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I've been struggling with a fuel supply issue. When my Series III XJ12 is angry at me, and it seems to be often, the fuel cuts off on me as I'm driving.
My mechanic has struggled to find the root cause. I've searched the forum and other sources of info, and am stuck.
We set up a spare battery in the trunk and when the engine cuts off, I hook up the fuel pump directly to this spare battery, and off I go. Yesterday, maybe due to the fact that I started looking at other used XJ12's online, my XJ12 didn't want to leave my garage. It cut off after a few seconds of idle. With the spare battery hooked up directly, it idled fine for a long time until I turned it off. I no longer go on long drives, and I'm about to store it for the winter.
The last thing we're about to try is to swap in a different computer. The alternate computer is used of course, for early Series III XJ12's (red dot on it), and I don't know if it's any better than mine. Considering what I would pay, and the questionable condition, I'm hoping someone here will have some other suggestion.
If the computer seems the way to go, does anyone have a trustworthy source of XJ12 computers, or even upgrades if they exist? I'd rather spend more on a reliable upgrade computer, than spend on another used ticking time bomb.
Any ideas?
Thanks,
ROD
Last edited by ROD; Nov 2, 2019 at 06:07 PM.
Reason: Should have specified 1986 XJ12 in title, but can't add it now.
your car is a Series III V12 Vanden Plas, not an XJ12, a model that went out of production in 1982. OK, enough pedantry. Let's go to the simplest solution as being the most likely: on the radiator rail facing the engine there is a bar of 3 relays just to the passenger side of centre. The cylindrical red relay is for the electric engine fan. The other two Bosch Relays look almost identical: one is the "main" relay (normally furthest to the right) and it powers the fuel pump relay (normally the dentre relay). The main relay is distinguished, or was when new, by a red diagonal strip on the top; this relay and the fuel pump relay are similar except that one of the (the main relay, as I remember) has an 87a contact in addition to the 87 contact ( the circuitry is on the casing). Given that the fuel pump works to start the car, it is likely that the main relay is good, but I would begin by replacing these relays - you can easily get generic relays with the same circuitry.
The other possible cause of the engine failing to procede is a faulty ignition amplifier module. The ignition amplifier unit is mounted to the top of the driver's side manifold, square black unit, Lucas label. Inside there is a bog-standard GM/Delco ignition module which in the v12 is at the limit of its performance capabilities -but it works fine until age and heat take their toll. You can replace it yourself in 10 minutes. The module (Delco 1906) was used in virtually every GM car (and others) in the 1980's. Most auto supply places will have these on the shelf. Do NOT buy an aftermarket module! They often fail right out of the box; get either a GM branded or Delco branded unit.
These fail frequently on the V12 and the early sign of failure is normal starts when cold, failure to start when warm, or simply failing while running.
One of your coils may also be failing (there are 2 on your car) and it is possible to replace these oil filled coils with one of the smaller resin filled coils that Jaguar used in the later V12 VDP.
can you please report the outcome of your investigations in light of these suggestions?
We set up a spare battery in the trunk and when the engine cuts off, I hook up the fuel pump directly to this spare battery, and off I go. Yesterday, maybe due to the fact that I started looking at other used XJ12's online, my XJ12 didn't want to leave my garage. It cut off after a few seconds of idle. With the spare battery hooked up directly, it idled fine for a long time until I turned it off. I no longer go on long drives, and I'm about to store it for the winter.
In addition to Gregory's advice....
Locate the orange wire at the fuel pump relay. The ECU/computer grounds this wire to activate the pump relay. Apply a constant ground to this wire and see if your problem goes away. If so, your ECU/computer is indeed on the fritz.
In the grand scheme of things these ECUs are amazingly durable and reliable but one known fault, somewhat common, is with this ground function.
Awesome replies. Sorry, I was out on a last minute business trip. I need to ensure I keep my income, not to pay for food and shelter, but to make sure I can keep this Jaguar on the road. Based on all my notes and receipts:
Relays:
One of the first things we tackled. Four relays changed in 2018.
Inertia Switch:
Also changed in 2018, but I don't think we were able to get the proper Jaguar part if I remember correctly.
Ignition Coil:
Changed to one DAC6093 back in late 2014 when one of my coils spewed oil all over the place. It failed on the morning of my Jaguar Club's Concours. And guess who the Concours Chair was... me. I ended up going in a Japanese alternative.
Ignition Amplifier:
This I changed myself even longer ago, mid 2013. Based on research, probably on this site, I did get a good GM one as opposed to a generic one. I remember it came with a grease which was for heat sinking I think. ECC 55-1513 from Car quest. Time to do this again? Should I get the Delco 1906 instead of the GM module?
Computer Grounding:
I want to try the orange wire to direct ground trick, hopefully tomorrow.
Thanks for the quick replies.
Oh, about the Jaguar name... thanks for the lesson. I like the full description of Series III V12 Vanden Plas, but I thought XJ12 VDP was a more humble way to say it. Kind of like when someone introduces himself but instead of a getting the expected first name and maybe a last name, you hear a string of 5 names and he ends it all with "the third".
On the opposite side of this is when I got my first insurance card for this Jaguar. I was excited about finally having the Series III, V12, with the proper trim and colors which I had wanted to own for a long time, and was hoping to see something cool on the card... but my card just said "Jagu". Man, that sounds like the ugly cousin of a Yugo. And we all know Yugo isn't good looking to begin with.
Ignition Amplifier:
This I changed myself even longer ago, mid 2013. Based on research, probably on this site, I did get a good GM one as opposed to a generic one. I remember it came with a grease which was for heat sinking I think. ECC 55-1513 from Car quest. Time to do this again? Should I get the Delco 1906 instead of the GM module?
It's impossible to say how long these ignition modules will last. Replacing one before it fails is purely a peace-of-mind thing and, at only about $50, a fairly painless one. My comfort level, personally, is 10 years.
The AC Delco D1906 is the "GM Module". Without getting into the weeds regarding the relationship between General Motors and AC Delco, if you order this part from a GM dealer or an AC Delco vendor, you're getting the same part.
Computer Grounding:
I want to try the orange wire to direct ground trick, hopefully tomorrow.
Worth a try; let us know what happens!
Oh, about the Jaguar name... thanks for the lesson. I like the full description of Series III V12 Vanden Plas, but I thought XJ12 VDP was a more humble way to say it. Kind of like when someone introduces himself but instead of a getting the expected first name and maybe a last name, you hear a string of 5 names and he ends it all with "the third".
Tricky business, Jaguar model designations.
Interestingly (well, to some) Jaguar dragged the "XJ12" model designation out of the closet,dusted it off, and put it back into use a few years later
Ignition Amplifier:
"This I changed myself even longer ago, mid 2013. Based on research, probably on this site, I did get a good GM one as opposed to a generic one. I remember it came with a grease which was for heat sinking I think. ECC 55-1513 from Car quest. Time to do this again? Should I get the Delco 1906 instead of the GM module"?
The last Delco 1906 unit I purchased was from Amazon for $30... Chain parts store have ""equivalents"" on shelf, but can order the Delco unit for you. O'Reily said next day by 10:00 am. Don't remmeber price, but It was higher than Amazon. I was not in hurry, as this was a spare (if you drive long distances on weekends, this is a good glove box spare part to carry).
I've been getting help from you guys recently on my 80 XJ6 for the same problem
I found that power stopped to the fuel pump when I let go of the key, so sticking a lawn mower battery in the trunk let her get a nice long run time.
However since it was running I had the chance to get it off the lift and into another bay.
Once my other jobs are done and the lift is open again I'll begin the trouble shooting again.
I'll be following this thread for sure to see if you find a smoking gun I can use.
Bobmo
Thanks Doug,
I've noticed a few items that dont exist on my 6 in the thread so far.
But my next adventure will be my 87 XJSC so the V12 info will come in handy there.
Bobmo
Ok, so my Engine bay won't impress anyone at a Concours… It looks like I have nothing stock (no cylindrical red relay, and no red diagonal strip).
So in my case, the second relay from the right (when looking from inside the engine towards the radiator) has an orange wire at the 85 pin. Doug, is that the one you meant?
I'll try to ground that directly next time but first I will buy an ignition module and replace it.
Today the temp was more mild so I got my new ignition module in.
Before doing any new work, as always, I got my baseline by trying to start the engine. I didn't want to attribute a temporary sign of cooperation to the module. Fuel pump did NOT kick in, and the Engine was dry so no start.
I changed the ignition module. I then turned the key two clicks. I heard the fuel pump kick in!... but it sounded like it was coughing. What I mean is that it was on for about a second, then off for half a second, back on, back off at a few irregular but very short time spans... and by the end of about 3 seconds total, it stayed off. I started the engine and it ran a couple of seconds with whatever fuel had gotten pressurized, and then shut off of course because the pump never came back on.
I double checked my work and I had forgotten to attach the black plug into the module case. I plugged it in, but this didn't help anything. Two clicks did NOT engage the fuel pump. I did find one of the white bullet plugs was loose. I disconnected it, squeezed the female side it a bit, and reconnected. Again, I thought I was onto something because the two clicks of the key seemed to cause some noises from the ignition module and the pump showed a bit of signs of life... but it didn't last.
Could I have fried the ignition module again, or should I assume that I still have a new and functioning module regardless of starting a few times with one plug not on, and the other slightly loose?
Egads, I meant to come back to this thread but it totally slipped my mind
Originally Posted by ROD
Ok, so my Engine bay won't impress anyone at a Concours… It looks like I have nothing stock (no cylindrical red relay, and no red diagonal strip).
So in my case, the second relay from the right (when looking from inside the engine towards the radiator) has an orange wire at the 85 pin. Doug, is that the one you meant?
Yes, that looks like it.
The pump relay has pink/black, brown, brown/slate, and orange wires. Sometimes the colors fade a bit but the orange wire is clearly identifiable. That's the one you'll want to ground
Today the temp was more mild so I got my new ignition module in.
Before doing any new work, as always, I got my baseline by trying to start the engine. I didn't want to attribute a temporary sign of cooperation to the module. Fuel pump did NOT kick in, and the Engine was dry so no start.
I changed the ignition module. I then turned the key two clicks. I heard the fuel pump kick in!... but it sounded like it was coughing. What I mean is that it was on for about a second, then off for half a second, back on, back off at a few irregular but very short time spans... and by the end of about 3 seconds total, it stayed off. I started the engine and it ran a couple of seconds with whatever fuel had gotten pressurized, and then shut off of course because the pump never came back on.
The engine did start, so we can assume it has spark....and thus assume the ignition system is at least fundamentally operational.
From your first posting we know the pump itself is operational.
The pump should run without sputtering for about 3 seconds whenever the key is turned 'on'. Since it doesn't, fix that problem first and then move on if needed
This is where grounding the orange wire would come in.
If the the engine starts and runs, and remains running, with the orange wired grounded then you know the the fuel pump circuit in the ECU is faulty. You get a different ECU (with some searching) or have yours repaired (which takes time) or leave the orange wire grounded and drive the car. This last choice is probably rather common but not fully desirable, as a fuel pump that runs continuously simply by having the key 'on' isn't optimal from a safety standpoint
I double checked my work and I had forgotten to attach the black plug into the module case. I plugged it in, but this didn't help anything. Two clicks did NOT engage the fuel pump. I did find one of the white bullet plugs was loose. I disconnected it, squeezed the female side it a bit, and reconnected. Again, I thought I was onto something because the two clicks of the key seemed to cause some noises from the ignition module and the pump showed a bit of signs of life... but it didn't last.
Could I have fried the ignition module again, or should I assume that I still have a new and functioning module regardless of starting a few times with one plug not on, and the other slightly loose?
Thanks,
ROD
I sorta doubt that you damaged the module. It is at least functional enough to provide ignition to start the engine briefly, as you describe above. I'd go back to the pump circuit and make it operational first.
Fuel delivery issue... Engine Computer?
