
belongs.

I can tell you this: Gene’s ECU functions fine as regards starting,
idling and throttling my 1990, so I do not think the ECU is at
fault.

-Mike–
Mike, 1990 5.3 XJS Conv., 5-speed, SE-ECU, TT Extractors
Lakewood, OH, United States
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151

In reply to a message from Gene Holtzclaw sent Sun 8 Sep 2013:

Remove this ground on pin 24 of the ECU…grounding an output
signal could be overloading the B amp signal internally in the ECU.–
The original message included these comments:

shielding on the EFI wire, pin 24 is grounded, but not the shields

–
John. '95 XJS 6.0L convertible. Southlake, TX
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152

In reply to a message from Pele sent Sun 8 Sep 2013:

''i.e. two conductors(black and light blue) surrounded by

braided shield - - --.’’

Pele,

but is that braided shield grounded??–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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153

In reply to a message from CJ95 sent Mon 9 Sep 2013:

Gene,

Lockheed furnished me a schematic for your '89. Pin 24 being
grounded is likely the reason the fuel injection is also not
working. It is the input to the EFI on the '89.

You have spark at the A coil. This means that mapping the crank
sensor wave pattern is merely interesting information, but it is
not what is causing your loss of B spark.–
The original message included these comments:

shielding on the EFI wire, pin 24 is grounded, but not the shields

–
John. '95 XJS 6.0L convertible. Southlake, TX
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154

‘‘i.e. two conductors(black and light blue) surrounded by
braided shield - - --.’’

Question for the EE’s here: How does “two conductors surrounded by
braided shield” – grounded or not grounded – compare to coax where
the surrounding braid is serving as one of the conductors? And is
there any signal in the Marelli universe that would be adversely
affected by the difference? Everything here is relatively low
frequency, is it not?

– Kirbert

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155

In reply to a message from mike90 sent Fri 6 Sep 2013:

Hi Alan,

That is interesting and very likely true. Gene’s testing never got far enough along to see if the firing on A bank
would also degrade but at a later time and with a higher rpm.

The idea that only B bank lost its firing signal and not A bank too became an anomaly that obfuscated the commonality
of the problem as that of both A & B banks and not just B bank.

Another one of those V12, Jaguar only scenarios that keeps the mystique there and instills pride of ownership and one-
ups-manship for those who have withstood the test and conquered the aggravation and travails in keeping these beauties
running. What a challenge, eh. But well worth it at the other end.On Sep 9, 2013, at 6:14 AM, Alan Erickson wrote:

Paul,

Thanks for the information. At the current time, my thoughts are as
follows:

  1. … Additionally, the engine may not start and idle/run on A bank only because the crank signal reference for the
    timing map may be degraded by the wiring at the higher frequency idle rpm as well (and certainly the speed signal) -
    forget about B bank because its timing cannot be computed if there is no A coil spark. I’m guessing that observation
    of the waveforms with an oscilliscope will show a decreasing amplitude with rpm increase for both the flywheel sensor
    and the crank sensor because of the improper wiring.

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

–
Ptipon
Modesto/CA, 90 XJS-V12 conv, United States
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156

Hi Kirby and All,

A quick primer on coax vis a vis shielded. First of all, the two do
not compare at all and it is very misleading to call shielded wire(s)
coax.

I will start out with coax because that is a special type of
‘transmission’ medium. Coax is designed with the inner and outer
conductor physically spaced in a circular configuration with a
dielectric medium between the two to maintain the physical spacial
dimensions at all times. Kinking or making right angle bends
deteriorates the transmission medium altogether. Therefore any
shielded wire or wires will never obtain the same transmission
integrity as coax. For transmission of signals at frequencies of
typically 1mhz and higher, coax must be used or the signal will be
degraded and also attenuated. This is why you see all radio, cable
and digital signals that go for any distance using coax.

In particular, the signal generated from the sensors is much more
signal than switching. The sensors do not put out enough juice to
also have the characteristic of being a switched signal and therefore
a low loss transmission of the signal must be employed. That is why
the leads from the sensors are transmitted via coax back to the ECU.
The sensor output is more a voltage signal than a switched current.

In addition to that, the signal may be low repetition but the signal
width is narrow and sharp. Something of the order of a square wave
but really pulses and not square waves. Because of that, the leading
and following edge of the signal has very abrupt rises and falls.
These very abrupt rises and falls mean that the signal is comprised
of very high frequency components. It is not a sine wave. Therefore
the means of transmission over a distance of these pulses requires
low loss high frequency integrity or the smoothing due to just plain
loss and lack of high frequency transmission integrity will cause the
signal to smooth dramatically. Such was probably the case with the
transmission of these pulses back to the ECU. In a high loss
environment to high frequency (RF signals) the pulses could easily be
degraded down to just large bumps and not square waves. Degradation
could also be to a degree where pulses are so degraded that the
leading and following edges disappear and one would see no
distinguishable signal at all. This is where the leading and
following edges melt into each other if you will. Actually a high
loss to RF frequencies transmission medium, integrates the pulses.
You can’t have this and expect to receive any kind of meaningful
switching signal at all.

Because I see that Marelli used coax about a foot after the sensors
for transmission from that point all the way back to the ECU, I
assume they were having a problem transmitting and delivering the low
level pulses back to the ECU and therefore used coax. There is no
other reason to employ coax other than to maintain signal integrity
and amplitude. The power of the signal from the sensors has got to
be pretty small and the only other way to handle this would be to put
amplifiers right next to the sensors but that’s a lot more technical
and expensive than just using coax, the best transmission medium for
low signal power which will also have steep rising and following edges.

Without getting too theoretical and technical, I hope this has helped.On Sep 9, 2013, at 7:48 AM, Kirbert wrote:

‘‘i.e. two conductors(black and light blue) surrounded by braided
shield - - --.’’

Question for the EE’s here: How does “two conductors surrounded by
braided shield” – grounded or not grounded – compare to coax where
the surrounding braid is serving as one of the conductors? And is
there any signal in the Marelli universe that would be adversely
affected by the difference? Everything here is relatively low
frequency, is it not?

– Kirbert

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

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157

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?

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?

– Kirbert

// please trim quoted text to context onlyOn 9 Sep 2013 at 23:06, Paul Tipon wrote:

158

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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159

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

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

Gene: did you tell us that you had the car started and running at
one time? Do you know, was it running on both banks?

-M–
Mike, 1990 5.3 XJS Conv., 5-speed, SE-ECU, TT Extractors
Lakewood, OH, United States
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160

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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