Hi everyone. I purchased a 1990 XJS V12 Jaguar that had a valley burn and previous owner began disassembly. I am not familiar with these cars, but I consider myself pretty adaptable and reasonably intelligent. I really need lots of help!


I have been searching the net for info and the best I could find was here in a few of the threads of similar topic. My first choice was to just buy a parts car and graft the wiring and parts I need and not bother anyone else with my problems. But I am having a really hard time finding a parts car, so I figure I might as well see if I might be able to put it back together with your help, hopefully.


I have found most of the harnesses I need except the digital ignition harness. I think I will need to rebuild it and it is the most burnt of anything else. some of the feeds and plugs that went to devices are completely gone. Using the schematic provided in timslayers thread, I can reason some of the wiring out, but I'm still in the dark about a lot of it. A member in that thread (Gene?) talked about coax in the harness. I have 2 coax wires in the harness, 1 goes to the crank sensor, the other appeared to exit the harness around mid valley and I have no clue where it went. It just stopped there, burnt.


I'm really sorry I have to bother you all with another valley burn thread, but maybe with a lot of help, and a little guidance I may be able to put this car back together.

 

I'm still here. If you had a burn, you are looking at building both the ignition harness, the FI harness, and maybe some? of the body harness. I tried salvaging a harness, but finally gave up and built it from scratch. There are actually 5 coax on the 90 according to my schematic, so if you have only two you may have a very very early 90 with the harness left over from a 89. There is coax going to both front and rear crank sensors, as well as both amps, and one going to the Lucas ECU. If you have lots of patience, just start from the plug at the Marelli ECU and work your way out. Same for the FI harness. I find all of it to be a stress reliever, lol.

 

I have the 89, and the 90,91 electrical guides if you can tell me how to send them to you. I have them saved to my computer. Send me your email via a PM, and I'll forward them to you. You may also scroll through the stickies and find the 89 supplement which gives a pretty good description of the Marelli ignition.

 

Thanks a million for your response. Only 2 coax wires for sure. All wires to the amps are stranded. I didn't have to cut back far enough to see the flywheel sensor coax but I did see its connector near the firewall. I do have a new injector harness and some new harnesses for some of the other damage. The mystery is mostly going to be the ignition harness and some of the micro switch wiring and little nic nacs.


I think my biggest problem is someone else started the repairs and gave up. If I had gotten it directly after the burn, I don't think it would be this hard. I think there may be things missing? I don't see an ECU in the valley for the coax to connect to. Is that where it would be? Is there a picture of it out there anywhere? Also, I am going off the schematics from jagrepair dot com. They are a little hard for me to follow when it comes to that module. I just am not seeing it.


Thanks for your help. I really appreciate it.

 

I looked over the supplements for 89. Lots of great info. But if the ecu is on the pillar post (which I am guessing is in the interior of the car) then why would the coax be exiting the harness midway of the valley? That's where I'm lost.

 

I think that will be for connection to injection ecu. From memory male and female spade, mine is cable tied about halfway back on L/H side fuel rail.

 

If you have a 1989 car, your engine will be running the Marelli ignition. If it is a Lucas ignition, it will have a box about as big as a pack of cigarettes on the LHS inlet manifold top to the rear. If it is a Marelli car, there will be two king leads on the dizzy top instead of one.

So if it IS a Marelli car, it does not have a coax to the ECU from the Lucas box. The wires in the centre of the valley behind the throttle capstan come from the throttle position sensor and the water and air temp sensors, and possibly the flywheel position sensor, which may be coax, not sure. They go across the back of the engine at the firewall area to the US passenger side of the engine bay. Here they join a huge loom and eventually, via the passenger footwell and the US passenger side interior of the car, end up in the boot by the RHS flying buttress interior, where they clip into the ECU.

If you have a Lucas car, this same bundle of wires includes a coax from the box on the intake manifold. It takes the same path to the ECU. On a Marelli car, there are also two cables, maybe coax not sure, from the dizzy running forward to the amplifiers on the radtop panel.
Greg

 

Mine does Greg (assuming we are talking about the same thing)
I am using coax to mean shielded and on my marelli (connector shown centre of pic) this shielded wire runs from pin 24 on ignition ECU to pin 18 on injection ECU. My diagrams show the same layout for shielded from injection ECU pin 18 other than marelli connects to ignition ECU and lucas to amplifier.

 

I don't want to sound ever remotely disagreeable with Greg, but the Marelli does have a coax going to the Lucas ECU. The 89 harness has coax going to the front and rear crank sensors, and in 90, the amps are fed via coax as well. No matter which harness you have, there are at least 3 coax. The one going to the Lucas ECU was white on my car. The schematics show shielded wire for all of these, but its coax. The car I purchased had been messed with for months by a shop that couldn't get it running. They even had a company build a harness to replace the Marelli harness. This is where it got really confusing. The new harness had shielded twisted pair wire feeding the sensors. That won't work....ask me how I know, lol. The harness also had the polarity reversed on the rear crank sensor. That won't work either (now I know, lol). I called the guy that builds harnesses and talked with him, and he assured me it didn't matter. I assure all of you that he is flat out wrong! Thanks to John is Texas for closely checking his sensors, and seeing that they are clearly marked with a + sign on one side (if you look closely, I didn't). Anyway, I can only say that there is/was a lot of redundancy wiring that I was able to eliminate, and make the harness much smaller, neater. The harness that came to me was so full of redundant grounds, that made it very bulky. I realize that auto harness manufacturers have to do certain things to make sure it can be assembled in a more timely way. I built a custom harness to the car. I eliminated the test plug and wiring, as well as the fuel octane rating bypass? while I was at it. I couldn't find anyone who had ever even heard or seen of the equipment to test Marelli ignition, nor anyone who had ever switched the octane wire on their car, so.....
I say all of this to say, find the Marelli ECU. It is extremely accessible. It is behind the right side footwell kick panel. The harness passes through the firewall behind the ABS pump. Don't forget in all of this, that there is also a hard vacuum line inside the harness. This feeds to the MAP sensor built inside the Marelli ECU. Won't work without that either. I am a builder by trade. If I can build a wiring harness, trust me, anybody can. It wasn't expensive at all, just time, thought, heat shrink, solder gun, heat gun, and time. Best I remember, I bought all tools, wire, and all for under 250.00. The automotive coax is readily available on eBay. Make sure it is automotive, as it has a twisted strand inner core that allows for vibration over time.

 

Baxter, SC6
My apologies to all for the confused wording. I meant to say there is not a Lucas Amp shielded coax, but that there were others. Completely understand there are shielded wires used in the Marelli setup, but I foolishly thought the OP was confused about the absence of the Lucas amp to ECU coax.
Great stuff on the loom you built SC6. The simpler the better say I, and the less junk in the system the better.
Greg

 

Mine is missing that box. I wonder what else might be missing.


*sigh* I guess I'll cry myself to sleep tonight.

 

Cat, which "box" is it that's missing?
If you mean the small white one attached to fuel rail in my pic, that is just an insulator for spade connection. Heat shrink or tape do the same job.

 

When I look at your pic, it appears the coax leaves the harness, spades into a small box. That box appears to be missing on my car. When it spades out of the box, does it turn back into coax, or regular stranded wire?

 

Ahhhh...


Ok, then it must turn into a single stranded wire at that point, leaving the shield behind as a drain....

 

You are in a bit of luck. One of the forum members is about to do a production run of the EFI harness, which should make your life a little bit easier. Here is the thread:

https://www.jaguarforums.com/forum/x...arness-159220/

You might want to raise your hand and let him know you are in the market.

 

That is an interesting thread. I am happy to see someone doing that to help poor fools like me. Unfortunately, it's too late for me. I was lucky enough to find an injector harness in mint condition. I also found a new right forward harness and a new XJS engine feed harness. The only thing I could use at this point is a digital ignition harness. But there are none to be found, so I have to fix mine. Which brings me here to bug the hell out of all of you. Which I really appreciate.

 

To the right of the connector in the pic (which is actually black, just looks white in pic) the wire is shielded and that is part of the marelli loom. To the left it is sheathed in a fibre tube and also shielded according to wiring diagram, that is part of loom leaving out right hand rear of engine bay, maybe the one you say you have.

 

In your photo the airbox has an AJ6 Engineering sticker on it. I wonder what modifications from AJ6 are on the car?

 

I have no idea. The only modifications I can see were done by a fire.


But there has been a development in my quest to save the Jag. Parts for this car are damn near impossible to find. I have been running a few ads in Washington and Oregon for the last couple weeks looking for parts or a parts car. I got absolutely NO calls at all, until yesterday. I checked my voice mail and there was a message from a guy who owned a 1990 XJS. He said there was a place in Oregon (a little farm house) that had several Jags that he had bought parts from. He gave me the info to contact this place. He said there was a 1990 XJS out there with the wiring intact, and I should look into it.


I talked to the owner of this little "Jag ranch" and he invited me over to remove and purchase what ever I wanted from the car. I went out there today, and for an hour in the pouring rain, I got the parts of the harnesses I needed to graft onto my harness. What a score... And when I got them home and prepared them for the graft, all my questions were answered about the coax situation. It makes absolutely no sense the way it was done. But I will consider that my first brush, with the quirkiness of the Jaguar that everyone talks about.

 

Welp... I finally got to drive my Jag yesterday. She purrs like a kitten. Smooth driving. It was the first time I had ever driven one. I still have a few vacuum and small electrical details to finish. I also need to re-install AC and cruise control. But I just couldn't resist taking it for a spin. Thank you to all the people who helped me out with what I'm sure were the same annoying questions answered day after day. Thank you for taking the time to help me understand some of the details where I was completely lost.

She sure don't look the same as the picture above...