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In reply to a message from jagwit sent Mon 9 Aug 2010:
A quick look at the wiring diagram shows the 2 sensors as
inductive. They will produce a sine wave. The frequency and
amplitude will increase as the engine speed increases.–
Greg 1985 XJS HE DANA rear
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In reply to a message from Greg Edge sent Tue 10 Aug 2010:
Thanks Greg
I understand that the Marelli cars have an ‘‘ignition module’’
in Jaguar speak it might be called something else. The
two VR sensors feed their signals to this ignition module
but this is not what I’m interested in.
This module then needs to send a signal to the engine ECU
telling it how many RPMs the engine is doing. This is the
signal I’m interested in.
Reading Kriby’s book it seems that there is little
difference between Marelli cars and Lucas cars ito engine
ECU, so I presume the signal being sent by the Marelli
ignition system should be similar to that of a Lucas system.
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In reply to a message from jagwit sent Wed 11 Aug 2010:
In the Lucas set up, the pulse to the ECU comes from the (-)
side of the coil through an isolating resistor of a few
k-ohms. 6k8 sticks in my mind but don’t hold me to it.–
The original message included these comments:
Reading Kriby’s book it seems that there is little
difference between Marelli cars and Lucas cars ito engine
ECU, so I presume the signal being sent by the Marelli
ignition system should be similar to that of a Lucas system.
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In reply to a message from JaguarJoe sent Wed 11 Aug 2010:
The Marelli ICU (probably the module you’re referring to) is
located inside the passenger compartment in the
passenger-side footwell I think, a friend of mine has an 89
XJ-S with the Marelli system and that’s where his is at least!
The two sensors are VR (Variable Relucance) sensors, they
produce a signal that looks very much like a distorted sine
wave (very close to a sawtooth waveform actually) and are
functionally equivalent to the sensor inside the Lucas CEI
distributor.
The two sensors connect to the Marelli ICU along with the
temperature switch and a vacuum line and have outputs
running to the two ignition driver modules (on top of the
radiator), the ECU and the tachometer. The engine speed
signal to the ECU should be a square wave and would look
VERY different from that of the Lucas CEI system. On a GM
HEI ignition system (which is what the CEI is) you’ll see
the following on the negative side of the coil during a
spark cycle:
Battery voltage while the ignition circuit is open
Ground (more or less) when the ignition circuit starts
charging the coil
Some voltage between battery and ground when the ignition
circuit goes into current limiting mode (more or less when
the magnetic field in the coil is at full strength)
A massive voltage spike when the ignition circuit opens
and the magnetic field collapses
Some voltage (possibly higher than battery voltage) while
the secondary coil fires the spark plug
Back to battery voltage once things have stopped oscillating
Most aftermarket ECU’s designed to trigger off the ignition
coil itself use opto-isolators to prevent the ignition
system from interfering with or damaging the ECU.
The part numbers of the 16CU ECU’s that overlapped the two
ignition systems are different according to the XJ-S book.
I imagine you might be able to drive a CEI version of the
16CU with a Marelli, but probably shouldn’t try driving the
Marelli version of the 16CU with a CEI ignition system
unless you filter it and add a zener diode to clamp the
inductive kickback from the ignition coil.–
1984 XJ-S V12 - Megasquirt-II injection
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In reply to a message from jagwit sent Mon 9 Aug 2010:
Philip:
Reading from the Jag manuals, the marelli ECU sends a speed signal
to the Lucas fuel ECU, and this signal has the following
characteristics:
peak voltage: ‘approximately 1 volt less than battery voltage’,
referenced to ground (therefore, the minimum voltage is zero).
positive going pulse, duration of pulse is 400 microseconds
period between pulses is proportional to engine speed, of
course, with six pulses per revolution.
Folklore has it that non-Marelli ECUs can be used in marelli cars,
and I think visa-versa, although I’m not sure I’d want to do this
myself, esp. the later case of a marelli-car’s Lucas ECU in a non
marelli car. The thing is, the Lucas fuel ECUs used for Marelli
cars are reported to have somewhat different input speed signal
conditioning (filtering, etc.), owing to the cleaned up nature of
the marelli speed signal vs. the coil signal off of non-marelli
cars.
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In reply to a message from mike90 sent Wed 18 Aug 2010:
Thanks Mike
This is really good news. Someone wants to convert a
Marelli car to a dual EDIS-6 setup using Megajolt and wanted
my DESC module for this purpose.
Despite my warning that this has not been done before, he
seems rather determined and with what you have shared with
us, makes me rather positive that it will work since the
DESC module puts out a ‘‘tacho signal’’ primarily intended for
a 12-cyl tacho (E-type and Pre-HE XJS etc) but it could very
well work for clocking the ECU of a marelli car as well,
since the tacho signal is very similar to the signal you
described.
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Lots of other good info there too and Rodger Bywater, the MD, is very
helpful should you wish to
ask his advice by phone or e-mail, contact details on their web page.
TTFN
Bill
Surrey, UK
86 Daimler 4.2, 84 Sovereign 4.2 5spd man, 88 XJ-S 5.3 OTS 5spd man
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Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
9
mike90 wrote:
Folklore has it that non-Marelli ECUs can be used in marelli cars, and
I think visa-versa, although I’m not sure I’d want to do this myself,
esp. the later case of a marelli-car’s Lucas ECU in a non marelli car.
The thing is, the Lucas fuel ECUs used for Marelli cars are reported
to have somewhat different input speed signal conditioning (filtering,
etc.), owing to the cleaned up nature of the marelli speed signal vs.
the coil signal off of non-marelli cars.
Supposedly, same signal (one pulse per spark) but less filtering at
the input to the EFI ECU itself in Marelli cars. What I’ve always
wondered is, might it be possible to ADD such conditioning
externally? There have been lots of Marelli cars converted to Lucas
ignition after a fire, and reports I’ve heard is that it’s common to
replace the entire EFI system, ECU and harnesses and all, along with
the ignition system to avoid compatibility issues. If it were
possible to just add a capacitor or some such into the line, it’d
save a LOT of money and hassle when doing such a conversion.
– Kirbert
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