I'm into the midst of replacing all the cabling that once sat down in the v...here are some photos of one of the harnesses I pulled off along with some questions.

The picture titled "shielding" shows a single wire that has a shield running most of the length of it. Is this shielding to protect the ECU or other critical parts of the car from interference, or just to lower the static on AM radio channels (it's a 1987, when maybe people still listened to AM). I'm trying to figure out if I need to duplicate this shielding. It will make the rewire more work, but if it needs it it needs it.

The picture "tied off" shows four wires connected somewhere in the harness. Just seems like an odd place to do it. Why not have just daisy-chained whatever these go to?

"What is this" is self-explanatory.

"New instruments" shows what I plan to replace my gauges (except for speedo and tach) with. I plan to monitor, independently, air/fuel mixture, coolant temperature, and (if I can find a probe up to the job) cat converter temperature for each bank individually. Obviously fuel and oil pressure and voltage will require just one bar display each. Not seen behind this pair are the two IC's and 4 resistors that make it all work.

I'm a visual learner and need pictures to understand things. In fact, I'm tempted to create "The Photographic Atlas of the Jaguar XJS", that will have an individual photo of every sensor, connector and component in the engine compartment to help others who are new to this sort of thing (as am I) to figure out what is what.

Thanks,

John
1987 XJS V12, 62,000 miles

 

ah yes...the infamous shielded wire! That buggar had me pulling my hair out for awhile! It is shielded because it is sensitive to the magnetic field created by the coil and plug wires. It is what tells the ECU when to fire injectors and controls the spark. You can save yourself a lot of agony by moving the ignition control box that is mounted atop the motor and placing it elsewhere. I put mine in front of the radiator. Then you can reroute that wire away from motor, thus eliminating the need for shielding. You just have to attach the black wire to the wire mesh shielding.

 

Your 2nd pic is pretty typical wiring practice by Lucas. I don't know why they did it, but you'll find a lot of it. When I rebuilt my harness I obviously deleted them and made a much cleaner set.

I'm having intercranial flatulence at the moment on the 3rd pic. I know it is necessary, but can't recall what it was for? Others will jump in to make up for my lack of recall I'm sure.

I'm not a big fan of LED indicator lights myself, I prefer good old needle gauges, but to each their own!

 

The third pic is either the full throttle vacuum switch, or a similar one which is part of the vac-tube system. I think from the metal fitting, it is NOT the full throttle one, rather, it is the one that is bolted at the front of the A bank side of the engine and which is part of the vac tubing cat's cradle of tubes etc which control the advance on startup and/or the cruise control vacuum release. I have removed all this stuff from my car so cannot be more specific.

If it is the full throttle vac switch (but mine has a different metal fixing) it should be bolted atop the A bank manifold, cabin end, and a vac tube from the manifold end connected to the spigot. The two wires go to the harness. When the throttle is pushed wide open, the vac switch switches as vac disappears, and this via the wires, does something to the fuelling (I think) or possibly the timing, meaning the engine cannot run lean and burn up the pistons at full throttle!

 

Best not to ask "why?", even regarding something as trivial as this. One "why?" always leads to another....and there are just too many of them to cope with on old Jags.

This might be a good time to remind everyone that there are no design flaws on these cars. Anything having the appearance of a design flaw is actually just "an interesting engineering feature".

The appropriate outward reaction is a moment of chin stroking followed by a chuckle...and then just keep working.

:-) :-)


Cheers
DD

 

British engineering. Those brown wires are simply part of an assembly that was terminated there. Bound together, taped off. Standard wiring practice in those days and found other British cars. Cheaper and faster than a junction box.

 

@John

I always like something different..... Your led gauges are a nice alternative to replacing the barrel gauges with "normal" gauges.

Will be interested to see a pic or 2 when they are done.

If you google "prosport gauges", these guys do exhaust temp gauges and i think you can buy the sensors separately

 

Don't forget that they had to put all the wiring in at build for all the options that were available at sale.
Front fogs, headlamp washers, heated seats, ejector seat.

 

One of the reasons I'm going with LED gauges is that there just won't be room enough for dials. Plus, they can be individually calibrated to whatever sensor they are hooked up to...for example, it's just a matter of changing one resistor if I want to go from measuring 0 to 1 volt (air/fuel mix via the oxygen sensors) to measuring 8 to 16 volts (alternator voltage)....I don't have to be bound by what the sensor manufacturer thought the "right" dial gauge would be. Well...at least that's the plan :-). I've not yet looked at the output of of coolant or oil sensor, but it should all be possible without spending a lot of cash.

All the above I can in the warmth of the house. The grunt work right now is degreasing the engine, getting ready to rebuild the distributor (it is gummed up), replacing injector hoses, and rewiring everything that was in the V. I wish I had a heated garage!

John
1987 XJS V12, 62,000 miles.

 

The first pic is the "speed signal" for the fuel injection (engine speed, actually primary ignition waveform) after passing through a resistor inside the amp. Keep it shielded, it's a high frequency signal that needs to arrive at the filtering circuitry in the ECM as Jaguar intended. Out of interest Marelli cars are different in that it is a logic level digital signal instead of a primary ignition waveform.

Pic two is the grouping of wires that I believe are for the extra air valve, distributor vacuum inhibit, and on pre mod (EVAP system purge mod) cars the electric purge valve (very rare to see anymore). The whole thing is triggered by the 45sec timer and it's even rarer to find one of these that works. Don't worry too much, it was all there for faster catalyst light off.

Pic 3 is the full load vacuum switch, the USA part (yours) is NLA, and the European spec is different. It is connected in parallel to the full load microswitch located on the throttle capstan. It can be tested with an ohm meter and a vacuum source, and should read infinity with vacuum and less than 1 ohm at atmospheric.

Sorry, I don't like the LED gauge idea. There is not nearly the needed resolution. Oxygen sensors are worthless for measuring AFM unless they are wideband, and the Voltmeter would be best having a digital readout. Water temp and oil pressure are best in analog because you want to see any nuances. Just my input.

While you are working in the Vee, do yourself a favor, obtain the TPS and throttle capstan from a Marelli equipped car (much easier to set up and more reliable...) base setting is 0.31 to 0.33 Volts, key on engine off measured between red wire and engine ground.

Good luck.

 

I can change the resolution of the LED display to make it read, say, 0.4volts low end, to 0.6 volts high end. In my case, the reason I want to have it is if I put my foot into it one day I'd like some indication if it suddenly goes lean, so it's more of a "get your foot off the gas NOW" thing than anything else.

Anyway, we'll see how it goes.

So a Marelli would be something later than model year 89 or 90 or so?

Ok, on to hunt for shielded cable. Would it make sense to ground it at both ends?

Thanks!

John

 

Oh...turns out one can buy just the EGT sensor from Prosport (I'm sure their gauges are nice, but no room)...thanks for the suggestion Sarc, I've got one on order. Now to figure out where to place it...

Quick research reveals that many like the exhaust manifold, but since I'm more concerned about fuel burning in the cat, I would think just aft of the cat(s) would make sense. Then again, I'm no engineer.

Thoughts?

Thanks,

John
1987 XJS V12, 62,000 miles

 

Yep, Marelli was introduce mid 89, it can be identified by the red TPS.

as for the shielded speed signal, do not ground it at both ends. I believe that will introduce a whole new set of problems!

 

that shielded wire has to be grounded to that black wire or the ECU will not get any feedback. as I said earlier, if you relocate the ignition control module, you can run a regular wire along the inside of the fender. This solves several problems. the ignition control modules had problems with heat from the engine causing them to fail. moving them to in front of the radiator like I did makes the engine look cleaner,, eliminates the need for shield wires, and keeps the ignition module cool.

 

I'm a little confused by this. Can you clarify, please?

Cheers
DD

 

Yes :-)

I have full appreciation for fun and interesting projects, but.....

Make sure your fuel injection and ignition systems are operating properly (which you'd want to do anyway, presumably) and you won't have to worry about fuel burning in the cats nor put a lot of effort into devising a monitoring system for same!

Cheers
DD

 

I'm working diligently on both, in the next week or so the fuel rail will go back on with new hoses, dish washers and injector "o" rings, new spark plugs and wires, properly operating centrifugal advance (thanks to 36 hours of soaking in WD40), new injector wiring harness, new rewiring of everything that was in the V, and a newly cleaned engine (one toothbrush stroke at a time) to boot...

BUT, the truth is, until the instrument panel looks like that of a Model 35 Learjet, I probably won't be happy :-). We all seem to love overkill in one form or another, and I won't be happy until I can take the pulse of everything the engine is doing. This car is a great way to merge my love for the vehicle itself with my interest in electronics.

My latest "victory" was finally freeing up the centrifugal advance, but now I'm wondering about the springs. I can't tell if I stretched them or not in the the removal process. I think Kirby mentioned that no stats were available on the length of the springs...I wonder if that's changed since the book was published?

Thanks!

John
1987 XJS V12, 62,000 miles

 

I sorted suspected that might be the case :-)

Good luck on the project !


Cheers
DD

 

Which bit of wiring are we talking about,please, Smokey?

 

sorry for the confusion, it's a little hard to go into much detail using voice recognition on the cellphone to reply.
that wire is shielded because it runs very close to the coils and spark plug wires which produce a fairly significant magnetic field. if you trace that shielded wire where it comes out of the fender you can re route it straight down towards the radiator. I place my electronic ignition in front of the radiator and that's where that shielded wire feeds in. by keeping it away from the coil and spark plug wires the shielding is no longer necessary.

did you just try to ground the shielding to the side of the car it won't run. it has to be grounded to that black wire. I only know this from experience, and I can only guess that it

I've done this with my car, and it works exceptionally well. is so the ECU get feedback.