1984 XJS Injectors not firing

The braiding goes to ground. The connection to the ignition amp on my car from the ECU is completed with a bullet plug if you unplug it what you have is a direct line to the ECU plug on socket 18 in the boot just like a bit of wire between the two points on the multi meter easy …OR is it me that’s a bit dizzy ? clive

I’m a bit confused here…
Test bulb in trunk one wire to positive on battery one wire to pin 18… OK

Then you ground the shield of the coax cable at the amplifier plug
If the light turns on you have a short

Then you ground the coax cable at the amplifier plug
If the light turns on, all ok
If not, you have a break

Either you use a multimeter either a test lamp, not both at the same time.

Any coax wire with about the same dimensions will do.
I think it has to have a braided core.

No need to replace the cable all the way back to the ECU, usually it’s the part in the engine bay that gets fried and breaks.
Many videos available on how to splice and join coax wires.

No. That sounds fine.

Braiding shield to Gnd, centre conductor with continuity. :slight_smile:

When testing the center conductor, that gets +12V, test it with +12V.

  1. Disconnect each end of the conductor.

  2. Connect +12V to the either end of the conductor.

  3. At the other end of the conductor, connect test light from that end to ground.

  4. The light should light.

You just made two tests:

  • the conductor has continuity
  • the conductor is not shorted to ground (which is also good to know)

Remove test lamp and remove +12V to conductor.

  1. With each end of the coax cable (which has an inner connection inside center insulation, with an outer braided conductor, all covered by an outer sheath for protection … I know, everyone already knows this, but you will see why I described it) still disconnected

  2. Connect +12V to the outer braided conductor … no sparking, arcing, etc, shows that the outer covering is not damaged such that the outer braided conductor is unintentionally grounded along its route, if the outer covering and braided conductor is damaged, there may be shielding losses

  3. At the other end of the braided conductor connect test light from that to ground.

  4. The light should light.

The above tests indicate that the shielded cable is in usable condition.

  1. Reconnect cable when desired.

An outstanding comment and right on the money.
I did say at the start that lots of so called mechanics had been poking around on the engine.
It turns out on closer inspection that the wiring was incorrect and that was why there was no continuity to the ECU. This is a big learning curve for me but I will not give up. There are wires going nowhere and one from the ignition amp had been cut off unbelievable how it ever ran in the first place is beyond me.
So at the moment I am attempting to rewire the ignition amplifier if that turns out to be a failure I will have to buy another unit, when I replaced the module in the amp I thought that was it being new I was unaware of what wiring should or should not be present but I know now thanks to all you guys out there.
I will keep you updated … regards clive


Aristides, I keep thinking about this and going back to it.

Why would it need a “braided” center conductor (i.e., “core”)?

Most if not all, coax I’ve worked with and seen have had a solid conductor as the center conductor, and I don’t recall having even seen a stranded conductor as the center conductor in a coax.

Which is probably why it breaks.

With proper ends/connectors on the cables, breakage of the center conductor has not been an issue that I recall.

That said, yes, if you connect the soild center conductor to something shaking, and do not also secure the cable to that shaking object, allowing the center copper conductor itself to become the ‘flexible joint’ … expect that copper conductor to break at some point.

A stranded center conductor would last longer, and a braided center conductor … may become questionable as, while being more flexible than stranded, has much finer (smaller) strands, which may break easier, counteracting the gain in flexibility?

The obvious solution is to secure the cable end to the shaking object, allowing a free length of cable (doesn’t even have to be coax, same thing will apply to any copper conductor) to slightly move along its length, thereby reducing any bending fatigue.

From an engineering point of view it would be a very bad choice to put a solid core on something that shakes and moves all the time…
In our cars they break because of the heat, it’s the insulation that gets brittle and breaks.

There are plenty with stranded cores to chose from:


Very good. Now I’ve seen stranded core coax.

Did you find any braided core coax?

Does the insulation crumble/deform or just cracks/breaks from being brittle? At only 12V, center conductor to braided shield conductor, if the outer covering/sheath is intact, there likely is any leakage current between the two conductors.

If the insulation crumbles or deforms from excessive heat, such as melts/softens at a bend, then the center conductor may contact to outer conductor. But if our cars got that hot, we’d have melted insulation everywhere in the engine compartment (not referred to melted insulation at connections which is caused by poor connection and the resulting resistance creating heat).

My theory is that the insulation just cracks/breaks from being brittle, creates a week point that kinks the inner core, and eventually it breaks, or it frays and shorts with the outer shield.

It’s hot in there, but not so much as to start melting cables, we would indeed have melted insulation everywhere.

It takes a lot to kink coax if reasonably routed. Sure, if installed in a sharp bend, it may already be kinked (by the installer), and that would not be great, but at only 12V?

At 120V/240V, yes.

At over 600V, definitely.

At over 1,000V … it come out to meet you halfway.

OK you guys I now have continuity from the ignition amp to the ECU and I thought great job done. … not so, there is now a major problem … for instance there is no power to the injector’s or the plugs any guidance on which way to go would be helpful. I checked injector’s and plugs with an independent power and they work fine, I have also run checks on the coil that also works fine. HELP please regards Clive

Thank you for those directions regards clive.

The Ignition and EFi ignition feeds are separate, so it sounds like there is a common failure, which would be near the Ign switch.

Is there any power at Fuse #4?



I have checked all the fuses as I did not know which is which so best do the lot so what comes next the whole thing is a mine field fix one and you get another hopefully one day it will be one goes wrong get one free regards Clive

The question was, does F4 have power?
If you look at the power distribution diagram in my previous post, it shows the +12v IGN feed goes to the Digital ignition unit, the Idle relay, EFI main relay and F4.

F4 should be an easy place to measure the power from, if it is there it “should” be at the other items.

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Just follow the diagram Clive.

Does the inertia switch work?
Does the Main relay get power from the battery?
Does the Main relay get power from the Inertia switch?
Does the the Main relay work, i.e. does it give power to the Injectors?
Is there continuity between the the Main relay and the Injectors (+12v)?
Is there continuity between the Injectors and the ECU plug (ground)?

Electrickery is tricky and needs a methodical approach.
Good luck.

Undoubtedly I will need it buffet for a 1000 [people three tier wedding cake no problem, but this is a brand new game. Thanks for the sequence clive.

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