[xj-s] Marelli question

XJ-S
221

In reply to a message from Kirbert sent Wed 18 Sep 2013:

Yes, they are the inductive type. Generate their own A/C voltage
without needing magnetic slugs in the flywheel ring gear or the
timing fingers. Just dug up some good info from the link below:

http://www.tiepie-
automotive.com/en/Measurement_examples/Sensors/Crankshaft_Sensor_Ind
uctive

If the link will not copy and work in your browser, just use
Google, or you favorite browser, and type in the following search
string - without the quotes:

‘‘measuring crankshaft rotation speed’’ Look for the Link to TiePie.

Here is some interesting and relative stuff for the inductive type
pickups Jaguar has used to measure the crank position and the rpm
of the flywheel � as well as ABS wheel speed via a reluctor ring
and similar sensor at each wheel, + the speed transducer on the
differential cover:

http://www.tiepie-
automotive.com/en/Measurement_examples/Sensors/Crankshaft_Sensor_Ind
uctive

Of note, is the use of the same sensor to compute both rpm and
crankshaft position. To use the same source for crank position,
one needs to knock out one or two teeth. Obviously, this presents
a problem on the flywheel ring gear as the starter would like to
have all the teeth present. I�m guessing that some sort of
reluctor ring is used in the above example, and it has a lot of
teeth inasmuch as the cycles are spaced very close together, or the
scope has a very slow sampling rate. But, the gap in the teeth is
obvious. A higher scope sampling rate would help space those close
order cycles out to where they would have a better shape to
illustrate the plus and minus swings of the A/C pulse. Also, the
information for this example states the starting voltage as low as
+/- 0.25 volts, but the running voltage could be as high as +/- 60
volts. Therefore a lower voltage range can be selected for
checking the waveform while cranking. But, if the engine is going
to run, idle at maybe */- 8 Volts, but at speeds higher than idle,
it could go as high as +/- 60 volts.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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222

Hi Gene,

In reply to a message from Kirbert sent Wed 18 Sep 2013:

I then checked the sensor across the two prongs, and got:

           1ms/div  .5mv/div   5V

This looks great, digital chips all use 5V as the switched or ‘on’
signal/voltage. Of course 0V is ‘off’ or not switched.

–
89 XJS convertible Marelli ignition

// please trim quoted text to context onlyOn Sep 18, 2013, at 4:02 PM, Gene Holtzclaw wrote:

223

In reply to a message from Paul Tipon sent Wed 18 Sep 2013:

I find it really interesting that these inductive sensors
appear to generate different voltages depending on how they
are tested. Now, even though I have not pinned down my
problem, I have another question to throw in. It seems there
is a polarity issue here as well. My harness was handmade by
someone. Is it important how the two wires are fed to the
ECU? I mean the factory had two different color of wires at
the multi-pin connector. Is there a magnet side/coil side?
Is my ECU receiving a bad signal due to improper polarity?
Since the voltage goes up with speed, and if the polarity
matters then the ECU kicking out starts to make sense.–
89 XJS convertible Marelli ignition
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224

In reply to a message from Gene Holtzclaw sent Thu 19 Sep 2013:

One more thing, as I read this article, I see that these
sensors are generating up to 80V AC???!! This would
definitely explain the coax vs shielded question. If the
signal going to the ECU is in AC, then the signals passing
those wires is in DC, then there really is no telling what
kind of mixed up signal is going on here. Looks like the
coax is needed if this is the case?–
89 XJS convertible Marelli ignition
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225

In reply to a message from Gene Holtzclaw sent Thu 19 Sep 2013:

‘‘I see that these sensors are generating up to 80V AC???!!’’

For that particular application example, it could be up to +/- 60
volts. I would imagine there are variations in inductive sensors
available in the marketplace and their applications which put out
different voltages, so I can’t say what they should be for the XJS -
only when you or somebody successfully scopes our application will
we know.

''If the signal going to the ECU is in AC, then the signals passing

those wires is in DC,’’

No, the inductive transducer outputs only AC voltage from the crank
and the flywheel, so the only voltage (between the center and the
outer woven mesh, or the wires) that should be in the coax or
wiring from the transducer is AC. I don’t believe there should be
any DC there.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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226

In reply to a message from lockheed sent Thu 19 Sep 2013:

Even though the senders are capable of generating high
voltage, the ECU may clip it to a useable range. It is
something to think about when testing the sender open
circuit, though.

The fingers for the front pickup are part of a single part
held to the damper hub with 3 bolts. If loose, it would
be very obvious. The fingers could be bent, but that
should have also been obvious when you were setting the
air gap.

Air gap makes a huge difference in signal strength in a
pickup. A small increase in the gap can make a large
voltage difference.–
John. '95 XJS 6.0L convertible. Southlake, TX
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227

Of note, is the use of the same sensor to compute both rpm and
crankshaft position. To use the same source for crank position, one
needs to knock out one or two teeth. Obviously, this presents a
problem on the flywheel ring gear as the starter would like to have
all the teeth present.

The Marelli system solves that problem by having a distributor!
Hence, it doesn’t NEED to know crank position, it only needs the
three firing triggers. The distributor decides which cylinder the
sparks go to.

– Kirbert

// please trim quoted text to context onlyOn 18 Sep 2013 at 22:55, lockheed wrote:

228

In reply to a message from Kirbert sent Thu 19 Sep 2013:

''The Marelli system solves that problem by having a distributor!

Hence, it doesn’t NEED to know crank position, it only needs the

three firing triggers.’’

Agreed, the distributor puts the spark to the proper cylinder.
And, the three trigger fingers provide trigger signal for the
sparks on initial start and crank position to the timing map for
proper spark timing in run mode.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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229

Is it important how the two wires are fed to the
ECU?

Ooooh, I’d bet it IS. Good catch! As I understand it, these pickups
work similarly to the HEI pickup in the Lucas CEI distributor, and if
you reverse the polarity on that thing it won’t work right.
Presuming that the electronics that READ these sensor signals are
similar, if I were you I’d be checking the polarity – or possibly
even just trying reversing one or the other and see what happens.

Since the voltage goes up with speed, and if the polarity
matters then the ECU kicking out starts to make sense.

With the Lucas CEI pickup, the voltage ramps up, suddenly reverses,
and then ramps up again. The electronics pick up the reversal. If
you reverse the polarity, the electronics are trying to find a
reversal in the middle of the ramping rather than in the sudden
reversal, and everything goes wrong. You may very well be onto
something here. The points on the CEI pickup are very discrete
indeed, so the reversal is very crisp. The gear teeth on the
flywheel are nowhere near that discrete, so the reversal may not be
as pronounced, so it would make sense that the system might still TRY
to work if wired backwards. And the crank sensor may respond to
incorrect polarity the exact same way the CEI does, trying to find a
reversal in the middle of 120 degrees of ramping up rather than at
the 8 degrees ATDC that it’s supposed to be seeing.

– Kirbert

// please trim quoted text to context onlyOn 19 Sep 2013 at 4:15, Gene Holtzclaw wrote:

230

And, the three trigger fingers provide trigger signal for the
sparks on initial start…

Only if the car starts on one bank and then switches to two-bank
operation, which I don’t believe it does.

– Kirbert

// please trim quoted text to context onlyOn 19 Sep 2013 at 12:58, lockheed wrote:

231

In reply to a message from Kirbert sent Thu 19 Sep 2013:

Guys, this is a whole new dilemma. Do any of you know of a
original wiring harness anywhere that could possibly looked
at to give me orientation of wiring? The boot on the
connector at the sensor won’t allow seeing which wire goes
to which side of the connector, but it is keyed so that it
will only plug one way. I still can see the original color
wires at the connector that plugs into the Marelli ECU, but
since everything has been rewired to every connector, there
is no way for me to determine this.–
89 XJS convertible Marelli ignition
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232

In reply to a message from Kirbert sent Thu 19 Sep 2013:

''Only if the car starts on one bank and then switches to two-bank

operation, which I don’t believe it does.‘’

From AJ6 Engineering:

‘‘Triggering is from a three toothed rotor mounted on the crank
pulley, providing a register signal via pin 1, for each A bank
cylinder at 8 degrees ATDC. B bank firing points are then
calculated via the speed signal, which is taken from the flywheel
teeth to pin 16. Both pickups are of inductive type and the air gap
setting is quoted as a rather unfussy 0.018 to 0.042 inches (0.45 -
1.0 mm).’’

I have again sent the JDHT reference to you - on the second page
18.3-4, it basically says the same thing - that B bank spark is
always computed in software from the A bank firing signal. I would
interpret that to mean that even during initial starting (with an A
Bank firing signal present) and continuing up thru the transition
to ‘‘run mode’’ at ~ 350 rpm, B bank spark will still be computed in
software as long as there is, and continues to be, a valid speed
signal from the flywheel sensor. But, if the speed signal from the
flywheel sensor becomes unusable at any time ( from initial start
and on up thru the maps), B firing signal will be lost.–
lockheed 92 XJS Cpe/97 LT1 Miami FL/ 96 XJS Cv 4.0 Austin TX
Austin, TX, United States
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233

In reply to a message from Gene Holtzclaw sent Thu 19 Sep 2013:

Homemade harness…very possible they reversed the wires!
The fact that the plug only goes one way says its gotta be
important which way the sensor hooks up.

Great thinking Kirbert!–
John. '95 XJS 6.0L convertible. Southlake, TX
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234

In reply to a message from lockheed sent Thu 19 Sep 2013:

Doh…I see that’s you that thought of that Gene!
Excellent!–
John. '95 XJS 6.0L convertible. Southlake, TX
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235

I have again sent the JDHT reference to you - on the second page
18.3-4, it basically says the same thing - that B bank spark is always
computed in software from the A bank firing signal.

That it does. What it does NOT say is that the A bank fires directly
off the crank sensor during startup. And the reason it doesn’t say
it is that it’s not true. It very clearly says that the timing
during starting is computed based on engine temp – and that cannot
possibly be true if it’s firing at 8 degrees ATDC based directly on
the crank sensor.

– Kirbert

// please trim quoted text to context onlyOn 19 Sep 2013 at 18:22, lockheed wrote:

236

Homemade harness…very possible they reversed the wires!
The fact that the plug only goes one way says its gotta be
important which way the sensor hooks up.

Great thinking Kirbert!

Hey, I’m not the one that thought of it!

– Kirbert

// please trim quoted text to context onlyOn 19 Sep 2013 at 19:17, CJ95 wrote:

237

In reply to a message from Kirbert sent Fri 20 Sep 2013:

I have contacted
WWW.macharness.com. A fellow has
responded that it sounds as if my
wires are reversed on the front
crank sensor. We shall see.–
89 XJS convertible Marelli ignition
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238

In reply to a message from Gene Holtzclaw sent Fri 20 Sep 2013:

A fellow named Kevin McKracken with www.macharness.com has
now sent me a detailed hand drawing showing the orientation
of the wiring on front and rear sensors. This is great info,
and has put a little ray of hope back in my situation.–
89 XJS convertible Marelli ignition
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239

In reply to a message from Gene Holtzclaw sent Fri 20 Sep 2013:

I have swapped another couple of
emails with Kevin at macharness,
and it turns out maybe all of us
may be correct. He states that he
has seen this problem before. He
states that although the car came
with coax, shielded wire ‘‘should’’
work. He says the problem is in the
polarity of the front and rear
sensors have to be opposite each
other in polarity for the ECU to
signal properly. He seems adament
that if the sensors read 700ohms
resistance, and the wiring, ECU is
good, it will fire properly–
89 XJS convertible Marelli ignition
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240

He says the problem is in the
polarity of the front and rear
sensors have to be opposite each
other in polarity for the ECU to
signal properly.

I woulda been cuttin’ and solderin’ connections before I even had the
time to type that, Gene!

– Kirbert

// please trim quoted text to context onlyOn 20 Sep 2013 at 9:55, Gene Holtzclaw wrote: