Hi all,

i've got a European 1976 Jaguar XJS here that won't run on petrol.
It is equipped with both a petrol and an LPG system, and it runs fine on the LPG system, which is installed a few years ago.
The engine immediately stalls though when switched back to petrol.
It also doesn't start in petrol mode. (ie. the main injectors don't fire, it stalls 1 second after starting, when the cold start injectors stop injecting)

Some info about the vehicle:
It is equipped with the early D type injection system with the Power Transistor Amplifier unit (NO power resistor unit)
It's equipped with the 25 pin ECU, with external MAP sensor
and the old OPUS ignition system from Lucas, with the in-dissy pick-up, the amplifier box, and the 3 resistor unit.
The LPG system is said to be entirely mechanical and, with exception of the ignition, independent of the ECU.

My observations:
-As mentioned before, the engine runs when set on LPG, so the fuel pump and the ignition system work fine, and the inertia switch is disengaged.
-The RPM gauge works as it should, so there is an enginespeed signal present. Though i'm not sure if this is the signal that the ECU uses.
-I do have sufficient pressure on the fuel rails.
-The ECU is physically able to fire the petrol injectors; I can hear them clicking when I move the throttle capstan, so at least the acceleration enrichment works.
-They also click when I fiddle with the MAP sensor

Based on the fact that the ECU does fire the injection when the throttle is moved,
but not when one attempts to start the engine,
my conclusion was that the ECU doesn't receive an engine speed signal, and therefore thinks that the engine running at 0 rpm, and consequently, doesn't inject.

I have googled the whole day, but I can't either find a pinout of the old 25 pin ECU, or a wire scheme of the wiring loom to it.
It is unclear to me how the ECU measures the engine speed, and on which pin.
I have found out that the newer ECU receives the signal via a coaxial cable on Pin 18, but this is different for the older 25-pin ECU.

So my questions are:
-On which pin does the older 25-pin ECU receive the engine speed signal?
-What signal does the ECU use to determine the engine speed? (ie. (in)directly the minus-terminal of the coil, or the amplified signal from the in-dissy pick-up, or something else)

My intended approach is to bypass the wire loom and clip a separate wire on the correct signal source for engine speed, and on the pin of the ECU on which is should receive its enginespeed signal.
This way I can either confirm or rule out this cause.

Feel free to ask for more information if needed.
Any information or help is appreciated,
Cheers!

 

Firstly welcome to the Forum.

When time permits please do an Intro in teh New members Area.

All my D Jetronic V12 books are in the shed, and its 1AM here, and 4degC, so it will have to wait until tomorrow.

MEMORY, is from the ballast on the RH side of the throttle capstan, but I dont remember the terminal of that ballast, probably the same as he Tacho feed????

It only signals Ignition activity, same as the P Digital system, not speed (revs) as such. Ho activity, no pulse, as you already said and have.

 

Found these, maybe keep you occupied until I go searching some more.

Might even answer the question.

D Jetronic 1.pdf

D Jetronic Jaguar info.doc

D Jetronic Injector wiring from the amp.doc

D Jetronic 2.pdf

D Jetronic wiring 2.pdf

D Jetronic wiring diag 1.pdf

D Jetronic wiring legend.pdf

 

Will do, shortly.


Okay this is really helpful, because first I assumed from info from newer(1989> jags) wiring diagrams, that the ECU uses the tacho signal(minus side of coil) to determine the pace of it's injections. But as I understand now, it uses the signal from the trigger unit. (that's the two reed contacts or hall sensors inside the dissy right?) And is it known what kind of signal the ECU should be getting from them?
Ie. like a pulsed 12V or more or less voltage? Then I can measure that signal on the 3 pins at the ECU to confirm that it either or not getting the correct signal.

By the way, a new dissy has been built in recently, which may have an optical trigger unit. I read on the interwebz that this could make for a lot of problems if the conversion isn't carried out correctly. So i'll have a look at that tomorrow, too.

A big thanks you for those wiring diagrams by the way! They're the very thing I was searching for for so long, but couldn't find.
I have scoured google and this forum for it, but without success. Do you happen to also have the wiring diagram for the ignition system?

I'm eager to hear more from you, but you've helped me a lot already!

 

The wiring diagrams have been of great help, (as I already mentioned in my first reaction to your answer, not sure why that hasn't been approved and released by a mod yet)
And I have been able to confirm my doubts. It is indeed just the engine speed signal.

I was able to get my hands on a similar distributor with a reed-contact, which i used to check the ECU's response.
I hooked it onto the Jag's connector which is supposed to go to the reed-contacts inside the dissy.

When the dissy is rotated around, I can clearly hear the injectors clicking, since the ECU thinks the engine is revolving.
I also hear the fuel pump getting activated as soon as I generate pulses with the test-dissy.

So this must be the root of the problem, the recently built-in dissy probably contains the wrong type of reed-contact, or maybe its just a dead-on-arrival unit.
Either way, the problem should be solved by replacing the dissy with one that contains working reed-contacts of the right type.

I'm curious what i'll find in the dissy in situ, as i haven't built that one in the Jag. But it probably has the optical sensor that they're talking about on the web.

 

MY BAD.

Brain fade after midnight.

YES.

The PreHE and all D Jetronic use a trigger board to activate the injectors, and have NOTHING to do with Ignition pulse at all.

Gotta love this old age rubbish.

I will look later for those other requests, just got a better offer????????

 

The original D Jetronic system in the V12 had a trigger board with 3 wires and Reed Switches. The magnet in the heel of the rotor activated these reed's simply by passing over them.

These units reeked havoc, and were quickly replaced by a 4 wire trigger board, which contained "hall sensors", and the 4th wire is a 12V Ign feed. This required the Ign to be ON for any activity to take place.

THEN THERE IS MORE

Jaguar changed the magnet in the heel. There is a squarish, and a round magnet. I forget which goes with which trigger board, BUT they are different. WHY, I have no idea, a magnet is a magnet, in my "grey matter".

To test a 3 wire trigger board, wave a magnet over each end, and LISTEN carefully. You will hear the reed's clicking if you are blessed.

The Optical system is NOT an issue. I have a Crane in our Pre HE XJ12, and the 4 wire trigger board is mounted as it was, and the car is happy. YES, the install is fiddly (polite version), and takes some hours to set up spot on. The 12v feed took some finding, but a relay from a relay sorted that clean feed. Now touching 540000kms, so care factor is low.

 

A little thought from someone with an LPG setup on a different car here...

The LPG system is supposed to be mechanical? If it is, check the splice into the injector system. My system is done with 2 ECUs grabbing a signal from the engine ECU and converting it into a different Signal for the LPG injectors to work.

This is only active when the LPG system is switch ON. When it is off, it will not interrupt the signal between the ECU and injectors.

As you have a Signal when rotating the dizzy but with no engine running, the signals from the ECU to the fuel injector rail works. But I assume when the engine has some turn impulses (aka is running and the LPG system is activated) and you switch back to petrol mode, the LPG system doesn't permit the signals to be forwarded to the injectors.

I don't know what type of LPG setup you have, but if it is mechanical, it is a rare and old type, but it must have some electrical connection into the injector harness with a 'switch' inbetween, which is either controlled by an ECU or a relay. Do yourself a favor, just to rufe out the LPG system, disconnect it and bypass the LPG wiring. Like that you can make sure it isn't the LPG setup.

My XJ8 had this problem as I Forst had the LPG system fitted. When the engine was running on gas, it would run perfectly. When I switched over to petrol, it would die. Turned out an ECU had a little crack in a board, enough to cause this issue.

My gut feel says something is to do with the LPG system.

 

Check the rotor arm. That triggers the injectors on pre HE. A v12 carb rotor looks the same but won't work. I ( think ) the correct one has a small magnet that goes over the trigger board.

 

First, apologies for reacting so late. I haven't been notified by email of activity in this thread, as most forums do. Since i'm doing multiple projects simultaneously, some things tend to be forgotten by me.

Anyway, by your writings here, I was made aware of the fact that there are two kinds of those dissy hammers, but I haven't checked yet which one is present in my dissy. Shame you can't check that quickly. But if the optical setup, or the hall switch setup also work for the older ECU, based on the 3 wire board, then the current dissy setup should be good to go. If it works at all, that is. So that is what i'm going to check first next time.

Is it known what kind of signal you should be getting from a working dissy?
Reed switches are just make/break contacts, so theoretically, they should give a square pulse signal between 0-12V.
Optical sensors however, might give a different signal, i guess. I do have access to an advance multimeter and a scope, so the only thing I need to know is what kind of signal i should be expecting. I hope you can help me with that...

 

The LPG system is said to be entirely mechanical, and independent of any ECU or the like. It only needs ignition. I have run it successfully with the ECU disconnected, so I believe this is true. The only electrical components present in the LPG system are the two safety shut-off valves.

I'm not sure what the fuel change over switch does though. The only thing I know is that it disconnects the fuel pump when changed over to LPG, and that it engages the shut-off valves to open. But I presume the switch disconnects more things from the petrol system, i don't have an idea about what though.

And like you said, I have disabled the LPG system, so it isn't interfering with the troubleshouting concerning the petrol system.
As a LPG car owner, do you have any idea what more the fuel change over switch might switch besides the fuel pump and lpg shut valves?
It'd give me a good idea for where to look. I'll have a look at the insides of the ECU, also.

 

To be totally honest, I highly doubt the LPG system in your car is purely mechanical. That would mean you'd need a totally different injection system like a Bosch KE Jetronic... I have my bet on the case, that your car has an electrical LPG setup.

Anyhow, the the LPG system ALWAYS needs a link into the petrol EFI. So it needs a switch (technically 4) to switch the injectors off. So it is spliced into the engine harness (EFI). It builds a 'bridge'. If LPG is on, the petrol injection is cut off. If petrol is on, the LPG is off.

This is normally done by either a timer, a temperature sensor or manually via a swotch. Normally the switch is an override and the LPG system turns itself on. So, your switch should only activate the LPG and DEACTIVATE the petrol. The LPG only cuts in on the normal 12 injectors. The 2 cold start injectors WON'T be wired into the system (they never are!). Again, I am really sure it is the splicing into the engine loom.

Do yourself a favor, and me at the same time, take some photos of the engine bay and upload them here. Also, follow the injector harness to the body and follow it further back. Somewhere along the line you'll find some control box which will be sploced into the setup. Older ones, no matter from when, do like to die and then only wprk on gas.

Do you know what brand of LPG system it is? There are loads of manufacturers and after searching, I couldn't find one that works mechanically.

As said, your LPG system has got a 'lofe of it's own' and isn't sending the signals from the Jaguar ECU to the i jectors. Instead they have been cut off. The 2 cold start ones though are still getting signals, which is why it will just somehow run. When the temperature or time period has passed, it swtiches to LPG and all is fine again, as the fuel injectors are cut off.

In my car, a lot easier than older systems, the LPG ECUs process the Jaguar ECU signals and switch off the normal EFI and over to LPG without even feelong it. No evaporators. No interferance with the cooling system (older vapor injection systems where the LPG is converted from liquid to vapor form have plumbing). Liquid injection is easier to service (doesn't requure any) and causes less problems with power loss...

 

That's not surprising, I also had my doubts, but it does run on LPG with the ECU disconnected, and not on the cold start injectors. I disconnected those to be sure.
And it is indeed an old LPG system. It is built in after purchse by some company. I'm not sure what kind, brand, or type it is. I'm not familiar with LPG systems. I'll make photos as soon as i get the chance.
If i remember correctly, the fuel changeover switch only sends 12V to one of two wires on that switch.
Those wires separately may switch more though. But I can't see that. I'll try to find out how the circuit goes further, but it's really hard to see. The loom disappears into the body and is hard to follow.

With your input though, I have more ideas where to look for possible issues, so thanks for that!

 

Hi, this is an old thread but contains the information I need. I followed the flow charts presented by Grant Francis, awesome info by the way.
1976 (late build) XJS V-12 (pre HE). Car sat for 19 years in a unheated storage shed.
The engine will run on carb spray indicating the ignition system is working as it should.
The fuel pump will turn on for a second or so when the key is turned to the on position, but not while the engine is being cranked over or right after releasing the key from the start position.
I've confirmed fuel pressure and delivery are fine. ( running off a separate clean tank and lines flushed)
The car is still fitted with three wire, reed style, trigger board. I ran tests on the board per Grant's post and it is working as it should.
I removed the throttle switch and confirmed it operates as it should. Also adjusted the throttle stop setting with a .030 feeler gauge.
I have battery voltage and earth at the injector amplifier mounted on top of the radiator support.
I have a few questions.
When the voltage is measured at both terminals of the injector connector, is the connector unhooked from the injector?
With the key on and the connector off of the injector I have 2 volts to both terminals of the connector.
The information I have learned the voltage should be 12 volts ( or battery voltage) at both terminals?
Even though the trigger board checks okay, can it still cause an issue with the "tach" signal input to the ECU?
Thank you,Tom H

 

Edit: I moved my post to a new thread.

 

Though I don't have one, I think the wiring diagram can answer some of your questions:

"Even though the trigger board checks okay, can it still cause an issue with the "tach" signal input to the ECU?"
Per the diagram, there is no "ignition/tach" signal from the coil or amp to the ECU. You only get the trigger board version of that triggered by the magnet in the rotor rear's action on the reed switches or hall effect upgrade board.

"When the voltage is measured at both terminals of the injector connector, is the connector unhooked from the injector?
With the key on and the connector off of the injector I have 2 volts to both terminals of the connector.
The information I have learned the voltage should be 12 volts ( or battery voltage) at both terminals?"

This test even on the HE probably confuses a lot of people. Generally injectors are supplied always-on +12V power, and the control side is open circuit until they need to be fired, in which case a ground is provided (with some current limiting). IF you test for +12V with ANY of the injectors still plugged in, the power will pass through the positive side, through the injectors, and into the control/negative side wire. You have do disconnect ALL injectors in order to see +12V only on the supply side wire/pins. AFAIK the same principle applies to the D-Jectronic injector amplifier box, so seeing 2V does not sound right. Per diagram the Main Relay should supply power to both the ECU (pin24) and EFI Amp (pin 10). Apparently the ECU can operate on power to pin 16 without the power to pin 24 being present, and thus the injector amp wouldn't work while the ECU would work (blindly attempting to trigger the injectors).

https://jetronic.org/index.php/en/d-...ungsversorgung

~Paul Kobres

 

The next chapter of that site specifically addresses the Jaguar varient:

https://jetronic.org/index.php/en/d-jetronic/102-jaguarte


Quote section 15.1: ---"Power transistors switch ground to injectors and power resistors permanently supply 3.5V to injectors"... So IDK. Perhaps that 2V is normal for the steady un-activated-state and I may need to read up on this more.


~Paul K

 

FerriGuy. Your information was spot on. I used your information and diagrams to locate low voltage at the main relay. Was able to repair it and have battery voltage at the injectors. I’m in the process of cleaning the injectors as the car has been idle for 19+ years.