Hello Fellow XJS, HE V12 Lovers
After ideas of intermittent power loss to the fuel pump ,?
On initial ignition key switch on ,fuel pump primes up ok ,engine starts and runs ok
Can drive the vehicle for a while and then intermittently ,engine has a stutter on acceleration and will lose accelerator pedal response for several minutes and then will operate correct
Currently searching for a wiring diagram suit 1986 HE XJS V12 ,as this vehicle has been in storage for a while and i know the area it come from and it is bad for those pesky little rodents that dine on fine English wire
I do know that when fault occurs have no power at positive wire on fuel pump and have power getting to f/p relay (currently searching for both relays)
Fuel pump test ok when ran with independent power supply
Cheers
PMD55 (AUS member)
Those symptoms often due to crud blocking the fuel surge tank, or overheating ignition amp on the inlet manifold. Although they do not recover in a few minutes as you report.
However, you mention no power to fuel pump and pump relay. That could be faulty pump relay or no relay drive from the ECU, both problems that turn up on this forum quite regularly. Not sure how you monitor the supply to the pump in the ( short ? ) interval when the stutter happens.
A stutter that appears to be mostly an acceleration issue can be the TPS under the throttle capstan, a notorious lifetime limited item.
Hello Richard
Have drained and cleaned surge tank as it was very dirty ,also installed brand new ignition Amplifier after several attempts at replacing condensers and ignition modules in original Amplifier
My thoughts also are leaning towards the two relays associated with the fuel pump operation as it does seem to be a heat related problem (and there was power at the relay plug when the fault last occurred ,but i could not measure it as only had a test light probe with me
As mentioned am trying to locate wiring diagram to see how the system runs power
through out circuit
(note - there is a repair tag on the ECU ,and the vehicle was recently bought at an auction)
At this stage no luck in finding new suitable relays or diagram
Cheers for your thoughts
PMD55 (AUS member)
Here is page from XJS 1989 MY manual.
A bit compact to easily read, might help.
I suspect 1986 fuel pump circuit the same.
There may be other forum members with correct diagrams for your car.
Phillip,
The wiring diagrams can be found in the Jaguar XJ-S Repair Operations Manual and the Jaguar S57 Electrical Guide for your model year.
Paul
Cheers Richard
Phillip (PMD55)
Hello Paul
Have been trying to buy Repair manual in with no luck too date
car with issue is an 86 ,I also own an 83 XJS ,HE V12
Cheers for replies
Phill (PMD55)
Phill,
Search eBay for “Jaguar XJ-S Repair Operations Manual” and you will find both new and used ones. If not, wait a week and search again or better yet put in a saved search. They are usually there.
Paul
Will do ,actually had a set promised to me from a fellow but he has misplaced them
during a house move.
Cheers
Phill (PMD55)
Phill, if you’ll send me your email via PM I can send you the S57 wiring guide
Cheers
DD
Phillip, some good info to be found at jagrepair.com. Won’t solve all your questions, but can add to your library
May I ask how old your fuel pump is? They can go bad within 20 years. Even sooner if they’ve been run dry. Intermittent fuel pump usually means bad fuel pump.
If this is original fuel pump, I’d replace fuel pump.
You did mention no power to fuel pump when it’s intermittent? How did you test for that?
Hello Greg
As far as I know the pump may be original
The issue is more the power supply to the pri have worked out
Appreciate your thoughts
Cheers 
Phillip D’Arro
Your issue sounds like it may be the fuel pump relay has burned contacts, or a poor socket connection and is failing to deliver enough current to keep the pump running.
Monitor the power at the pump and relay to be sure.
It’s interesting that the O2 sensor heaters are also fed by the FPR.
Don’t forget to check Gnd is being supplied from Pin 15 of the Ecu as that is the FPR control…
Only on this forum would a poster ask for a wiring diagram and be told to buy a manual…
Can someone please host the wiring diagrams for all to download as required.
Thanks to Richard Dowling for posting up the EFI diag.
I would appreciate a My 1989 ignition circuit diagram for a V12 Cab.
I have sourced one online, but it isn’t a genuine item, though should suffice.
Without a tacho output the injection Ecu stops delivering fuel, as it uses the tacho input to know engine speed, thus basic fuel requirement.
It would be worth ensuring the ignition amplifier has a good Gnd, preferably made through some heat conductive grease.
Thank you
Diesel man
I am thinking alone the same area as I did find when the fault occurred that I had no power at the positive wire at the f/ pump
Cheers
Phillip D’Arro
Battery is in the trunk, fuel pump also, what could be easier to test than this?
Rig up a temporary supply with maybe a 20 amp fuse to the fuel pump and let it idle, if good go for a drive. No more guessing.
You have just ruled out power to the pump and pump itself.
Electric fuel pump is at least 35 years old? My money is on a bad fuel pump. They only last so long.
When the car loses power there is no power being supplied to the pump.
Firing the parts canon isn’t the way to diagnose a fault.
That’s when you trace an electrical issue back to the source.
In the case, the fuel pump and source are both physically and electrically close, with one exception (as I recall), that being from the fuel pump relay to ignition and back, while taking the scenic route over to the fuse block an inertia switch on the way.
Simplistically speaking:
- Battery to ignition switch
- Battery to fuel pump relay
- Ignition switch to fuel pump relay via fuse block and inertia switch
- Fuel pump relay to fuel pump
- Least we forget: fuel pump to ground
Please correct where I made an incorrect “simplistic” path.
And as the O/p suspects, Fuel pump relay Gnd to ECU.
The behavior indicates burned relay contacts overheating in use.