Good evening all.

I’m new to the forum having volunteered to help a friend get his ‘76 V12 XJS running that has been sat idle for while. I’ve never worked on a Jag before but I’m fairly handy with a spanner, fancied an oily challenge and have been given access to an XJS Haynes manual – dangerous stuff indeed!

So the story is thus…. the motor cranks fine, fires up for a second or so, then attempts to run but is too lumpy to sustain itself. The fuel pressure checks good. The initial second or so run seems to be enabled by the hot start injectors (one in each manifold that are not connected to the ECU). A hard suck on the manifold pressure sensor, albeit rather unpleasant, indicated the diaphragm is still good as no air was being drawn in.

Manually operating the throttle with the ignition on (but not cranking) shows that six of the injectors are squirting but the other six are not – so we appear to have identified the (a) symptom of the non-starting, though not yet the cause.

Digging into the manual, the wiring diagram of the D-Jetronic ignition indicates that post the ignition amplifier, the injectors are arranged into four separate groups of three and all three injectors in a group are connected in parallel. Pre-amplification, the four groups are connected into pairs – ie one group of six injectors fires simultaneously and then the second. A little on the crude side IMO but this is one of the earlier FI systems yet it’s still good for 150mph!

The six working injectors are all nicely contained within their designated two groups of three. In addition, they also service the first six cylinders in the firing order. So what we have is one set of six injectors (two groups of three), not operational, that services the last six cylinders in the firing order. OK – we’re getting into some more detail but the cause still evades.

Several lengthy web searches indicate one common electronic cause of injector malaise is failure of the FI trigger board at the base of the dizzy housing. While this seems plausible I’m still a little uneasy with it. My tacit assumption is that all twelve injectors would typically fire on manually operating the throttle with the ignition on but no cranking. Would trigger board issues have any impact on the injector operation when the motor is not turning?

Before I charge in with spanners flying I thought it might be wise to consult the learned counsel of this forum for guidance. Any help on narrowing down the root cause, along with a way of testing before the disembowelling commences would be much appreciated.

 

Hi I had the same problem on my 1990 XJS V12 (marelli ignition) Six injectors were stuck as my Car had been standing for 16 years and the Injectors were gummed up and wouldn't squirt.

So I removed the Fuel Rail with all of the Injectors Still attached and then filled up the Fuel Rail with Wynns Carburetter Cleaner and then attached my compressor to the Fuel Rail at a very low pressure (as in almost no pressure)

Then using a Spare Injector Plug, that I had kicking around (The Electric Plug) that goes on the Injector.

I got a PP9 9volt Radio Battery and then gave each injector, a 'One Second' dab of Power and got 10 of them Squirting almost straight away.

The last two were a bit Stubborn, so I squirted the Carburetter Cleaner (From an Aerosol Can) directly onto the end of the Tips of those Injectors (Head on to the Tips) in fact I did it with all of them just to be sure.

As I Squirted the Carburetter Cleaner directly onto the Tips, that the Injector is supposed to Squirt Petrol out of, I could hear the Tips Clicking as the Aerosol Pressure forced the Internals of the the Injector Needle Back.

If I remember correctly a couple of those Injectors needed a dab of 12 volts rather than 9 volts, plus a Tap with a Hammer on the Metal Part.

Eventually they all gave up and all Started working, so I put the Fuel Rail Back in the Car and then I turned the Key and she ran like a bird.

'As soon as I realized my deliberate mistake of connecting the wrong plug to the wrong coil' as there are two on a Marelli and I think you only have one.

Only give the Injectors a 'One Second' dab of Power, to prevent burning out the Coil in the Injector.

But before you do any of that take out the Resistor Pack which is the Silver Box down by the Headlamp Nacelle and thoroughly Clean both the Plug and Socket, as a dirty plug and socket can cause all sorts of problems with the Injectors not firing.

This is worth doing anyway, as it may just be all you need to do, although if She has been Standing a while, then maybe the Injectors are 'gummed up'



Resistor Pack on a 1990 V12 XJS (Marelli)
A very similar set up to the Lucas, make sure the Plug and Socket are nice and clean




A Dirty Plug and Socket on the Resistor Pack, can really play Havoc with the Injectors Firing.


You can see how I cleaned my Injectors here which was quite easy to do with no special tools:

https://www.jaguarforums.com/forum/x...-122634/page8/

 

OB is correct the injectors do stick. Before removing the rail I would check the injector harness.

Turn the ignition on DO NOT start the car. Check for 12 volts on BOTH wires on the injector with it still plugged in.

How the system works...
Turn ignition on - all injectors fire a priming pulse, this is independent of any feedback control.
As soon as the engine cranks the tach in the dissy provides injector timing to the ECU. The injector now fire once every engine cycle until the engine starts.

 

Thanks Guys,

I'm sure the injectors could do with a good clean though I suspect an electrical issue somewhere is the most likely cause at this stage - as all the non working injectors are not randomly spread across injector groups but lie nicely within their electrically encapsulated groupings. You can hear the working injectors 'clicking' (while not cranking) - the non-working do not click. Checking for a permenent 12v at the injector connector with the ignition on would indeed be the simplest next step - as I understand the injector circuit is closed by the amplifier connecting the injectors to negative rather than positive.

 

When you mention the amplifier are you referring to the power resistors located behind the RH headlight?

The ECU (in the boot/trunk) grounds one side of the injectors.

 

The injector amp is electrically placed between the ECU and the injectors - the ECU does not have enough grunt alone to power all the injectors. Physically it's an alu' box about 8" x 4" fitted on the body crossmember above the radiator grille and under the hood. Theres a B&W annotated photo of it on this page JAGUAR V12 FUEL INJECTION 1975-1980 - D Jetronic / AJ6 Engineering

 

Just got a brick on the roof from Greg to wake me up.

Our Internet is down in most of OZ, and when its up, its SLOW, as in dial up was blistering as compared.

ANYWAY.

D Jetronic.

The trigger board is the WEAKEST point of these systems, BUT, the original baord was a 3 wire unit, with reed switches imbedded in the mastik.
The later is a 4 wire unit, with Hall Effect sensors imbedded in the same mastck.

The 3 wire is easy to test. The 4 wire is bullet proof, and harder to test.

The Ign ON engine OFF test you are doing is great, but I dont remember any of mine ever having the same issue as yours. The injectors clicked at each phase of the switch as it opened.

The 6 out you have is almost pointing me at an amp issue, or the wiring coming out of it.

Let us know if that trigger board is a 3 or 4 wire, and I will keep remembering as long as I can stay on air.

 

Yep - The next test will be the amp, or more precisely, the power transistors in it. One pair is firing but the other pair does not appear to be operational. This might not be the amp but simply an absence of a trigger pulse from the ECU to the amp for that pair. To identify which, I'll be bridging / swapping the trigger pulse (wire for ECU) from the working pair across to the non working pair - then repeat the ignition on, no crank test with manual throttle operation. If all injectors then click, then we'll know the amp is good and the fault lies somewhere else in the system before the amp.

I've been holding back on pulling apart the dizzy until I'm more confident that's where the problem lies. It's well buried in there below the fuel lines and HT cables - let sleeping dogs lie etc. In fact I've not been able to identify the trigger wiring with certainty from simple observation as its burred so deep in the V and obscured by other components, plus the ignition connection unhelpfully has three wires of similar type. However if I need to dig in there with leather apron, rubber gauntlets and welding goggles, so be it.

PS +12v tests good to all the injectors - so they all have power, awaiting operation by closing the circuit to earth via their respective amp transistors.

 

I am not familiar with the D Jetronic as my car is an 89.

If 6 injectors are clicking when you crank and 6 are not then the trigger in the dissy is working.

You can test the transistors in the amplifier with an ohm meter, or most of the new DVM's have a diode test function you can use to test transistors.

To test the transistor, OUT OF THE CIRCUIT. This is important as other components could cause the readings to be different. The resistor and capacitor across the Collector - Emitter will cause the readings to drift until the capacitor is charged and then the meter on ohms will read the resistor value. If the Transistor is short circuit between C-E then this will show short circuit, although this is unlikely with the symptom you have.


Positive Meter lead on the base
Negative lead on the Collector or Emitter will read open circuit ohms or diode mode

Negative lead on the base
Positive lead on the Collector or Emitter will read less than 1.0 ohms or in diode mode 0.6volts

Collector to Emitter should read open circuit BOTH ways. Out of the circuit only, the capacitor and resistor WILL change these readings.

.