Grant, didn't you have a write-up on dealing with that dreaded shielded wire that goes to Pin 18 of the ECU? if so, can you post it again?

My V12 project is at the point where it should start, but won't. No injector pulse. The shielded wire is on my checklist and I wanna make sure I have my head wrapped around things properly.

As I recall you've simply cut back any broken wire, soldered-in new, and eliminated the shielding. Is that correct?

Cheers and thanks
Doug

 

Hello Doug.

Dont remember a write up specifically, but probably part of other write ups.

Originally I stripped the coax in the engine bay, all the way to the rear of the brake booster (RHD), and replaced it with standard TV coax, coz I knew bugga all in those days.

Later, I removed the amp, and mounted it out the front, near the 2nd coil. I ran the wires up the RH inner guard channel, and used standard wire, and spliced into the thin centre core of the factory coax inside the panel near the RH door hinges (XJ-S). Soldered and shrink wrap sealed. I made very sure the earth wrapping coax was stripped waaaay back, so it had no way of shorting that centre wire.

Mine had that coax outer earthed AT the ECU, some are earthed at the amp. It matters not, as long as it is earthed at ONE end only.

Since you are doing what you are with an XJ6 car, I would simply run a decent standard wire from the amp to the #18 pin, and dont worry about the shielding.

My understanding is that the coax was to prevent HT signals from the 6A and 6B spark plugs messing with the pulse signal, was the main reason for it. If you keep that standard wire away from that area as best is possible, I see no issues at all.

Hope that makes sense????.

 

It wouldn't matter which coax you used as long as its only grounded at ONE end.

 

What is this wire for?

 

It is the ignition pulse signal wire for the V12 HE P Digital EFI system.

Without that pulse getting to pin #18 of the ECU, the engine is DEAD.

 

Is it a Lucas OPUS cable or on both OPUS and Marelli engines? And where does it run? Just so I know what to look for

 

Its only on the HE V12 with the Lucas igniton system, and the "Black Box" bolted to the LH inlet manifold.

It is inside the wiring loom at the rear of the engine bay.

Marelli cars do not have that wire.

 

Phew! 'Luckily' mine is a Marelli engine... 'Luckily' bä

 

Grant I believe that wire does still exist on marelli cars.(at least early digital ignition cars) It is certainly present on my '89 xjrs but rather than connect to the amp on L/H intake the shielded wire loops back on itself and connects to the ignition harness by male/female spade which is cable tied to fuel rail.

 

I transplanted an entire V12 harness into the car, so I have the shielded wiring whether I like it or not

I really should've given this wire my attention before I installed the harness. Sometimes I'm not very bright !

As for the no injector pulse I've gotten as far as making sure the injectors have 12v supply and that all F.I. ground wires are grounded before running out of time. It'll be a few days before I can do more....but the 'dreaded wire' is high on my list

Thanks for the help

Cheers
DD

 

Doug if you have a Scope, test for a signal at the ECU when cranking, use the ECU case as ground NOT the shield. Also use a DVM on AC volts and measure the shield both ends to the ECU case and centre pin it should be 0volts, any AC voltage here could be an indication of noise being induced into the wire messing up the signal.

 

Just had another thought, You could run a bit of coax outside the car bypassing the loom coax. If you have injector pulses then the dreaded wire is at fault.

 

Is the case grounded via the bundle of ground wires....or should it be grounded at the actual mounting points?


Will do, and thanks

One more question to clarify things in my head:

Should shielded wire be grounded to the ECU case itself? On my harness the shield terminates with an eyelet which I simply attached to a convenient ground point. Seemed logical, but I should've taken better notes when I was removing the harness from the donor car.

Cheers
DD

 

The Marelli cars definitely have the shielded wire. It is what gives the Lucas ECU the EFI signal

 

What happened to my Series III v12 was the plastic got heat cooked and brittle near the amp on the engine. The wire is a multi layer affair: signal wire in the center, a layer of PVC, shielding braid, then more PVC.

As the PVC gets brittle and cracks, the shielding wires can touch the center signal wire, shorting the signal.

I cut it back until I reached flexible wire again and soldered on a new piece of wire to the signal wire and ran it to the amp. I think I left that piece unshielded, and it ran fine.

it's not essential that the shield should ground to the ECU case. There is nothing going through the shield, it's just protecting the signal wire. If it was a common sensor ground then yes.

Did you check upstream too? Is the amp connected correctly, as well as the distributor? If there is no pulse from the star wheel and pickup, there will be nothing from the amp either.

 

I just barely began checking things before running out of time. After voltage supply to the injectors the 'dreaded wire' was my first thought.

Never had a problem with it on my XJS V12 so I never learned much about it. Must've been lucky for those years

I'll keep everyone posted

Cheers
DD

 

The ECU case should be grounded to the chassis, the shield can be connected to the chassis as well, but only at one end, doesn't matter which end, but I would ground at ECU end. If its grounded at both ends current will flow in the shield and can cause erratic issues.



Any ground point should do

That'll work

 

Yes it does it goes between the Marelli and the Lucas. The Marelli ECU provides the tach signal to the Lucas ECU.

 

So where does the Marelli wire run then? The ECU is in the boot right? I assume the wire is there too!?

 

From engine loom back to EFI ECU in the boot is same for lucas and marelli. The differences are the lucas simply has the shielded wire hooked up to the amp on the L/H manifold while on the marelli car it connects to a similar shielded wire which runs to pin 24 of the ignition ECU in the footwell inside.
You can probably find the insulated male/female spade connection tied to L/H side of fuel rail near throttle pedestal/coil assembly.