Well you know how the story goes…you go out one Sunday afternoon and a 1985 (non running) Jaguar XJS follows you home. You tell the wife, it’s no biggie, I can get it running, then you sleep on the couch for a week. I have read the book, and reviewed the Haynes manual, and I know it’s electrical. Replaced Relays and ECU. The fuel pump will only get 12 volts to power when the starter is engaged. I’m at my wits end here, trying to track this down. Obviously, with power only happening to the pump while cranking, she tries to start but not enough pressure. Really don’t want to jump relays to “short” the connection between the pump and relay, does anyone else have an idea on where to track down the connections. Is there any other relay other than the 2 in the trunk I should be looking for?
Well you know how the story goes…you go out one Sunday afternoon
and a 1985 (non running) Jaguar XJS follows you home.
I hate it when that happens!
The
fuel pump will only get 12 volts to power when the starter is engaged.
What’s the status of the impact switch in the driver’s side footwell?
– Kirbert
I looked for that, but being new to Jaguar, didn’t find the switch…Haynes manual says Passenger side, your post says Drivers side. just took a peak and don’t see it on either, but we know it’s there right? Driver side shows fuse panel, then above that a few connectors and relay. Passenger side reveals smaller fuse panel. Also looks like DPO somewhere added something as I have a wire held in with a fuse (will have to track that down later). Any suggestions on where to track down the switch?
Thanks for the help BTW. I have worked on MGB’s for the past 20+ years, so British cars are all the same right? 
So just a little update, I went ahead and bypassed the relay on the fuel pump with a jumper on pins 86 and both 87, and still the same symptoms, crank, sputters but not running but I do hear the fuel pump running now. I will try again tomorrow and post further updates. I believe it’s something to do with a switch or relay somewhere else as when I jumped the relays, the _fuel pump came on with out the key in the ignition turned _, which to me, means another switch/relay is at fault. quoting the book…“If you’d like to test this circuit to determine if this is your problem, simply remove the fuel pump relay in the trunk (the one without a red mark on it) and put a jumper into the socket connecting terminals 86 and both 87’s. This will allow the pump to run whenever the ignition is on”. How about if the ignition is off? Anyone else experience this?
Mike,
I don’t know if your car is a 6 or a V12, but at least at my 1986 XJ V12 SIII:
If I jump the fuel relay the pump will run regardless the ignition position.
If the impact switch is located at the right side between the fascia and the passenger door, and if energized, the fuel pump and starter are disabled, i.e. you can’t start the car.
The fuel pump is controlled by the ECU, so maybe there lays the problem ?
A test with a known working ECU would clarify things…
Hope that helps,
Aristides
…still the same
symptoms, crank, sputters but not running but I do hear the fuel pump
running now.
Have you checked the dreaded skinny white coax wire from the ign amp
rearward and across the rear of the engine?
“This will allow
the pump to run whenever the ignition is on”. How about if the
ignition is off?
So you’re saying you jumped it as described and the pump runs with ignition
off? That’s bad. That means there’s a short somewhere, I think. Are you
sure you jumped the correct terminals? Because there’s another pair of
terminals in there that will run the pump constantly.
– Kirbert
@Kirbert, yes. I jumped pins 86 to 87(a) and 87(b) and the pump comes on before the ignition switch in energized. I will take a look at the dreaded ignition wire hopefully this afternoon or the next day. Perhaps go as far as posting a video for those smarter than me (99% of the population! 
@Aristides, thanks for the reassurance, yes the vehicle is 1985 V12. However what you say and what @Kirbert says points out a discrepancy. The plan of course is to not keep the jumpers in place, but get the vehicle back to normal operating conditions, aka at least running!
I bought this because it has 45K miles, fully documented over the years, and the PO was smart enough to have it taken to Ziebart for rust proofing, thus the underside is incredible clean. There is a few spots of surface rust, but nothing major. Hopefully a little engine work to at least let me wife know that “that thing” in the driveway can and will come back to life for the summer of 2018.
more details to follow…
Does anyone have a recommendation on where to purchase a ECU that can be trusted, or know of someone who rebuilds them?
The answer is simple, Roger Bywater:
Steve
Maybe all this time I was jumping the wrong relay (main relay instead of fuel pump relay) ?
Quite possible as they are side by side, will have to check this out.
Aristides
I’m still thinking you jumped the wrong terminals. 87 and 87a are one contact terminal, but 86 is a coil terminal. If you merely connected the two contact terminals – 87 and 87a to 30/51 – the pump would run forever.
Just a tad update for future reference for those with the same issue. Turns out after replacing the relays, I decided to put a known good ECU in that I got from a reputable supplier and with fingers crossed, turned the key over and no luck…same symptoms. No fuel pump, no go…lucky for me someone had installed a battery shut off switch in the boot, so I put the key in, and went back to energized the system to listen to the relays… Then by chance i hit the plethora of cables going into the ECU and guess what kicked on? Turns out either a short or crappy connection as the little car now runs. Thanks for all the help and where to look. Will start tracing the connection tomorrow