[v12-engine] Weird ignition problem - v12 6.0L

V12-Engine
1

Gentlemen:
Does anybody know what part is interchangeable with the
DAC11520 for the Marelli cars? Lots of info in the archives
on Lucas ignition and the GM part that fits, but very leads
when it comes to the last of the V12. The OEM unit is NLA
and very, very expensive. This might be my problem, but I
am not sure and do not want to swap parts for no good
reason.

I’ve contacted some off-line already and got help, yet, the
first start of the day is still problematic (perhaps related
to the temperature is dropping, summer time seemed OK).
I need to crank long (7-10 sec), sometimes it dies and
flashes the FF77 code (lost engine speed).
Runs very well once awaken. Warm start and idle are fine
(barely noticeable shaking on P and N, but no exhaust
popping), no error codes. Did a long trip in the summer,
got 18 MPG, passed emissions in September.

What I’ve checked already:
-AAV was cleaned and idle speed set.
-Throttle plates cleaned and confirmed adjusted to 0.002’’.
-All injectors click.
-Spark plugs are new (1000 miles), new wires.
-Sparks from both coils are strong.
-Revs easily to the red-zone.
-The CTS which should provide fuel mixture enriched is OK (2
kOhms cold, drops to 200 when warmed).
-I also looked at the vacuum tube that goes to the ECU in
the boot – it holds vacuum and is not blocked.
-vacuum tube to the Marelli unit in the passenger boot also
checked

I have not checked the TPS voltages and the microswitch at
the throttle plate though. The ignition amps are not checked
either.

As some of you know, I became a Dad a few months ago and
since then have not driven the car much. Aside from the
obvious hypothesis ‘‘bad gas’’, what else I should be thinking
about?

Best regards,
Steve

PS BTW, when I was checking for vacuum leaks, I forgot to
reconnect the vacuum tube to the ECU in the boot, yet, the
engine started and ran ‘‘normally’’ …? How is that possible?–
'95 XJS V12 6.0L, saphire/tan
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2

Hi Steve,

I presume you are talking about the Marelli ignition module (DAC11520).
What makes you suspect either of the two ignition modules is bad?

I don’t have a Marelli car, but my understanding is that the car will
start and run smoothly (just VERY low power) even if one bank is
getting NO spark. Also, given the nature of amplifiers, when they have
intermittent problems, it is more often when hot rather than when cold.

I presume that the CTS upon which you tested the 2,000 ohm cold & 200
ohm hot resistances is the left/B bank CTS? Did you check the
resistance of the A bank CTS (ignition timing CTS)?

Are you sure that the vacuum line that was left loose was the one to
the EFI ECU, as I have a hard time believing the engine would run
normally with that open?

If the only time you have starting or performance problems is at the
first cold start of the day, worse the colder the ambient temp, then
the first place I would focus is fuel delivery.

Have you tried spraying ether/starting fluid into the left air cleaner
horn immediately before/during starter cranking on cold start?

Have you applied a ground jumper to pin 85 of the Fuel Pump Relay 1 to
see if Fuel Pump 1 runs continuously when the key is in the run
position, and tried to do a cold start after the pump has run for 5 to
10 seconds before cranking?

If you don’t have the Electrical Guide for your 95 6.0L model, you can
download it from this site and see the fuel pump circuit and relay
location (Fig.25 for circuit):
http://www.captainjaguarscathouse.com/Electrical_Reference.htm

George Balthrop, Clifton, VA USA
89 and 85 XJ-S Coupes; 89 XJ40 VDP----Original Message-----
From: sbobev shelxtl@yahoo.com

Does anybody know what part is interchangeable with the
DAC11520 for the Marelli cars?..

first start of the day is still problematic (perhaps related
to the temperature is dropping, summer time seemed OK).
I need to crank long (7-10 sec), sometimes it dies and
flashes the FF77 code (lost engine speed).

Runs very well once awaken. Warm start and idle are fine
(barely noticeable shaking on P and N, but no exhaust
popping), no error codes. Did a long trip in the summer,
got 18 MPG, passed emissions in September.

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3

In reply to a message from George Balthrop sent Fri 2 Nov 2012:

Thanks George:
The Marelli modules and the TPS are the only two things I
have not ruled out.
Answers to your questions:

  1. CTS I mentioned is in the B-bank thermostat housing
    (DBC3728)
  2. Have not checked the A-bank thermal sensor
  3. Have not sprayed ether but have tried turning the key
    on/off 3-4 times (without cranking) to prime the fuel rail.
    That makes no difference
  4. Have the JDHT ROM with all diagrams and have spent the
    last 3-4 weekends checking everything I could think off.

Last, the vacuum line to the ECU in the boot – yes, I
removed it from the unit and from the A-bank manifold and
tested by first blowing through it and then by caping one
end with a vacuum gauge and pumping in the other. I am
absolutely certain the line is not compromised and I have a
video of the car running with the line removed from the ECU
in the boot…?

I will measure the resistance of the A-bank sensor and will
report back.

Thanks much,
Steve–
The original message included these comments:

I presume that the CTS upon which you tested the 2,000 ohm cold & 200
ohm hot resistances is the left/B bank CTS? Did you check the
resistance of the A bank CTS (ignition timing CTS)?
Are you sure that the vacuum line that was left loose was the one to
the EFI ECU, as I have a hard time believing the engine would run
normally with that open?
If the only time you have starting or performance problems is at the
first cold start of the day, worse the colder the ambient temp, then
the first place I would focus is fuel delivery.
Have you tried spraying ether/starting fluid into the left air cleaner
horn immediately before/during starter cranking on cold start?

–
'95 XJS V12 6.0L, saphire/tan
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4

In reply to a message from sbobev sent Fri 2 Nov 2012:

I would be looking at grounds, and maybe a crank sensor.

Dave
Atlanta
1994 XJS Coupe 6.0L–
Penfold99
Atlanta, GA, United States
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5

In reply to a message from Penfold99 sent Fri 2 Nov 2012:

Hi Steve

The only thing that would cause FF77 fault code would be a
faulty FLYWHEEL sensor (mounted on or near the bottom cover under
the flywheel) This is a coil and if showing signs of age will go
higher in resistance with cold weather . When engine heat warms the
coil it may expand a little and make contact . I would first check
the contacts where this sinsor plugs into the loom for porrs
contacts and if ok replace the flywheel sensor.

When I had a faulty coil the engine would start ok , run a bit
lumpy , smell of petrol and NOT REV OVER 1200rpm as at this speed
will SURGE BAD!. If the computer generates FF77 you must look into
this speed sensor and plug first as they are old and been well
cooked over the years with a V12 Heat.

I hope to get you the TPS info soon for you .–
95 XJR6, 94 XJS 6.0 coupe, 04 XK8 ,99 Ka
Croydon , SURREY, United Kingdom
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6

In reply to a message from Penfold99 sent Fri 2 Nov 2012:

Dave:
Thanks for the suggestions. I have 2 redundant grounds to the
engine, doubt this is the problem – plus, once it starts, it
runs very well and can be restarted normally.
The CPS could be a problem, I’ve got a new sensor CPS and will
try it, although I don’t see how a bad CPS will not cause
other issues but a long cranking
Steve–
The original message included these comments:

I would be looking at grounds, and maybe a crank sensor.
Dave

–
'95 XJS V12 6.0L, saphire/tan
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7

In reply to a message from 540itouring sent Fri 2 Nov 2012:

Hi Dave:
The FF77 appeared when I cranked for 3-4 seconds and
stopped. The engine stalled and the DIC showed up. I
immediately cleared it by removing the ground from the
battery and cranked it again – started at ones and run
without any codes. Did not touch the CPS.
The videos I sent earlier with the long cranks did not
produce FF77.
Will have some time to do some more tests tomorrow morning,
but since this happens only after long periods of
inactivity, troubleshooting takes forever.
Thanks much for your help.
Steve–
The original message included these comments:

The only thing that would cause FF77 fault code would be a
faulty FLYWHEEL sensor (mounted on or near the bottom cover under
the flywheel) This is a coil and if showing signs of age will go
higher in resistance with cold weather . When engine heat warms the
coil it may expand a little and make contact . I would first check
the contacts where this sinsor plugs into the loom for porrs
contacts and if ok replace the flywheel sensor.

–
'95 XJS V12 6.0L, saphire/tan
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8

Hi Steve,

I just realized you had stated in your original post that you had FF77
code:

“sometimes it dies and flashes the FF77 code”

Could you elaborate on WHEN you see the FF77 code?

Does that appear any time after you have to crank the starter for more
than ~ 2 seconds before it fires?

Reason I ask: It is my understanding that the FF77 code is set when
the “Engine Speed Signal” from the Flywheel Sensor (NOT the CPS) to the
ignition ECU is interrupted.

It is also my understanding that lack of either the flywheel sensor
signal, or lack of a crankshaft position sensor signal, will prevent
the ECU from providing the trigger signal to the amplifiers, and you
will therefore get NO spark from EITHER coil during starter cranking
when that fault (which may be intermittent) is occurring.

So try the test I suggested offlist, where you disconnect both coil
king leads, anchor their metal ends 1/4" from ground, and see if you
get any spark from either king lead during starter cranking on cold
start.

I would suggest doing that test BEFORE you separate the Flywheel Sensor
connector and clean the contacts on either side of the connector. Such
cleaning, or cleaning the face of the sensor if dirty, may restore
proper operation without sensor replacement. Doing the test first, if
it shows no spark from either coil on cold start, may allow you a
greater confidence that you have identified the cause of the problem.

George Balthrop, Clifton, VA USA
89 and 85 XJ-S Coupes; 89 XJ40 VDP-----Original Message-----
From: sbobev shelxtl@yahoo.com

The CPS could be a problem, I’ve got a new sensor CPS and will
try it, although I don’t see how a bad CPS will not cause
other issues but a long cranking

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9

In reply to a message from George Balthrop sent Fri 2 Nov 2012:

Hi George:
Out of 6-7 tries in the last 2 months, FF77 has come twice.
NOT during cranking. I will crank, the engine would start
and then stall – FF77 comes. I clear it and it starts.

I am sorry I confused the CPS with the flywheel sensor. If
indeed signals from both sensors are needed for proper
ignition sequence you and Dave might be onto something.

Check my photo-albums. I have a small unresolved oil leak
in the back of the engine (you responded to that post from
ca. 1 year ago) and there might be engine oil covering the
flywheel sensor.

I will report on the spark from the coils tomorrow.

Many thanks for the careful reading and for the sharp eye.
Steve–
The original message included these comments:

I just realized you had stated in your original post that you had FF77
code:
‘‘sometimes it dies and flashes the FF77 code’’
Could you elaborate on WHEN you see the FF77 code?
Does that appear any time after you have to crank the starter for more
than ~ 2 seconds before it fires?
Reason I ask: It is my understanding that the FF77 code is set when
the ‘‘Engine Speed Signal’’ from the Flywheel Sensor (NOT the CPS) to the
ignition ECU is interrupted.
It is also my understanding that lack of either the flywheel sensor
signal, or lack of a crankshaft position sensor signal, will prevent

–
'95 XJS V12 6.0L, saphire/tan
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10

In reply to a message from sbobev sent Fri 2 Nov 2012:

Sorry, I call both of those sensors crank sensors, I should have
been more specific… the flywheel sensor is what I meant. I think
that will resolve your issue.

Dave–
Penfold99
Atlanta, GA, United States
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11

In reply to a message from sbobev sent Fri 2 Nov 2012:

Hi Steve-
Seems like the flywheel sensor might be a clue. My car does
not have error codes, but the following might be useful,
it’s something that I did which I think is pretty slick but
nobody seems much impressed when I bring it up :slight_smile:
1 - purchased a $100 oscilloscope (Velleman HPS10) with an
LCD screen, it’s a bit bigger than a multimeter and I keep
it in the boot.
2 - pulled the passenger side kick panel to access the
Marelli computer.
3 - Located the following wires per the ROM: 1) input from
Crank Position Sensor 2) input from Crank Speed Sensor 3)
Output to Fuel Injection ECU
4- Using an old 4-position radio shack female plug (like
you’d see on a car stereo harness) with about 18 inch leads,
tapped in (soldered) to each of these wires, and also added
a ground wire. The plug is tucked up under my glove box.

The ROM illustrates the wave form that you are supposed to
be able to read off of each of these wires. By using my
scope, I can go between each of the wires and ground and
check the wave forms. If I ever encounter a no-start, I can
quickly check the obvious Marelli suspects without even
opening the bonnet. Since I am checking right at the
computer, if I get a good form I know that the sensor, plugs
and harness are fine. If I don’t get a wave, I can work
outwards. If I get good input waves but no output to the
fuel injection ECU, the Marelli unit is suspect. Checking
the sensor resistance is fine, but it’s not definitive. I
have absolutely had sensors which were intermittent yet
passed the resistance check every time. Having said all
that, you’d need to be watching the wave forms while you
were experiencing the no-start (cranking.) Obviously if the
engine is running the waves will look good.

Regards
Bob–
89 XJS Coupe, Mesa AZ www.goflyrc.com/projects/XJS/xjs.htm
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12

In reply to a message from BobPhx sent Sat 3 Nov 2012:

Steve-
I just realized that in your very first post you said ‘‘all
injectors click.’’ If you are 100% sure they are clicking
during the no-start situation then you do not have a crank
position sensor or a flywheel speed sensor problem. A
failure of either of those sensors means the Marelli ECU
does not output a signal to the Lucas ECU and the injectors
do not click.
Be 100% sure, lots of noise during cranking and unless you
are listening with a steth or screwdriver in your ear it’s
difficult to be sure… never mind how I know that :slight_smile:

Bob–
89 XJS Coupe, Mesa AZ www.goflyrc.com/projects/XJS/xjs.htm
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13

In reply to a message from BobPhx sent Sat 3 Nov 2012:

I have worked with electronics for 35 years and always used a scope
for faults but these days the scope is only used on cars . Another
way but not quite as good is to measure the voltage from the sensor
on a multimeter AC RANGE = AC volts as most sensors give a AC PULSE
and not DC .

The best tool for measuring voltage / resistace /current and
waveforms is a SCOPEMEATER . Do a google search and you will fine
many but are expensive new but older models go cheaper and great
for car use all in one unit. I think the FLUKE models are best and
are well built.–
The original message included these comments:

it’s something that I did which I think is pretty slick but
nobody seems much impressed when I bring it up :slight_smile:
1 - purchased a $100 oscilloscope (Velleman HPS10) with an
LCD screen, it’s a bit bigger than a multimeter and I keep
it in the boot.

–
95 XJR6, 94 XJS 6.0 coupe, 04 XK8 ,99 Ka
Croydon , SURREY, United Kingdom
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14

Hi David,

I have very little electronics knowledge and no experience in that
field. I’ve always wondered if one could not get 99% of the benefit
that a scope provides for most automotive diagnostics applications by
simply using a mulitimeter to measure AC voltage of the output signal.

The types of sensors or pulse generators on the XJ-S that come to mind
are:

CPS to Marelli ECU
Flywheel Sensor to Marelli ECU
Marelli ECU to Amps
Marelli ECU speed signal to EFI ECU
Lucas ignition amp speed signal to ECU
Diff Pulse Generator for Speedo, Cruise, etc. (?)
ABS Wheel Sensors (?)
O2 Sensors (?)

My questions are:

Can one at least determine if there is wire continuity via the shielded
cable (I believe all of the above use shielded cable for the signal
wire) using a multimeter to see there is an AC voltage?

If so, what should be voltage range be?

If the sensor or pulse generator were to fail, at least for most common
faillure modes, would the presence or absence of AC voltage indicate
that, or do you really need to “see” the quality of the wave form on a
scope to diagnose the fault?

Thanks for any education you can provide.

George Balthrop, Clifton, VA USA
89 and 85 XJ-S Coupes; 89 XJ40 VDP-----Original Message-----
From: 540itouring d.reacher@hotmail.co.uk

I have worked with electronics for 35 years and always used a scope
for faults but these days the scope is only used on cars . Another
way but not quite as good is to measure the voltage from the sensor
on a multimeter AC RANGE = AC volts as most sensors give a AC PULSE
and not DC .

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15

Hi Bob,

I AM impressed, and think that’s slick.

Problem is: I’m cheap and don’t want to buy an oscilloscope for auto
diagnostics if I don’t really need it.

I just posted another reply in this same thread, questioning whether a
multimeter AC voltage test of the shielded cable will provide all of
the needed diagnostics, or if “seeing” the quality, shape, amplitude,
etc. of the waveform is really necessary.

Please enlighten if you can.

BTW, I’m so cheap, the multimeter I normally use cost $5 from harbor
freight (one for each car at that price :wink: You can’t even get a
replacement battery for that meter (A23 12V) for $5. I do have a $60
multimeter from Radio Shack with all the bells & whistles, but the $5
cheapies are easier to use, and accurate enough for auto diagnostics.

Even the $60 Radio Shack meter has a label on the back saying: “Custom
Manufactured in China for RadioShack Corporation.”

I paid (new) less than 12% of what I paid for the first Mobile Pentium
IV Dell laptop 10 years earlier. Cheap I say – CHEAP ;-0

George Balthrop, Clifton, VA USA
89 and 85 XJ-S Coupes; 89 XJ40 VDPSent from my Lenovo Chinese (used to be IBM Thinkpad) laptop, for which

-----Original Message-----
From: BobPhx brackney67@gmail.com

something that I did which I think is pretty slick but nobody seems
much impressed when I bring it up :slight_smile:

1 - purchased a $100 oscilloscope (Velleman HPS10) with an
LCD screen, it’s a bit bigger than a multimeter and I keep
it in the boot.
2 - pulled the passenger side kick panel to access the
Marelli computer.
3 - Located the following wires per the ROM: 1) input from
Crank Position Sensor 2) input from Crank Speed Sensor 3)
Output to Fuel Injection ECU
4- Using an old 4-position radio shack female plug (like
you’d see on a car stereo harness) with about 18 inch leads,
tapped in (soldered) to each of these wires, and also added
a ground wire. The plug is tucked up under my glove box…

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16

In reply to a message from George Balthrop sent Sat 3 Nov 2012:

Hi George

The best wa to test sensors like wheel sensors is first by
testing the resistance of the sensor including the cable from where
it plugs in to the loom. Sensor resistances of inductive types will
be from about 400 ohms to about 2000 ohms but will alter with temp
etc. The next stage is to read another wheel sensor on the same
car as they will all be VERY NEAR the readings you get. That them
almost confirm they are ok apart from a signal test The metal gear
or magnet that induces the pulse may have come off or moved away
from the sensor (on some car makes) and so resistance would not
find the fault. The next test would be to repeat the last tests but
with the meter on low AC mV range and this will display a small AC
voltage reading as you spin the road wheel of the sensor you are
testing. The other sensors of inductive type can be tested the same
way apart from electronic sensors usind a HALL SENSOR. These
sensors often have a GROUND , +12 volt supply and a PULSE output
pin. These sensors will give a output (amplified) of about a 5 volt
to 12 volt pulse which cn be seen on a DC range eg later cars
GEARBOX SPEEDO SIGNAL = 12/0/12/0/12/0/12 is the output as
the car is moved forwards a few feet.

Any signals you need to test FROM one ECU to another ECU should
almost always be done with a SCOPE so you can see the waveform
shape to be sure .

I use a FLUKE SCOPEMETER for car work as they can read AC and DV
volts at the same time or DC volts and SCOPE at the same time .
They also record voltages and store waveforms to recall late. You
will find these on ebay as mine must be over 15 years old now but
works great. The one I have is a PHILIPS PM97 so do a search for
the spec and you can read up on them but this is made by FLUKE and
just rebadged

This is a link to show you what it looks like but you will find
them cheaper than this price if you look about.

http://www.ebay.com/itm/Philips-PM97-50MHz-scopemeter-9444-000-
97031-probes-/110966036403–
95 XJR6, 94 XJS 6.0 coupe, 04 XK8 ,99 Ka
Croydon , SURREY, United Kingdom
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17

In reply to a message from George Balthrop sent Fri 2 Nov 2012:

Thanks for all suggestions.
I started with the simple test George recommended yesterday
– spark from the coils king leads.

It checks out:
https://dl.dropbox.com/u/10323653/startup%20movies/MVI_1986.
AVI

At this point, given that FF77 does not come under any other
circumstances, I am willing to rule out the flywheel sensor.

George also suggested measuring the resistance of the
temperature switch at the A-bank thermostat housing. The
CTS at the B-bank thermostat housing gives 2 kOhm cold and
drops to below 200 Ohm when hot, i.e., must be OK

Here it is my new question – my JDHT disk shows a thermal
switch DBC12183 at this location. However, the sensor I have
there does not look similar to DBC3728/LHE1600AA (the CTS).
It has 2-spades and its cold resistance is only 7 Ohm.

Any suggestions/recommendations?

Steve–
The original message included these comments:

It is also my understanding that lack of either the flywheel sensor
signal, or lack of a crankshaft position sensor signal, will prevent
the ECU from providing the trigger signal to the amplifiers, and you
will therefore get NO spark from EITHER coil during starter cranking
when that fault (which may be intermittent) is occurring.
So try the test I suggested offlist, where you disconnect both coil
king leads, anchor their metal ends 1/4’’ from ground, and see if you
get any spark from either king lead during starter cranking on cold
start.
I would suggest doing that test BEFORE you separate the Flywheel Sensor
connector and clean the contacts on either side of the connector. Such

–
'95 XJS V12 6.0L, saphire/tan
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18

In reply to a message from sbobev sent Sat 3 Nov 2012:

I can not see why out engines would have two CTS as one will give
the info to the ECU to control mixture etc. The Coolant temp switch
i can see in my circuit and parts list but I am sure the other CTS
is only fitted to the NON CAT VERSIONS.

If FF77 was detected I would check out the FLYWHEEL SENSOR first.
This is easy to get at and after a clean of the connector plug I
would remove sensor and give this a good clean. I could not get any
thing from your link and you need to check that.

The spark test you have done is not valid as how can you confirm
the signal from this sensor is clean and constant ? If we say NO
SIGNAL from this sensor = NO SPARK fine BUT THE ENGINE FIRES so
their must be a signal of some type PRESENT ! . Clean this sensor
first and then see what you get. I remember a Renult model that
have cold start problems and all that needed was a clean of the
flywheel sensor.–
The original message included these comments:

At this point, given that FF77 does not come under any other
circumstances, I am willing to rule out the flywheel sensor.

–
95 XJR6, 94 XJS 6.0 coupe, 04 XK8 ,99 Ka
Croydon , SURREY, United Kingdom
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19

In reply to a message from 540itouring sent Sat 3 Nov 2012:

Dave:
After I researched this a little bit, it appears that the
thermal sensor on the A-bank is for the operation of the
air-pump. Equivalent to the EAC3762 sensor on the XJ40

Question – is 7 Ohm cold normal?

That sensor is not input neither to the Marelli computer,
not to the EFI ECU. Regardless whether or not this will
have to be replaced, it cannot be contributing to my cold
start problem.

On the FF77 code, you are absolutely correct. Page 18.3-14
from the ROM says ‘‘TEMPORARY loss of signal may cause
stalling or starting difficulties. COMPLETE loss of signal
will prevent running’’

Still, FF77 was shown only 2 out of 6-7 times I had cold
starting difficulties. Also, the engine has not stalled
while warm.

I will report back on what is the condition of the flywheel
sensor.

Steve–
The original message included these comments:

I can not see why out engines would have two CTS as one will give
the info to the ECU to control mixture etc. The Coolant temp switch
I can see in my circuit and parts list but I am sure the other CTS
is only fitted to the NON CAT VERSIONS.
If FF77 was detected I would check out the FLYWHEEL SENSOR first.
This is easy to get at and after a clean of the connector plug I
would remove sensor and give this a good clean. I could not get any
thing from your link and you need to check that.
The spark test you have done is not valid as how can you confirm
the signal from this sensor is clean and constant ? If we say NO
SIGNAL from this sensor = NO SPARK fine BUT THE ENGINE FIRES so

–
'95 XJS V12 6.0L, saphire/tan
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20

Hi Steve,

Sorry for the fire drill. When I asked of the CTS you had checked
resistance on was the one on the B Bank (EFI), I didn’t realize that
your 95 didn’t have the A bank CTS for the Marelli ignition that the
5.3L Marelli cars had. It appears that no 6.0L car had the A bank CTS,
but rather used an Air Temperature Sensor for the Marelli digital
ignition.

In any event, I would doubt that ANY temperature sensor has anything to
do with your delayed cold start problem.

George Balthrop, Clifton, VA USA
89 and 85 XJ-S Coupes; 89 XJ40 VDP-----Original Message-----
From: sbobev shelxtl@yahoo.com

George also suggested measuring the resistance of the
temperature switch at the A-bank thermostat housing. The
CTS at the B-bank thermostat housing gives 2 kOhm cold and
drops to below 200 Ohm when hot, i.e., must be OK

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