1984 XJS Injectors not firing

I have a 1984 XJS with 69,870 miles on it. Last year, I lost the pulse to all the fuel injectors. I replaced my original ECU (DAC 2597 / 6 cu fuel control unit) with my friends ECU from his 1986 XJS (DAC 4118 / the upgraded ECU with 16 cu fuel control unit). I also swapped out the original 1984 Fuel Interface Relay with his 1986 relay. This instantly solved the problem, and the car ran perfectly for 8 months. I now have the same problem again. The car starts right up and runs fine for a few seconds using starting fluid. I have good spark, no timing issues, and good fuel pressure (cleaned both tanks and replaced all filters over a year ago). I secured a second used 16 cu ECU from a parts yard (used, not rebuilt or reconditioned), and still have no injector pulse. Everything else is functioning, and the car starts right up and runs with starting fluid. Is it possible that my replacement 30+ year old ECU’s may need to be repaired or reconditioned? We have done no recent work on the car, so no wiring, relays, or connectors have been disturbed. Any thoughts or ideas would be greatly appreciated.

Do you know about the small white shielded wire that goes from the Lucas ignition amplifier back to the EFI ECU to provide the pulse to fire the fuel injectors. This is a regular source of problems and lots of posts about it in the archives. The wire has a conductor core, then an insulating covering, then a shielded braid around it to protect the signal. It the wire breaks, if the grounded shield gets broken, or if the insulation breaks and the conductor core contacts the shield then the signal from the ignition amplifier to the EFI ECU is lost and tge fuel injectors won’t fire. Since the wire runs in the spark plug valley for a while it sees some pretty high temps.

In order to see if this is your problem you will need a multi-meter and verify that you have continuity between the wire as it leaves the ignition amplifier until it reaches the pin in the EFI ECU connector and also that you do not have continuity between that pin and ground. If you find that you have a broken wire or continuity between the core and ground, then you have found the reason that the injectors are not firing.

There should be a lot of other helpful info in the archives as this problem comes up regularly. It is also covered in Kirby’s Palm’s book.



Thank you for your quick reply and the information. I have heard about this “wire” problem on the V-12, and it sounds like a good place to start. I believe the ignition amplifier on my car (1984 XJS) is located on the top left side of the engine compartment. I am not sure where the EFI / ECU connector is located in the engine compartment, but will research the topic in the archives as you recommend. I will also check out the availability of Kirby Palm’s book. Is this a single stand alone wire or is it part of a harness? I hope a suitable replacement wire is available since this is a common problem. My car is in a storage garage in town, so I will check all this out tomorrow. Thanks again for your time and assistance.

Jim Foxgrover

Oak Harbor, WA

The Lucas ignition amplifier is a square shaped black metal box that sits on the top of the left side (B bank) of your intake manifold.

I have not had a problem with that white shielded wire in either of my two V12 Jaguars, but others on this list have and they have posted about how they did the repairs, so.the archives should be a great place to start. I doubt that a replacement harness is available new but others might know of a source. I think those that had this problem repaired the wires, but because it is a shielded cable it is not a simple repair.

You can find Kirby Palm’s “Experience in a Book” on Jag-Lovers and download it for free.

Your EFI ECU is probably located in the same place that the one in my wife’s 1990 XJ-S convertible is. It is over the right rear wheel well with access through the right side of the trunk. You will need to remove the carpeted trim panels to find it. You will need to remove the large connector in order access the pins to test the wires.

I was a pilot in the U.S. Navy and had some short visits to NAS Whidbey Island in the 70s-80s. A very beautiful area.

Good luck with your car.


Just to add to what Paul said.

If the dreaded white wire is bad then the injectors will fire a priming pulse when the ignition is turned on this is independent of the tach signal (white wire).

Most likely the issue will be the connector at the ignition amp. Check this first as it will be an easy fix.

If the wire is bad it can be replaced with any coax, RG6 is cheap and will work fine. If you replace the whole wire make sure the wire is grounded at ONE END ONLY. does not matter which end.

Just in case you were wondering, the shielded wire from the ignition amplifier goes to pin #18 on the EFI ECU connector.


1 Like

Paul, Thanks for the EFI / ECU pin information. I plan to spend some time with the XJS over the next couple of days. Do you happen to know if there is a diagram or picture that shows the pin identification of the EFI / ECU? I do not see one in Kirby Palm’s book. I thought it may be marked (or printed on a label) on the unit itself, but don’t see anything on either the large plug or the ECU. Thanks again for your help.

Warren, Thanks for the suggestion and the information. I plan to spend some time with my XJS over the next couple of days. I have down-loaded Kirby Palm’s XJS book, and was looking for a diagram or picture that identified the pins in the ECU / EFI unit. This would be a big help if I need to do any of the additional testing recommended in Kirby’s book (like checking the “white wire”). Thanks again for your time and assistance.

All the pinouts on the ECU are in the electrical guides and ROM. If you don’t have a copy get one it’s invaluable for working on the car.

Jim, You can do a Google search for “V12 Jaguar pinout” and it will come up with all the info you need. You will have to sort out the photos to make sure they are correct for your year model. In the interim…If you were to remove the ECU, hold it in your hand with the pins facing you, AND the three mounting lugs down.(two of the mounts facing you and one in the rear of the ECU), there will be 18 pins on the upper row and 17 pins on the lower row. The numbering starts at the top row left side with pin # 1 and reads to the right to pin # 18. The lower row starts with pin # 19 at the lower left and reads to the right to pin # 35. Cheers.

SD Faircloth

My recommendation is to spend the $90 bucks and order the manual from http://www.store.otpubs.com/brands/Jaguar.html. Great resource. I’ve also have the 4 volume OE manual for the XJ-S and I compared it to the book, and it is the original, and not the abridged version, which in my humble opinion is useless. For Example: Alternator replacement.
remove alternator:

  1. Slacken the belt,
  2. remove the fasteners
  3. disconnect the electrical connections
  4. remove alternator

Install alternator:
See remove alternator and do in reverse order. Seriously? thank you for that captain obvious…

So what’s missing? Well for starters, Which way out through under the car or out through the top?
short answer - out the bottom.

My point is get the OTPubs.com variant. it has all of the good stuff and more.


Thank you for the info. I inspected and cleaned all the under-hood plugs and contacts today as recommended on the site. I also unplugged the shielded white wire from the amplifier, and did the “continuity ground test” at Pin #18 of the ECU plug back in the trunk. As it turns out, the shielded wire is grounding out somewhere. I plan to replace the section in the engine compartment. Interesting since my car (and the wiring) appear to be in excellent condition. Thanks again.

Jim Foxgrover

That stinkin’ little coax wire strikes again!

Finally had a chance to spend some quality time with my 1984 XJS today. I had already checked Pin 18 at the ECU connector in the trunk (after unplugging the white wire at the amplifier), and did have ground continuity (a short somewhere in the circuit / wire). Usually this points to the shielded white wire up front.

Today I isolated the shielded white wire from the harness all the way over to the right fender. All the wires were in nice shape with no sign of heat damage or deterioration. I cut the white wire over by the right fender, carefully stripped the end of the shielded wire going back to the ECU (rear of the car), checked for ground continuity, and everything was fine (no continuity - no short). I then checked Pin 18 at the ECU connector in the trunk, and still had ground continuity (even with the white wire not connected to anything up front).

After this, I checked the PLUG on the white wire that is only about 2" from the Ignition Amplifier (it plugs into the white shielded wire), and had ground continuity at the Amplifier Plug.

So … I have ground continuity at Pin 18 (with the wire disconnected up front), ground continuity at the shielded wire plug (on the Amplifier side), but no ground continuity on the shielded wire over by the right fender. All the other wires (and plugs / ground) going to the Ignition Amplifier have been cleaned and check.
Is it possible the Ignition Amplifier has an internal short? Why would I get ground continuity at back in the trunk at Pin 18 (with the white wire disconnected), but not have ground continuity on the cut white wire up at the right front fender (which is what I would expect if there was no short in the shielded wire)? Is there a junction box (or connectors) up front on the right side (back towards the fire wall) the white wire runs through that may have an issue?

Any thoughts or ideas would be greatly appreciated. Thanks for all the help and suggestions.

No one brought that information to the discussion thanks BETTER LATE THAN NEVER MAY 2024

You probably know by now that I am a Brit and as such need things explained in detail so here we go.
To check continuity from the Lucas amplifier to the ECU unit please explain one of the probes on the multi meter goes to pin 18 on the ECU plug which one goes to the pin and where does the other one go. What settings do I put the meter at and what am I looking for and what does it tell me. … Even when you have explained the procedure I’m sure I will need more detail we are not verry bright over here. Kindest Regards to all Clive

To measure resistance use the Ohms range on your meter. It probably has a continuity setting whereby it bleeps when the two test leads have continuity.

One lead should be at the unplugged Ecu connector, pin18, the other should be on the also disconnected white wire at the ignition amplifier. If the wire has continuity it the meter should read 0 Ohms and may bleep.

It’s probable that your meter leads aren’t long enough to reach both ends of the white wire, so try this trick.
Connect a test lamp to batt +ve and Pin 18 of the Ecu connector… Go to the ignition amplifier and with your meter, or another test lamp, with the negative lead connected to Gnd (bare metal), connect to the white wire.
If there is continuity the lamp will light, or the test meter will display +12v. If the wire is broken, there won’t be any voltage present.

1 Like

If you are just testing for continuity the polarity is irrelevant, as Diesel states your DVM (digital volt metre) should produce a beep when set to the ohms setting, if the internal battery is good.

We have no idea what nationality you are or where you live since this is an international list. You can put that information in your profile or provide it in your emails if you wish to. Thanks for providing that information.


He wants to test continuity of a wire and appears to be able to understand English.

He did say he is a Brit. I took that to mean he is British. :wink: