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#61 Re: V12 ECU

Posted: Sat May 09, 2020 9:52 am
by MLBS3V12
With the fact that nothing seems to happen with the vacuum applied manally, I'm looking now for a new Marelli ignition ECU.
Hope to find one soon.

#62 Re: V12 ECU

Posted: Sat May 09, 2020 12:19 pm
by MarekH
Dear Michel,

The reason Philip wanted to know whether the injectors were clicking was to ascertain whether they were simply being held fully open all of the time. This can happen if there is a short in the wiring or if the ECU commands a very long opening time. The very long opening time is called for either if the load is very high (e.g. throttle wide open - electrical signal from throttle pot) or if the pressure sensor inside the ECU sees no vacuum at all (e.g. throttle wide open - vacuum signal from manifold) or if the ECU thinks the engine is cold.

The full load vacuum switch is not the most important component here and not important in my opinion. You can disconnect it and mimic its working by either leaving it open circuit (disconnected), or closed (by joining the two wires which go to it). As you know it simply adds 15% fuel if it is "on".

More interesting is that there needs to be a variation in the output of the manifold air pressure sensor sitting inside the ECU proportional to the vacuum applied to it.

If no vacuum is applied to it, the ECU thinks the throttle must be wide open, i.e. the air pressure in the manifold equals outside air pressure.

You can stick a meter across the relevant electrical contacts of the ECU's barometric sensor and verify that it does do something as you vary the vacuum. By this I mean does the electrical resistance vary with pressure (do this with no voltage applied - just measure the pins on your workbench). If it does appear to work and you have no change in your fueling, then you should look for a dry joint in the ECU that stops the signal from getting any further in the ECU. If there is no resistance change across any of the pins as you vary the pressure, then that sensor is probably bad and a new one needs to be swapped in. (For example, if the sensor casing is cracked or broken then it probably doesn't hold vacuum and so always thinks it is at full throttle.)

As Philip said, the main coolant sensor, if bad or accidentally replaced with a non correct but similar sensor, can fool the ECU into thinking it is colder than it really is and if this is your problem, then there are workarounds. It is not obvious to me that you have the correct coolant sensor for the Marelli ECU. (I have seen this done once before when a very experienced etype guy fitted a new coolant sensor (the correct sensor!) but to an ECU which had been remapped while the old coolant sensor was fitted. The ECU always overfueled exactly with the same symptoms you have.)

Firstly, check the ECU coolant sensor resistance is high when it is cold - it'll likely be well over 5000ohms. It will also likely be only 200ohms or so when hot and maybe about 2000ohms at 20'c. Those are just guesses, but if that information doesn't come through the loom to the ECU, then the ECU will always think it is very cold and always overfuel.

You can mimic this sensor's resistance by simply using a variable resistor pot instead in its place (for testing purposes only). You should be able to change the fueling to be lower by fooling the ECU that the engine is hotter than the original sensor thought it was. Another way of doing that is to put another second resistor in parallel with the ECU coolant sensor to mimic a cooler temperature.

Depending on how these tests work out should give you an idea of whether you are looking at an electric fault or a physical fault with the pressure sensor in the ECU.

(The other coolant sensors do different things and ought to be wired into different locations. Roger Bywater's website shows the ECU connector pin numbers.)

kind regards

EDIT:- when testing the air pressure sensor in the ECU, make sure the full load vacuum switch is not part of your circuit and that you are testing only the sensor in the ECU. Also if the hoses are split or cracked, then off course it will think it is 100% air pressure and no vacuum, so eliminate those from your test.

#63 Re: V12 ECU

Posted: Sun May 10, 2020 8:50 am
by jagwit
MLBS3V12 wrote:
Sat May 09, 2020 9:52 am
I'm looking now for a new Marelli ignition ECU.
Why? I have not seen anything you wrote to suggest the Marelli ignition ECU is faulty. Perhaps you meant "an EFI ECU for a Marelli car"?

Thanks for your input Marek!! I did not know that the pressure sensor output on the Jag ECU is resistive (like an oil pressure sensor). I only know the active MAP sensor used in MS which has a voltage output. Learnt something new :-)

#64 Re: V12 ECU

Posted: Sun May 10, 2020 9:31 am
by MarekH
I don't know whether it is resistive either, but I am suggesting that there ought to be a variation measurable with change in air pressure. Firstly I'd try to do that in a non-invasive way, via checking resistance on an unpowered unit, then if that is not conclusive, I might go on and try to measure the outputs of the baro sensor versus ground when the ECU is powered up, since it ought to be able to trace where at least some of the tracks it connects to.

If the baro sensor does respond linearly to pressure, but the ECU output doesn't, then it is very unlikely that one one the pcb tracks is broken, but it is quite possible that a dry joint stops the information from either going in or coming out of the ECU somewhere.

Either way, I think Michel wants to work out whether he has an electrical fault or a physical fault. The ECU obviously works, but not the way it is expected to.

If the car can be made to run on a cold engine with its current "bad" input, it can then be fooled into thinking it is hotter than it is and made to run better by making the coolant sensor input reflect a different (hotter) temperature.

From experience, the most likely list of electrical faults are first in moving parts, then connectors, then harness and finally the sensors. The harness may be higher up that list, as it has been moved about and changed.

If Michel has a track pump for his bicycle, then he can pressurise the MAP sensor line to a modest pressure, e.g. 1.25 bar and see whether it holds pressure. If it holds pressure, it should hold vacuum.

kind regards

#65 Re: V12 ECU

Posted: Sun May 10, 2020 7:13 pm
by MLBS3V12
Hi Marek and Philip,
I thank you so much for your deep explanation. It is always helpfull to get information from you. You both have a serious experience on the V12.

For weeks, I was cleary turning around. I've verified all 2 or maybe 3 times, the wiring harness, all the connections, all the modifications I've done on the wires as I've reduced or extended some of them, the coolant sensors the air sensor have been checked too.
Lately, because of your remarks I've looked to the Map sensor and the vacuum hoses. All seemed to be ok. Yes the Map sensor from the Ignition ECU Ignition because Philip, there is one into........ But not the one from the EFI ECU!!!! :banghead: :banghead: I just simply do not connect a hose from the manifold to the EFI ECU!!!
I've looked again the doc I have particularly on the "Marelli" engine. It is cleary shown the tube to be connected to the ignition ECU. Nothing about the EFI one. This is normal because this doc talk about the Marelli, not the EFI..... :doh:
I do also have the spare parts catalogue from the XJS. The ignition EFI is shown with the small tube to be connected to. Not the EFI !!!
I' m sorry guys :shrug:
I'll test a tube connected to the EFI tomorrow and post the result. I need first to clean one again the plugs

#66 Re: V12 ECU

Posted: Sun May 10, 2020 7:28 pm
by jagwit





Its always good to find the "smoking gun" or, in this case...... the missing pipe.

#67 Re: V12 ECU

Posted: Mon May 11, 2020 5:06 pm
by MLBS3V12
The engine runs now fine, smouthly, very regularly on all the 12 ctlinders without exess fume compare to last week. great!
I fine that the AAV I ve restaured using a bulb which works @85°c is a bit long to allow the engine to comes to idle. I will find another one.
I ve another topic to work on: the 2 fans do not work....

#68 Re: V12 ECU

Posted: Mon May 11, 2020 7:45 pm
by jagwit
Excellent news!

Congratulations Michel!

#69 Re: V12 ECU

Posted: Mon May 11, 2020 8:08 pm
by MarekH
All's well that ends well.

There ought to be plenty of threads about Otter switches, the fan relay, the wiring (including the fuse above the battery) and about the fans themselves, so checking them off systematically should be no problem.

kind regards

#70 Re: V12 ECU

Posted: Wed May 13, 2020 8:51 am
by MLBS3V12
Not one but 2 problems on the fans. Bad contact into the connector from the Otter switch and a grilled ground cable connected to the relay.

Yesterday afternoon I ve done ~1km on the road. All sounds good. A few meters on the 5th.

Next step is the bonnet replaced anr the interior.

#71 Re: V12 ECU

Posted: Wed May 13, 2020 9:19 am
by MarekH
Le chemin est claire maintenant - at long last!

kind regards