[xj-s] Marelli question

Switching stuff around was my first wave of attack. No
matter what I do, the problem is always in the B side of the
ignition. And it drops out at the output of the ECU on the B
side.

Here’s what I asked: If you connect the B output from the ECU to the
A amp and the A output to the B amp and don’t change anything else,
is it still the B coil that stops firing? Or is it then the A coil?
The two sentences above indicate two different answers; I wanna know
which answer holds up.

I am certain now, that something is happening, that
causes the ECU to stop sending a signal to the B side amp.

I still don’t see what the crank or flywheel sensors could have to do
with that. If they quit working, both the A and B banks would quit
firing.

– Kirbert

// please trim quoted text to context onlyOn 15 Sep 2013 at 16:01, Gene Holtzclaw wrote:

In reply to a message from Kirbert sent Mon 16 Sep 2013:

Kirbert, the coil receives its firing signal from the amp.
Switching amps, does move the problem, because the signal
goes from the ECU, into the amp, and then back out, and to
the coil. B signal comes from pins 9 and 10, so no matter
which amp is plugged to it, that is the amp that stops
receiving the signal, and hence, firing stop. I wish the
idea of switching amps helped, because I could easily
replace a amp. It would seem my problem has boiled down to a
input problem. The data the ECU is ‘‘seeing’’ doesn’t fit in
the parameters of its tables(?) and drops one bank off. I
want to feel that its something to do with the front sensor,
since it is the one that controls timing. The rear sensor
controls speed of firing (so it seems). I guess at this
point, for one last stab, is switch my sensors, and see what
happens. The sensor Mike sent me is used, but it could be
borderline bad I guess.

Having never owned a Marelli car, how often to the sensors
go bad?–
89 XJS convertible Marelli ignition
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In reply to a message from Gene Holtzclaw sent Mon 16 Sep 2013:

Gene,

I am curious, have you tried scoping the ECU output to the B amp
right at the ECU connector across pins 9 and 10?

By scoping the B output directly at the ECU, it will once and for
all eliminate any possibilty of the harness downstream of the ECU.

As far as upstream (inputs), one thing we have not looked into is
the power supply. If you put a meter across pins 12 and 13 at the
ECU, does the voltage remain constant before and after the B spark
drops off?

I apologize that I am at a loss, not having the JDHT info for the
input pulses you are mapping. But, off hand, the shape of the
front triggers are shaped far different from the flywheel teeth
providing the rear triggers…so I don’t think they will ever look
the same on the scope, although I would think the amplitudes could
be identical. If the amplitudes are different, I would also think
you can adjust that by adjusting the air gaps.

Finally, the sensors on my car have never gone bad.

John–
John. '95 XJS 6.0L convertible. Southlake, TX
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In reply to a message from Gene Holtzclaw sent Mon 16 Sep 2013:

‘‘The rear sensor controls speed of firing (so it seems).’’

Gene, according to the documentation from AJ6 Engineering and JDHT,
the flywheel sensor merely provides an engine speed reference (rpm)
to the PCMI. The speed and timing reference of the firing signals
are provided by the front timing fingers passing the front sensor.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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In reply to a message from CJ95 sent Mon 16 Sep 2013:

''By scoping the B output directly at the ECU, it will once and for

all eliminate any possibilty of the harness downstream of the ECU.’’

Yes, but it could also indict the harness if you find that the
signal at the power amp is degraded from the signal at the PCMI.

‘’ But, off hand, the shape of the front triggers are shaped far
different from the flywheel teeth providing the rear triggers…’’

No, they are not ‘‘far’’ different, but there is a slight difference
in the waveform, but the biggest difference is of frequency per
crank shaft revolution - 3 pulses per revolution vs 160 pulses per
revolution. See the waveforms for each from JDHT that I have sent
you off topic.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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I am curious, have you tried scoping the ECU output to the B amp right
at the ECU connector across pins 9 and 10?

I dunno the inner workings of this thing, but I wouldn’t expect the
signal to be between 9 and 10. I’d expect the signal to be between 9
and ground, or POSSIBLY between 9 and +12V (Jaguar likes ground
switching stuff!).

By scoping the B output directly at the ECU, it will once and for all
eliminate any possibilty of the harness downstream of the ECU.

Actually, maybe not. If pin 10 is actually a feedback signal from
the amp back to the ECU, then a fault in that circuit could be
confusing the ECU.

As far as upstream (inputs), one thing we have not looked into is the
power supply. If you put a meter across pins 12 and 13 at the ECU,
does the voltage remain constant before and after the B spark drops
off?

I asked that early on. I certainly HOPE that power supply has been
verified by now!

Perhaps a better avenue for investigation would be to scope the
difference between the #4 terminal on each amp and pin 13 on the ECU,
as well as between #2 on each amp and pin 12 on the ECU, just to
check for stray voltages. Both those checks should come up with
absolutely nothing, but if there’s noise it’ll probably cause
trouble.

But, off hand, the shape of the front
triggers are shaped far different from the flywheel teeth providing
the rear triggers…

Are these the kind of things where a ferrous piece of metal is coming
closer and then is moving farther away, and it’s the point of
switching polarity – from getting closer to moving farther away –
that provides the actual triggering?

– Kirbert

// please trim quoted text to context onlyOn 16 Sep 2013 at 7:09, CJ95 wrote:

In reply to a message from lockheed sent Mon 16 Sep 2013:

I was thinking we already know that the signal is degraded
at the amp, or the B spark would continue sparking.
Checking at the ECU defines the problem as an ECU input,
vs. a harness problem in the output.

Alan sent me the waveforms. Their shape explains why the
shield wire is not grounded.–
The original message included these comments:

Yes, but it could also indict the harness if you find that the
signal at the power amp is degraded from the signal at the PCMI.


John. '95 XJS 6.0L convertible. Southlake, TX
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In reply to a message from CJ95 sent Mon 16 Sep 2013:

‘‘I was thinking we already know that the signal is degraded at the
amp, or the B spark would continue sparking. Checking at the ECU
defines the problem as an ECU input, vs. a harness problem in the
output.’’

Just to make sure we are on the same page here, we suspect that the
speed signal output from the flywheel sensor arriving at pins 3 and
16 of the PCMI has been degraded by improper wiring in the
harness. Likewise, we can suspect that the crank sensor signal to
the PCMI (pins 1 and 2) may be degraded for the same reason, but to
a lesser degree because of the lower frequency. Bad stuff in, bad
stuff out. Only when we are sure that what is going into the PCMI
is good, can we expect proper output. Mike has already verified
that your PCMI is working correctly.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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In reply to a message from lockheed sent Mon 16 Sep 2013:

I hear you, but, like Kirbert, I am still wary that the
input is the issue. Gene has a continuous spark at the A
side. We currently know we have enough input to drive the
A spark…so the B output is still very suspect.

Now that Gene has the scope, it only takes 5 minutes to
verify that the B output of the ECU is indeed good prior
to the harness. 5 minutes with the cool scope could save
hours of splicing unknown coax into the myriad of other
unknowns.–
John. '95 XJS 6.0L convertible. Southlake, TX
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In reply to a message from CJ95 sent Mon 16 Sep 2013:

''I hear you, but, like Kirbert, I am still wary that the

input is the issue.’’

That’s what I said: ‘‘the speed signal output from the flywheel
sensor arriving at pins 3 and 16 of the PCMI has been degraded by
improper wiring in the harness. Likewise, we can suspect that the
crank sensor signal to the PCMI (pins 1 and 2) may be degraded for
the same reason,’’

GIGO - Garbage in, Garbage out.

‘‘Gene has a continuous spark at the A side.’’

During cranking yes, but as of yet, we know nothing about idle or
above when the crank signal rises to a higher frequency and goes
thru the same sort of suspect wiring as the flywheel speed signal.

Anyhow, it will be interesting to find out how Gene is doing with
his ‘‘O’’ scope.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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In reply to a message from CJ95 sent Mon 16 Sep 2013:

To be honest with you guys, I feel like I have the deer in
the headlights look. I am getting some great info, but I
lack the confidence to KNOW that replacing the shielded wire
with coax will actually fix the problem. I suspect it
strongly, but I agree with the idea of having A side to
continue firing means that the shielded wire must be doing
at least something right.

I have followed advice given here, and must be doing
something really wrong. This oscope is neat, but I still
don’t know what I am looking at. When I change the settings
to the point that I can ‘‘see’’ a signal on the rear sensor,
the same setting won’t allow me to ‘‘see’’ anything really on
the front sensor. I know this has to do with time. The front
sensor is giving 3 signals per rev. If the engine is turning
300rpm with the starter, then that is 900 signals per
min(?). Likewise, the rear sensor should be sending 160
signals per rev, and 48000 signals per min?

Obviously, if I don’t change the scale on my meter, then I
won’t see anything that even remotely looks like what I see
from sensor to sensor. And so far, I have YET to see
anything that looks exactly like what I see in the manual.
And if I don’t verify that I am getting a proper signal TO
the wiring, and then AT the ECU, then how in the world is it
going to help to change my wiring? I may do all of that, to
still have my problem. And then, even though I know I am
getting a signal TO the ECU, what kind of signal should I be
seeing OUT of the ECU, on either side? (meaning what sort of
signal should be going to the amp(s). So far, all I really
KNOW, is that the analog volt meter shows that a signal is
going into the ECU from each sensor, but B side output drops
out. But even then, I am reading from one pin to the other,
not pin to ground, as has been suggested–
89 XJS convertible Marelli ignition
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In reply to a message from Gene Holtzclaw sent Mon 16 Sep 2013:

‘‘but I agree with the idea of having A side to continue firing
means that the shielded wire must be doing at least something
right.’’

Gene, The A side spark timing is directly from the front crank
during initial starting, and then shifts to the timing map at
around 350 rpm according to documentation. The flywheel speed
input to the PCMI is apparently not needed initially to generate
the A spark. But, it may be needed for the timing map lookup when
the engine goes from the crank to ‘‘run mode’’. That’s something we
don’t know yet because your engine has not started yet, although
you indicate it did fire some, but never accelerated to idle speed.
The documentation also tells us that the B timing is always, repeat
always, computed from the A timing by the PCMI software which needs
the flywheel speed input to do that. The frequency of the crank
pulses are much lower than that of the flywheel - 3 vs 160 pulses
per revolution. So it would seem reasonable to guess that the
lower frequency crank pulses are degraded less than the higher
frequency flywheel pulses by the lack of proper coax wiring. And
therefore, you still have A sparks while cranking but the B sparks
go away as the speed signal becomes unusable when the engine begins
to turn faster, and the PCMI software cannot compute the B spark
without a useable flywheel speed signal.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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In reply to a message from lockheed sent Mon 16 Sep 2013:

Gene can you entertain us all here for a
minute and first charge your battery
before going any further. Then swap the
front to back sensors, left to right amps
and front to back coils. Also make sure
you dig out your ignition ecu and it’s to
hand. Then with a fully charged battery
and leads going to the top pf the
distributor removed and positioned 1cm
away from say air rails have someone
cranking the engine while you try and
wiggle first the rear sensor wire which
can be reached just behind the
‘‘turntable’’ and then the front sensor
wire which is just left and forward of
the crankcase ventilation pipework (that
mushroom at the front of B bank, you got
your banks right, right? Sitting in the
car right is a left is b, right) and see
if that changes anything in sparks coming
from those two main leads. Unplug and
plug the sensors while cranking see what
happens.
You are way off in suspecting the type of
wiring used here as this is a really
primitive computer that only cares if it
sees 12v going on and off on a particular
input (or wire) coming from your sensor.
Even a coat hanger could provide it with
a good ‘‘signal’’. Maybe if nothing but the
connector on the ecu that’s dirty or not
seated properly? The long one with 20-
something pins. I think you need to have
a break and think about the problem a
little. Go back to basics as they say.
Also don’t forget to try the 3rd spare
sensor you have. In both locations. But
if you already tried swapping sensors
then that can be eliminated as your
problem. Maybe the little vacuum tube is
disconnected from the ecu? Now that your
ecu is out and at hand you can verify
that by the person who was cranking the
engine sucks or blows on it on one side
and you trying to feel the air flow on
the other by holing the end of the tube
to your cheek or lips. And then well go
from there.–
'91 xjs v12 5.3 early facelift in BRG
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In reply to a message from Pele sent Mon 16 Sep 2013:

Do Gene a favor and go back and read the history - you will see
that he has already done all of what you ask. He has spent enough
time repeating answers he has already given.

''You are way off in suspecting the type of

wiring used here as this is a really

primitive computer that only cares if it

sees 12v going on and off on a particular

input (or wire) coming from your sensor’’

Obviously, Jaguar and Marelli (with their electronic engineers) do
not agree with you. For one thing, the signals from the sensors
are not 12V going on and off, but much smaller, probably less than
1 volt with waveforms - can you tell us the voltage exactly? And,
knowing how cheap Jaguar was with the XJS and how near to going
broke they were, what makes you think they would wastefully spend
extra money to put in coax that wasn’t needed? Furthermore, they
added additional coax lines, beginning with the facelift in '92, to
the signal lines between the PCMI and the Power Modules. Have you
written to tell them how you know more than they do about this
subject, and how foolish they were?–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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In reply to a message from lockheed sent Tue 17 Sep 2013:

Alan, I follow your logic, except I have never seen a starter turn
an engine above 350 RPM. 150 to 200 rpm would be about
right…unless maybe when all the plugs are out you may push 300 or
so.

Gene, your assessment of the difference between the scope readings
on the front vs rear sensors is right on. To make them look the
same on the scope, you will have to reduce the ‘‘ms’’ value of the
scope by a factor of about 53 times (160 teeth divided by 3 teeth)
when you change from reading the rear sensor to the front sensor.
I would think the auto function should get the scales close.

Once you take a snapshot of the pattern, you can adjust the scale
values on the snapshot to get a better picture of the pulses.
Larger ‘‘ms’’ numbers will compress the pulses into spikes.
Small ‘‘ms’’ values will stretch them into longer patterns. The
voltage (vertical) scale can be adjusted on the snapshot to bring
the height of the pulses into a convenient size on the screen.

Just to play with the scope and get familiar with it, you can read
the wall outlet voltage. It will give you a nice sine shaped wave
you can use to play with the different settings on the scope. Once
you feel comfortable taking holds from the steady wall voltage, you
will be confident on the car.–
John. '95 XJS 6.0L convertible. Southlake, TX
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In reply to a message from CJ95 sent Tue 17 Sep 2013:

‘‘except I have never seen a starter turn an engine above 350 RPM.’’

John, you are probably right about that - I’ve never watched it
that close. But even then, 200 rpm of the flywheel would be giving
(200*160) 32,000 cycles per minute or (32000/60) or 533.3 Hz. It is
still conceivable that the signal could still be screwed up even at
some rpm below 350 such that the PCMI is getting a speed signal it
cannot use to compute B spark just above the initial cranking speed.

‘‘you can read the wall outlet voltage.’’ Don’t know that you want to
tell him that, unless you know the input limits of his scope.
There is info. on it at: www.velleman.eu Look for the link to
HPS140i. Maybe suggest checking the output ripple of his cell
phone charger, or something of lower safer voltage.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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In reply to a message from lockheed sent Tue 17 Sep 2013:

As fate would have it, I have received a bad oscilloscope. I
talked with Velleman today, and they want me to send it
back, as they have not ever seen one of their oscopes do
what mine is doing. According to the directions, their
website videos, as well as what I could see on youtube,
there is something wrong. The tech guy at Velleman asked me
to send it right back. Just when I thought I was gonna
figure this out…–
89 XJS convertible Marelli ignition
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In reply to a message from lockheed sent Tue 17 Sep 2013:

Good point…from what I read, the X10 switch has to be on the
probe before you try it.–
The original message included these comments:

‘‘you can read the wall outlet voltage.’’ Don’t know that you want to
tell him that, unless you know the input limits of his scope.


John. '95 XJS 6.0L convertible. Southlake, TX
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In reply to a message from lockheed sent Tue 17 Sep 2013:

I can tell you the voltage exactly - it
is DC as specified per the manual. DC on
this car is 12V and that’s what the
sensor will give off but since they say
DC that means anything will work. Servos
are like that, they just see and count if
something triggers their input or not and
the spec on ye olde ‘‘computers’’ would be
something like 3-12V DC - just like your
bog standard LED. You will of course need
a resistor in series with that to get any
meaningful current out of it but that’s a
different story altogether. And I don’t
need to suspect anything as he has the
exact same wiring on his car as I do on
mine with twin conductors and a shield -
that is the original marelli wiring on
the sen8 s sensor. Arguing whether
marelli installed the correct wiring on
their own product is what you lot have
been arguing on here since forever. And I
'm saying that there is no need to. And a
coat hanger would demonstrably carry a Dc
voltage to where it needed to get to.
If you have an intermittent problem in
the wiring that means wiggling the wires
in question could restore function. He
doesn’t have an intermittent problem as
his coils fire for a few seconds and then
one goes berzerk. That could mean the ecu
gets reset. Maybe power or ground for the
ecu are not ok? All I’m saying is that he
needs to stop and give it a rest as
debating about cables has stopped being
productive ages ago.
And just because I didn’t quote my degree
and professional membership info doesn’t
mean I don’t have the stamina or
intellectual and technical prowess to
analyse the issue currently at hand. I
thought we were supposed to keep this a
hobby and fun.–
'91 xjs v12 5.3 early facelift in BRG
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In reply to a message from Pele sent Tue 17 Sep 2013:

I sincerely appreciate any and all
input here. I also wish my problem
were as simple as a loose wire, or
even a broken wire. I feel that I
have eliminated that possibility,
due to the fact that I have ohmed
out every single wire, every
ground, voltage tested every 12V
lead, switched components, had my
ECU checked by a fellow lister, and
now purchased an oscilloscope. The
only thing I know for sure at this
point, having my wiring harness
laid wide open, is that my problem
is still here.The ECU drops out the
signal going to the B side amp, for
whatever reason. It is down to
either a bad signal getting to the
ECU, or improper wiring, or too
much resistance causing the ECU to
stop sending a signal.I do know for
a fact, that the wire going to the
Lucas ECU is coax. The workshop
manual shows this wire being a
shielded wire, but its not. Whether
all of the other wires are shielded
vs coax, I don’t know, as this car
does not have the original harness,
nor do I know where to see a
original harness. Either way,
something is wrong, and I can’t
figure it out, yet. Thank you.–
89 XJS convertible Marelli ignition
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