HI All

I have a 1978 XJS V12 which has been very reliable for the past 35 years I have owned it, last week after a 4 or 5 mile run the engine cut out whilst idling and hasn't started since.
When you initially turn it over, it fires once then nothing. A auto electrician has found several cracks in the cabling of the injector harness which have been temporarily repaired.
There is a spark at the plugs, the fuel pump runs when the ignition is turned on, there is fuel in the injector rails, with the ignition on if you turn the throttle linkage you can hear and feel the injectors firing, we have shorted out the coolant temperature sensor and checked the inertia switch by the door pillar.
At present our thoughts are the ECU which is marked 3CU83477B which I believe is part number AEU1121E from SNG Barretts, can somebody confirm this, and has anybody had a similar experience or any further suggestions.

Thanks

Gary

 

Gary,

Welcome to teh Forums.

When the beast is sortted, please do an Intro in teh New Members Area.

The 3CU is pure analogue, and I have never seen a dud.

The trigger board under the rotor is the first thing on my PreHE list,

followed by:

The magnet imbedded in the rotor heel.
The OPUS ign amp is fickle, BUT, you have spark, so leave alone for now.

The fact you have Injector "click" when opening the throttle indicates the ECU, and the EFI Amp, are talking.

Sooooooo,

Is the trigger board a 3 wire, or a 4 wire. Look at the rear of the distributor, just under the cap edge, and count the wires in that silly rubber grommet, and let us know what you have.

Then we start the process.

 

Hi Grant,

Thanks for the reply.

What I didn't mention in my post was that the car actually broke down at a garage after a road test to check a gearbox leak!!!

I cannot contact them now until Monday unfortunately, also I had the "Luminition" ignition system installed some years ago, but I believe that, and the trigger board you are suggesting are two separate items.

Is the trigger board rectangular in shape mounted at the four corners sitting just below the rotor?

Be in touch Monday

Thanks again

Gary

 

Gary,

OOPS

Weekends, the curse of the Jag that wont run.

YES, that is the trigger board.

The original was a 3 wire thing, reeked havoc. Had reed switches imbedded in the mastic stuff,

The replacement, along with a matching rotor, for reasons I have never understood, is a 4 wire, and has "hall effect" sensors imbedded, reliable as any Jag on the road, fact. Mine is topping 700000kms and still fine.

ALSO

Check the loom plug has not fallen out of the MAP sensor, on the RH side of the radiator top panel (memory, as mine is an XJ12, and on the LH side). The vac hose that runs TO it may also have fallen off, of simply split.

The loom plug INTO the TPS can fall out, but they will run and drive with that unlugged, just not super quick off the mark.

ASSUMPTION, on both our parts.

You have spark, CHECK, but is it actually AT the spark plug, or is he testing the coil lead only.

REASON

The carbon brush in the centre of the cap, transmit energy TO the rotor for distribution, has a habit of crumbling, MAINLY the HE engine, but I have a few PreHE do it. So, when the cap is off to check that trigger board, turn the cap over and ensure that brush is intact.

Next, before coffee.

The ignition switch electrical section may be getting hissy. VERY common due to age. Volt meter of the +ve of the coil, turn ON the ignition, and should show a voltage (not sure what the Lumenition does here), but then "go to start" and note the volt, It should NOT drop very far at all. If it does, that switch needs to be removed, easy on RHD, and cleaned, just remember to do the opening of said switch INSIDE a freezer bag, so the SMALL spring does not go AWOL.

 

Gary
If you are saying that the Jaguar OEM ignition has been changed to a Luminition system, then Grant's advice will not necessarily be entirely appropriate. If I remember correctly, the Luminition uses a trigger wheel cutting a light beam as the switching mechanism. This system might have failed for a number of reasons: most likely being the trigger light emiiter has failed, or the Luminition amplifier-equivalent box has failed.
Greg

 

Grant,

Thanks for your second reply, managed to get the garage to look at the distributor today, it does have the original three wire trigger board as I thought.
I have ordered the replacement 4 wire unit which is being delivered directly to the garage tomorrow, hopefully fingers crossed that's it, if not I will start with your other suggestions.
I believe the new "Fourth wire" needs to have 12 volt applied when the ignition is on, can this be taken from the side of the coil where you suggested to do a voltage check to prove the ignition switch was ok?.
I will keep you informed of progress

Thanks again.

 

Greg,

Thanks for your post, you are correct on the operation of the luminition system, however I do have a spark at the plug which indicates this side of things are working.

All suggestions are appreciated

Gary

 

Gary,

Well done so far.

I would frame that trigger board, its probably the OLDEST operation board in existence, ha.

Yes to the 12v Ign feed, and NO to the coil (MY OPINION). I like a clean crisp Ign 12v to these things, and the coil is not as clean as I would like.

You could use a feed (trigger) wire FROM that coil +ve TO a relay, that then supplies the clean 12v for anything requiring 12v with Ign ON, including the Lumenition maybe.

Mine was an XJ12, so the relays were all under the bonnet, and I used a feeder from the Main Relay on the radiator support panel, to the "Ignition Relay", took all of 5 minutes (1 beer).

I know most instruction pamphlets state to connect to the coil, and many have done so without issues, I am just a tad different, and like to keep the spark side of things quite seperate from anything else, makes trouble shooting later on easier.

You mentioned EFI loom repairs, that may also be a root cause of your issue, as that loom was fickle right from the packaging way back.,

BUT

I have never known an EFI loom to stop the engine dead, they usually go erratic, overfuel, hard to start, etc etc, and eventually die.

 

Hi Grant,
Good and bad news, replaced the trigger board and no go,, checked the ignition switch as you said still no go, however removed wire 85 from fuel pump to depressurise the system and it started, obviously a bit lumpy but going none the less, have you come across this, fuel pressure valve??

 

Mmmmmm.

No fuel pump and it starts, odd at best.

Yours has the twin round section fuel rails, and ONE FPR (Fuel Pressure Regulator) per half rail.

The FPR do die, they are old, but 2 at once, about the same odds of winner the lottery.

ASSUMPTION from Downunder.

1) Yours WILL have 2 cold start Injectors, dead centre of each Inlet Manifold, inboard of the throttle body. Disconnect them from the fuel rail, plug the rail (EFI hose with a bolt stuck up its end and clamped. Leave the electrical plug for now, as I suspect these Injectors are "dribbling" seriously. Jag fitted them, removed them, fitted them, etc etc throughout the life of the V12, and I remove ALL of them on any V12 that appears. The engine will start just fine cold without them.

2) I still suspect loom faults. Holding many Injectors OPEN, and thus flooding the engine. No fuel pump = NO fuel, and the engine is running on residual for a few seconds.If the engine is running for more than a few seconds with that wire OFF, then you have something still operating the fuel pump, or the relay is jammed closed, or the wrong relay has been pulled.

The Injectors are fired in 4 banks of 3:

1A 3A 5A
2A 4A 6A
1B 3B 5B
2B 4B 6B.

Not sequential, just back fired based on the rotor over the trigger board. When the loom goes fickle, this is messed up, and some dont fire at all, and others stay open, BUT, it still needs fuel pump running to maintain any running past a split second "puff" Once the rail is empty, depressurized, whatever, the engine will not fire. No "go juice".

AND

When the Ign is first turned ON, do you hear the fuel pump whir for 1 to 2 seconds, then stop. If so GOOD, that is normal. If the pump continues to run, then someone has bypassed the ECU control of that relay, and that could explain why its still getting fuel with the wire pulled. The pump is being powered direct from Ign live wire somewhere.

This could explain why it running now, albeit rough, as the over fueling that MAY have stopped it has cleared, but the spark plugs are fouled with soot.

 

Had a coffee and a bit of thinking time!! here's a bit more info.

When I checked the ignition switch I had 12v on the positive side of the coil, this dropped to 8v when cranking, thinking this could be the problem a connected a cable directly from the positive terminal of the battery to the positive side of the coil, effectively bypassing the ignition switch, still no start. I then removed this temporary cable.

When you turn the ignition on the fuel pump runs for 1 to 2 secs then stops, I read somewhere to make sure the pump starts running again when cranking,(which I don't think it does) when you stop cranking it runs for 1 to 2 secs again ??

As I said, I decided to depressurise the fuel system by removing wire 85 from the fuel pump relay, turned the ignition on (no pump running) cranked to depressurise and it started. Running rough on tickover but sounds ok at 2000rpm, turned engine off tried to start again (wire off) won't go. put the wire back on let the pump run for it's 1 to 2 secs won't start, took wire back off starts again!!!.
Then with the engine running (still can't hear the fuel pump running) put the wire back on, engine cut out straight away and fuel pump ran for 1 to 2 secs.

Its as if you need the fuel system to be up to pressure without wire 85 on and it starts (can it be sucking fuel through?) once the engine is stopped and the pressure has gone it won't go until the wire is put back on to raise the pressure and then removed again .

I have noticed a smell of petrol in the boot recently with no sign of a leak anywhere.

Oh the joys of an old Jag

 

I had some weird run behaviour when depressurizing the fuel system a while ago. Turned out to be a bad pump relay. Cheap enough to test.

 

OOPS.

I will think on this, as its way after midnight here, and I need the beauty sleep.

Relay points are welded shut, BUT, the 1 to 2 seconds would NOT work, as that is the ECU cutting the earth supply to that relay after the 2 seconds, and I dont remember any of my PreHE doing that 2 seconds after shut off pump run. They ALL did it if the engine failed to start, and you released the ign switch from the start position.

When it died, did it stop, like flicking a switch, or splutter out??

 

The engine was running and when i connected wire 85 the relay clicked, the engine stopped like a light switch (The original fault) and the fuel pump started.

Its enough to drive you to drink

 

OK.

Studied the wiring schematics, and drank coffee, lots of coffee.

Terminal 85 of the fuel pump relay is an Orange wire, and goes to Pin 19 of the ECU, which control;s the earth of that relay. Nothing sinister. With that wire pulled, that relay should be dead, so should the fuel pump.

The Main Relay, adjacent, has the earth wire from Terminal 85, and is Black, and goes direct to earth, alongside the battery from memory.

The Main Relay turns "on" the fuel pump relay via a KB colour wire, to terminal 86 of the Fuel Pump relay, among other things, so, a dead Main relay would be a dead Fuel relay, dead ECU, etc etc.

The fact the pump is working, and cycling for the 2 seconds, means that the ECU is still controlling a fuel pump relay, but I know not how. Unless there is some strange wires in the area that have by-passed that relay for another hidden somewhere, NOT likely, but who knows. If there are NO strange wires there, forget this line of thought.

I have scanned the wiring diagram related to this area, and attached for you.

#14 = ECU, 3CU.
#16 = Main Relay
#17 = Fuel pump relay
#18 = Fuel Pump.

The pump MUST be running with the 85 unplugged, but how (see my thoughts below). Any EFI engine will stop instantly the pump stops, no doubt at all.

Thoughts, and there are MANY:

The relay has somehow linked itself internally, and age related, not human messing. I have seen this a few times with those older Lucas Relays.

Soooo, cheapest is replace that fuel pump relay, and see what changes.

IF nothing changes, then I suggest unplugging the Orange wire from Term 85, and tuck it aside, then fit a "new" wire from terminal 85 direct to earth, thus removing ECU control of that relay. CAUTION HERE, the fuel pump will/should run whenever the Ign is ON, so be aware of that PLEASE.

I am still thinking that the fuel pump relay is internally having issues.

 

Thanks, I have ordered a relay and will try that first, I was also thinking about connecting the relay terminal straight to the neg on the battery (great minds think alike!!) the only other thing the fuel relay does is to supply power to the contacts of the cold start relay, I may disconnect that wire as well if nothing changes to eliminate anything there. As I said the car is in a garage 1 hours drive away so probably wont be able to get there again until next Saturday, if you think of anything further please let me know.

Speak soon

 

Hi Grant

Did my weekly visit to the car this morning and not a lot of luck I'm afraid.

Changed the fuel pump relay still no change from before (Take wire 85 off and it runs all but unevenly, sometimes having trouble ticking over) put 85 back on the pump starts and the engine stops!!

Put wire 85 aside and connected relay terminal directly to the neg of the battery (Pump runs all the time the ignition is on) Won't start, take wire off and it starts.

When I say starts it does run but nowhere near right, there is a very strong smell of fuel from the exhaust and you can see a hint off black from the pipes.

The wire from terminal 87 to the cold start relay, still the same.

Even left relay fully connected and took off wire 87 to the pump still the same.

Almost as if there is to much fuel !!

 

PS got some video clips but cant attach them for some reason

 

Bugga.

Something is running that pump, it has to be, or there would be no fuel flow or pressure, to start the engine at all, rough or otherwise.

The smell and black sooty stuff is overfueling, and the fueling measure is controlled by the ECU, and the EFI Amp.

When its running, and running for a few seconds, you must be hearing the pump, or it will not sustain running. Without the pump, there MIGHT be a kick from the engine, but certainly NO running in the true sense at all.

That 3CU ECU now becomes suspect, but it does NOT answer why the engine runs with the pump relay dead, and stops running when the earth wire is attached. The running of the pump when the engine dies at connection is the built in circuits running the pump via that relay as it would if the engine was cranked and failed to start.

I cannot picture anything in that EFI loom that has been "patched" causing this, but maybe there is a cross feed of circuits, but that is FAR fetched in my head at the moment.

Something is running that pump, and the engine IS overfueling, sooooooo, ECU, or EFI loom, and I am thinking EFI loom, as you have had repairs of sorts to it recently. Once they start to fail, it is a domino thing, and patching them rarely sorts anything.

I am assuming the 2 FPR's are OK (they are purely mechanical and rarely fail),but the Injectors are being held open by loom issues, and this will overfuel a PreHE in seconds, AND soot the plugs badly, so much so, that removal and cleaning of the plugs will be required, as the PreHE will not "burn clean" a sooted spark plug.

1) Loom crossfeeding may be confusing the ECU, highly likely.
2) This confusion could affect the Orange wire (I doubt it, but ????).
3) With that Orange wire unplugged the ECU has NOTHING to do with the fuel pump circuit, that pump relay is DEAD, something else is running that pump, OR earthing that relay to run the pump.

I'll keep thinking, but am running out of logic real fast.

 

hopefully this will attach