Diagnostic Tree?

Everything buttoned up. Cranked it over for a start … No go. Everything working but no spark on the A bank, probably the B bank too since they are duplicates. I’ll check the B bank later today.

Does a diagnostic tree exist for chasing down ‘no spark’ ?

It’ll definitely help to tell us which ignition system you have.

I would hazard a guess that it is a Marelli system seeing as only A bank appears to have no spark. but maybe not.

Ptipon, if it is a pre 1988 engine than its a lucas system, if it is 1988-1989 it could be either, and if its a 1990 and up its a Marelli system.

On the “old” site which many hated, there were signatures:

Sonora/CA, 90 XJS-V12 conv, United States

If this is the same car Paul is still working on, then NO START is most likely caused by the CPS (crankshaft position sensor)

Spark on either side missing is either coil or ignition amp, BUT, the V12 will start and will idle on one bank only.

True! regardless it is a Marelli system and I would have to agree with you that it is probably a CPS problem if both banks are indeed not sparking. question is, is it the front sensor or the rear one? I would assume the front one due to it being the position sensor and the rear one is the speed sensor, right?

To Kirby: It is a Marelli System.

Further testing finds that there is also no injector clicking at all.

You can use the “Location”, “About Me” and “My Cars” fields in your profile to give some detail - this is helpful to other users as it allows them to see what year/model your Jag is.

What were you working on before you “buttoned it up”? Thatugh be the primary suspect.

A common causes of a no-start problem with no spark and no fuel injector clicks in a Marelli ignition car, as detailed extensively in the archives, is a bad Crankshaft Position Sensor (CPS). Take a look at the the sensor head and connector for issues. If the CPS is original or suspect get a new one and swap it out. They are not that expensive and it is good to have a known good spare CPS handy just in case.


No spark on A or B bank along with no injector clicking on both banks.

Yes, this is I


Sonora/CA, 90 XJS-V12 conv, United States

As already pointed out – on a Morelli, the only thing that will not give you spark on both banks is the CPS (front sensor).
Since you did heads, everything was disconnected – please check the connector from the CPS (near the B-bank/your driver side thermostat) – any chance you forgot to hook it up?

Hi Steve,

Not everything was disconnected. I only did the A Bank, right head and the CPS was never disconnected. Along with no injector clicking, it seems like all engine electrics are dead.

This is a picture of the ECU and just below it what I would call a liquid trap for the vacuum feed from the engine. As can be seen, it is now separated into two pcs. I’m going to be ordering a new one tomorrow, Monday. Any comments, anyone? It does not have any break points or surfaces but then it doesn’t click back together either and I assume that it is probably air tight. A real mystery as to what happened here.

No spark will result in no injection, as the injection is triggered off the spark.

Still thinking CPS, or connector to same. Might as well unplug, clean up, and reconnect the cable to the sensor at the crank damper.

That chamber in the vacuum line is an issue, but I don’t think it’d stop the engine from starting. You might even just bypass it and connect the vacuum line up straight and see what happens.

The spark and injector clicking do in fact get triggered together under constant and regular throttle action with no throttle stomping. However, when one tromps on the gas there is a micro relay on the turntable on the pedestal that has the rods to the throttles. When you turn the
turntable to engage the micro switch it triggers all of the injectors at the same time to enhance the gas mixture for engine acceleration, independent from the CPS signal. I have used this technique to check the clicking for years and several times each year to verify that injectors are able
to click.

Doing that now gets no injector clicking at all. I use this technique to verify that if there is no injector clicking then the culprit must be the CPS because this test tells me that all injectors are clicking and are able to click if the CPS is working. Since I now
cannot get this test to pass it tells me that the injectors will not click, CPS or no CPS. It also means that there is no injector trigger voltage at all regardless of CPS or not.

So I need to check for voltage at the ECU and at the injector firing circuit. Where do I find those
points to check for +12V? Where are the ECU and injector power supply relays?

Just checked the voltages at the micro switch and the vacuum switch. Two photos. First photo shows the turntable and the lower microswitch or the switch closest to the bottom of the frame is the microswitch for fuel enhancement. The 2nd photo of the tan plastic gismo is the switch that works off of vacuum and enhances the fuel when the vacuum drops at the intake.

Both switches are supplied with the same voltage to trigger the injectors to enhance the fuel when the intake pressure drops or one stomps on the throttle. Neither have any voltage at any of the terminals. Therefore there is no fuel enhancement or injector clicking occurring independent of the CPS and there should be injector clicking, all of them at the same time/in unison.

So, no power to the entire EFI system? A malfunctioning EFI relay could explain that – but it doesn’t explain the lack of spark. Unless you have two separate failures, we need to look for something that would kill both. Not much past the ignition switch will do that.

I’d like to treat it as two separate systems. In tracing down just the injector system, I may find a common fault. I’ll just have to wait and see.

Starting with the injector system, does anyone have at lease a block diagram of the system? Like where is the EFI relay? And what and where are the components that make up the EFI system?


What about a tripped inertia switch?


The hard copy versions of the XJ-S Repair Operations Manual (ROM) and S57 Electrical Guide that I purchased on eBay have all that information and a whole lot more on the EFI system. I would be unable to keep my wife’s 1990 XJ-S convertible running without them and the XJ-S Parts Catalogue.

Someone recently posted a link to an online source for the S57 Electrical Guide. Search the archives and you should be able to find it. I don’t have digital versions because I much prefer the hard copy documents.