http://www.jag-lovers.org/xj-s/book/InjHarnessRebuilding.html
Hey Kirbert,
Thanks! Perfect lead. (sic)
I pulled the injector harness out of the car and am pulling the wrapping off the wires. Iād like to repair what I have now just to test it but Iām already finding issues with it.
Wes
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Wes,
I did a complete engine bay cleanup and restoration of my wifeās 1990 XJ-S convertible last year, including rebuilding the engine bay harnesses. A lot of the connectors were fragile from heat damage so I replaced them. I pretty much cleaned, painted, polished or replaced everything on the top or around the engine. While I was in there I adjusted the valve clearances, replace the cam cover gaskets, and replaced all the belts and hoses. I posted several times with pictures as I struggled with problems but eventually got everything back together again and itās running great now. The new OEM radiator made a huge difference in coolant temperatures.
Take a look in the archives for my posts and pictures.
Paul
Hello Allā¦
There seems to be a lot of wisdom and experience with the folks who participated in this thread so I thought I would āwake it upā for a problem Iām having with my 1990 Marelli v12ā¦ Hope some of yāall will see the smoke signals.
Iāve been working on this car a while and come a long way. Super pleased. Must have clocked about 1000miles this summer up and down the East Coast. Iāve run into a problem.
Description. I got to twisting and turning the looms leading up to the plates where the amps are mounted on the radiator plate. Unbolted them, pulled the rubber strip that allowed me to lift the harness out of the gap the harness runs thru to get the amps to their mounting points. Then bent and twisted (stupidly) the looms under the high pressure side AC hose and then bolted them back down and replaced the rubber strip. Seemed simple enough, but seems to have opened up a can O whooop ass.
2 days later I started the car and it started as normal and began to warm. I drove away and within a mile things started going funny. Tach was allllll over the place and not coinciding with engine speed at all and or bouncing. Loss of power, bad, and (unless I imagined it) a backfire or two. Never stalledā¦ The tach would read 1 or 200, then 2.5 to 4k - having nothing to do with engine speed. Had to drive home and park it up. Havenāt driven her since.
Thereās a lot O wiring associated with this system. Looking at the EDiagram most of the wiring having to do with the tach is with coil A and Power Module A. Iāll attach a picture. I am sure that my monkeying with the amp wires and units had something to do with this but Iām wondering where YOU would begin in cutting into the harnesses (if at all) and what you would be looking for? Are there some tests that can be run. I see in the Diagram that there is a āDiagnostic Socketā but know nothing about the readings I would get - even if I knew where it was. More, the Diagnostic Socket seems to be most associated with Power Module B, not A.
Below is the EDiagram page I will be working with and I am terribly nervousā¦ I would really appreciate any suggestionā¦
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Diagnostic Socket calls for PDU. There are only two people here that I know off that have the system - John6 and Veekay.
For the commoners - no joy - forget about running any test through it.
As far as your Marelli question. The engine should start and run on one bank only. Start with swapping coils and/or amplifiers, making sure the opposite site is disabled. This should tell you if you have a bad component.
The other common problem - flywheel and crankshaft position sensors. With a scope, you can read the waveforms. If any of the two is ions, engine wonāt run. If they are intermittent, youāll see it right away.
Good luck!
as per the schemlatics, the tach signal comes from the ābank Aā amp :
so my guess would be first checking this area
you can swap the amps to be sure the āA sideā amp is not involved, but my bet is on a brocken wire in the loom around coil, amp and distributor
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And the test for faulty CPS is to observe the tach when cranking. If the CPS is bad the tach wonāt show anything hardly at all (i.e. movement/reading)l while cranking. 
Wow, Paul #2 ā¦ and no exception for the car, no matter how old? No emissions ins. required here in TX after 25 years. 
I wonder, though, Palmdude ā¦ couldnāt the holes just be filled with something like QuikSteel putty (assuming you donāt ever want to go back to the OEM system), or would the exhaust force blow the āpatchesā off the manifold?
Paul,
California used to exempt all cars 30 years old or older from emissions testing but stopped that in 2005. Since then all gasoline powered cars built in 1976 and newer had to be tested every two years and there are no plans that I am aware of to move that year forward. 
So our 1957 MK VIII and 1969 E-Type are exempt, 
but our 1990 XJ-S convertible and 1990 V12 Vanden Plas need to be tested every two years as long as we live in California (not much longer as we plan to move out of the state in 2021). Over the past 20 years our two former XJ6s and two V12 Jaguars passed CA smog easily but I must admit I was always concerned about a surprise failure during those 30 or so smog checks. 
The states that we are likely to relocate to do not have a vehicle emissions test requirements. Those states are very different than California when it comes to a lot of things. 
Paul
Solved
Well, a tentative āSOLVEDāā¦
So I went out this morning (after finishing with the motor mount install on the XK8) and started cutting off the cover for the wiring to the A and B bank amps. Last week, I (foolishly) really gave them some good twists and turns rerunning them to make room for a the low pressure A.C. hose that I needed to bend DOWN some due to the inlet port on the low side fouling the bonnet when closed. Anyways
The wires were soft, flexible and looked damn near new. I disconnected the amps, leaving them on the rad cover, and removed the covering as far back as that 90degree hook in the long L shaped fuel pipe that they run under just before the FPR on the LH side. The wires looked great so I stopped
I then turned my attention to the amps, removing them and studying the plugs. B on the left hand side looked good. Both amp and plug. The A on the RH not so much. The plug was a bit discolored on the two connectors on the LH side of the plug as they would be when the plug and amp are in place on the rad cover. So, I went about giving them as good a cleaning as possible. Small plugs and small spade didnāt make this easy. I plugged everything back up and started the car. She fired right up BUT the problem persisted. Idle rough and unsteady, tach needle not corresponding with engine speed. I turned her off
About two months ago I cam across and new genuine Marelli Amp on eBay and took it CHEAP. So, I went and plugged that in on the RH side replacing the A Bank amp. I learned that the A Bank amp, unlike the B Bank Amp, work thru the coil to produce the signal that runs the tachā¦ So, my focus today (thanks to yāall and the ED) was only and mostly on the A Bank part of the ignition system. Plugged in the new amp and started the car, also built a new plug for the CTS during this, and started her up. She went to idling smooth as silkā¦
Failed amp? But why? The reason I say tentatively SOLVED is that I have a HAUNTING suspicion that a failure in MY rewiring new coil plugs and messing with and rewrapping wires in the RH corner at the rear of the engine bay a couple of weeks ago may have caused this failure.
I donāt think Iām out of the woods just yetā¦
Tomorrow (after installing a new door handle in the sweet running 928) I plan to start really really really giving another and BETTER go at cleaning up messes I myself have made in this wiring systemā¦ I think itās faulty wiring or shorting somewhere that put some stress or extra load on that ampā¦ I could be wrong but Iād rather have a better look.
Another foolish thing that may have contributed to the failure of that amp was that when I moved them last week to rerunn the wire under that low pressure A.C. hose, I didnāt disconnect the amps from the connectors and knocked them around in the process. Who knows, them things could have been original. 30yrs old, and why would they take well to being rough-housed. Stupid move.
Below is a picture of the āDiagnostic Socketā the ED talks about. Also, I am pretty sure that that common ground point for the ignition system in the ED is one of the two on the LH fire wallā¦
How does one TEST the quality of a ground point and wire? Measure for resistance?
Both of my CPS test at 742ohms exactly and the plugs are clean.
I am ALL ears for pointers and suggestionsā¦
What years were the Marelli car in production anyways? Just a few years of the xjs run? I could just be me but there seems less info on Marelli than the other XJS years.
āDiagnostic Socketāā¦ No clue what to do with this. Can I test for good ground here?
I was HAPPY at the wires conditionā¦ Clean, soft and flexible. Felt new-ish.
YOU were so right! Thank you
Well come on down to TX, pard ā¦ ! 
Or, IIRC, Florida eliminated their emissions requirement some time back. 
āThe states that we are likely to relocate to do not have a vehicle emissions test requirements.ā
Paul . . .
As a snowbird, I can recommend Florida. No emissions tests, no mandatory safety checks, lots of British car enthusiasts, active Jaguar clubs . . . how much better could it be? Oh yeah - no forest fires, no earthquakes , but the occasional hurricane.
. . . Ron
FWIWā¦
Washington state recently discontinued the emission test requirement. Even when it was in place, it was a county-by-county thing. Not statewide.
In Oregon emission testing still existsā¦for certain counties. Iām in a āno testā county.
Cheers
DD
Having spent 30 years in Los Angeles I well remember the anxiety over āpassing smogā every-other-year.
Cheers
DD
My interaction with US smog requirements was CA in the 1990s, and the vehicle was an aged and terrible Ford Escort. It failed. I was given a bill to spend some dollar amount to fix it. I asked them what they were going to do for this money, and it was basically nothing. I was essentially just paying a fine/tax to get the paperwork completed. I checked the rules and they were correct, all the law stipulated was a minimum spend figure. Pay the money, and youāre good to go, no matter how bad your car had failed, and will continue to fail.
But what a revenue generator that amount was to be paid every time the car was due an inspection which is why they could care less if the vehicle was properly repaired or not.
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Correct. I stopped paying the tax by getting rid of the horrid 1980s Escort, swapping it for a 1965 Fastback Mustang - no smog check for historic vehicles. It was also a terrible car, but thatās a different story.
Actually, we had a pretty massive forest fire perhaps a decade back. I dunno if weāll ever have another, though, because the forestry managers have taken to a serious program of prescribed burns ever since.