I have recently saved a 1990 XJS from the scrapyard as a project to work on with my 16 year old son over the winter. He wants to be a mechanic, and this is a wonderful project. We have replaced Plugs, Fuel Regulators, Distributor Cap, Fuel Filter - all the basics - and the engine runs. There is no power, it does not drive. I noticed that the exhaust on the left side was blowing cold air, the right a bit of smoke. I think its running on one bank. I stopped the car and reviewed the ignition work of the previous owners.... i see just one coil... and its the Marelli system. Easy mistake - one input, one coil.

I have ordered two replacement coils, but all the diagrams I find show the Coils connected together in parallel (almost like its a primary and backup) but no HT lead from the second coil. I see notes regarding a mysterious Y-HT Lead that connects the two outputs to one ... I also see notes like 'Sealed'. The diagrams I managed to find are attached.

Looking for advise on How i wire these up! Does the Y-Connector exist? or are these diagrams correct.

 

Typo in title - Marelli... not Marinelli.....

 

I think I have an answer.

On the Jaguar V12 there isn’t a Y-connector on the HT side in this Lucas CEI/AB14 twin-coil setup. The photo above shows the two coils in the bay: the main coil’s single HT lead goes to the distributor, and the auxiliary coil’s HT post is capped/sealed—no Y-join feeding the cap.

What’s going on electrically:

• The primaries of both coils are in parallel and fire together from the AB14 amplifier (shared +12 V and trigger). This shares current and builds more primary energy.
  
• The aux coil’s secondary is not used; its HT outlet is sealed. Jaguar’s own documentation and community write-ups note that the auxiliary coil’s HT “is sealed off,” i.e., no output lead—exactly what your diagrams show
  

So... how does the second coil helps at all - it would build up a charge, but with no HT lead can't discharge.
Both coils are connected in parallel on their primary (low-voltage) windings — that’s the 12 V side. When the ignition amplifier triggers, current flows through both primaries simultaneously. This parallel connection effectively doubles the available magnetic core area and halves the current through each winding, allowing both coils to charge and collapse their magnetic fields in unison. So the magnetic fields from both primaries collapse together — but only one coil (the main coil) has a usable secondary circuit to convert that into high voltage.

The second coil still contributes magnetically to the system because its primary winding is carrying current, and that magnetic coupling stabilizes voltage and current rise time for the main coil.

When you run two identical inductors in parallel:

• The total inductance decreases slightly,
  
• The available current capacity increases,
  
• The system can store and release energy faster without saturating a single core.
  

This means the main coil:

• Charges faster (shorter dwell time needed),
  
• Runs cooler (lower current per winding),
  
• Maintains spark intensity at high RPM where single coils struggle.
  

 

Haven’t read your entire posts but the diagrams you posted are not for Marelli ignition!

More later

DD

 

You have the wrong diagram for the Marelli ignition, which, if a 1990, your car will have. The diagram you have posted is for the previous Lucas ignition. If you have in fact got Lucas ignition, you will see a 3 inch square black square box on the top of the cabin end of the B bank (US driver's side) intake manifold. If a Marelli ignition, you should see two HT leads going into your dizzy cap from the coil and a double rotor. Best to renew both.

As your car has no spark on one bank, it is because the Marelli ignition has failed on one bank. This is quite common, do a search on marelli V12 ignition failure on this site. It is caused by one of these things:

• amplifier failure (you have two on the radtop panel, one for each bank)
• wiring failure from amps to coil
• distributor cap or rotor failure (they can burn out the cap or rotor insulation inside)
• also you MUST chnage the plugs and HT leads as the burnout in the dizzy insulation is often caused by poor spark transmission.

 

Ah - the old One input vs Two Input.

The 1990 Jaguar XJS V12 has two coils and one input to the distributor.

 

Or ... maybe this is more of a Frankenstein car than I thought...currently one coil installed.

Photos of my engine bay may help. Warning.. not pretty.

 

You have Lucas ignition on your car.

From the pics I'd say you have an early 1989 MY car. Not 1990.

The switch from Lucas ignition to Mareli was at VIN 156xxx which was about half way thru the 1989 model year (for USA market cars)

Cheers
DD

 

I would fit new plugs, new HT leads, new coil or coils (later Lucas versions only had one squareish looking coil), new dizzy cap and rotor and try again. You should, after all this, have a fat bright blue spark at the spark plug end of the HT lead. if not, then a new HEI module in the Lucas box may be needed. Also check that the lead (two wires) coming up from the dizzy base to the plug that goes into the side of the Lucas box is intact.
Finally, you have a vac capsule going into the dizzy, this may well have a fractured diaphragm and not be working, if so, that needs changing and the dizzy spindle lubricating so it can turn about 15° and snap back on its spring. CAREFUL removing the rotor, do NOT just pull it straight up or somerhing will break lower down. Rock it with a hand each end in a see-saw motion and it will come free with patience.

The top photo (says power resistor on it) is not the amplifier, it is part of the injection system. No need to touch that. The next photo, black box green script saying "electronic ignition amplifier" does the spark signal to the coil. But where is it mounted, as I cannot see it on the manifold in your photo of the engine?

 

That is defiantly a Lucas setup. Early cars had 2 coils in parallel, as low resistance coils were not available. The second coil was non-firing, but using Ohms law connecting the coils in parallel was a way to lower resistance. Later cars had a single coil, once low resistance coils were available.

Inside the Lucas box you will find a GM HEI ignition module. It should be mounted on the intake manifold, as that is a heatsink as the module does produce a fair bit of heat. It also needs a good ground. If you replace the module, use only a genuine GM/AC Delco part number D1906. The V12 pushes that module right to it's limits and the aftermarket ones generally don't work as well - or at all in a V12. Ask me how I know!

 

Thanks for all the help. Given it’s a Lucas, makes more sense. It’s a 1990 for sure, but I guess built with Lucas or converted later.

still have one exhaust hot and one cold .. fuel pressure good, new plugs,leads,distributor cap.. but now don’t think I have a missing coil…so I need to check bank 1 has sparks ….

 

If it runs, you have spark on both banks….1990 would have been a Marelli ignition system that probably had the dreaded meltdown, and had a Lucas system fitted. The next diagnostic step that I would perform would be to connect a noid light to each injector plug on the dead bank.
Injectors are batch activated, there are 4 groups of 3 injectors, and each group has a initial “on” circuit and a “hold” circuit, the later going through the power resistor block and all controlled by the ECU in the trunk. Could have an open in the wiring, the resistor, or the circuits in the ECU that could kill 3 or 6 injectors at one time. Also do some investigation of what ECU you have; an earlier 6CU ECU could have been substituted at some point, and those are prone to injector control circuit failures (but usually short to ground, which holds a batch or batches of injectors open). Good luck and a good learning experience for you and your son!

 

As mentioned, with a Lucas system, if you have sparks, there's no reason why the cylinders on one bank aren't working. And even the banks of 3 injectors are spread across the cylinders on both sides, as I recall.

I would start by carefully seeing if the exhaust manifold is getting hot on that side. If not, I'd then check the plugs for sparks.

Good luck

Paul

 

Just a note, others may have already mentioned it if I missed it in the comments, or you may already have found this through your own research but the spark plug gap is critical and needs to be .025". This is due to the high compression lean burn strategy of the engine. Almost all plugs come pre-gapped at .035" and the aftermarket stores will have that larger gap in there system. This is a case of 'garbage in garbage out' with computers which they rely on for everything.
As mentioned by others looks like your car is an early '89. You can verify the build date on the drivers door vin plate. It will give a manufacture date which will often be prior to the model year. My '95 has a date of 5/94 while my '94 has a date of 3/94, just two months earlier. I'm guessing that yours will have a date in '88.