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Discussion Starter · #1 ·
Hello, new to the forum, looking for some guidance with my newly acquired but far from new 1985 XJS V12. She hasn't run in several years from what I can tell, and has many issues. My current plan is to install a more modern ignition coil and remove the two coils wired in parallel, I'll bench test the Lucas igniter but if it doesn't work I'll probably use an MSD box, and for the sake of simplicity I'm planning to retain the stock fuel injection, but run it open loop without O2 sensors, vacuum retard for warm up, or any of the emission controls. No emissions requirements for this beast where I live. Basically it will have the MAP, IAT, ECT, and TPS sensors. Apparently this car is supposed to have a Crank Position Sensor, but if it does I have been unable to find it, and I believe that it actually uses the distributor pick-up for it's RPM signal. I don't care about warm up time or fuel consumption, but I'd like decent driveability from it. I paid $300 U.S. for this car if that gives any indication of it's condition, I'm a very experienced auto mechanic with some experience on older inline-six Jags, and I intend to make this old girl run on a shoestring budget before I make any more long term decisions about the project. Any advice, input, warnings, or comments are very much appreciated!
 

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It has a Lucas Ab14 ignition amplifier that powers the rev counter via (I think) white and black lead.it doesn't have a crank pos. Sensor. Which is prob a good thing as the pully can have a spasm with the woodruf
 

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Discussion Starter · #3 ·
I figured it must not have a CKP sensor, because I can't see one anywhere, but the wiring diagram shows something called a "trigger" in addition to the distributor pick-up, and the parts catalogs list the CKP. I have it turning over with 225-270 psi. compressions, 25 psi. oil pressure, repaired my starter, (for the time being at least it works and sounds much better), installed a new set of plugs, and it sounds great cranking now. It will live again! I'm going to rig up the ignition system on the bench, and from there work out the fuel injection and vacuum arrangement.
 

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Has anyone here ever replaced the Lucas ignition amplifier with an MSD or other aftermarket ignition box? Worried that the extra power output could damage the ECU for the fuel injection.
Why would it damage it? I can't see how improved power output could cause any damage to the ECU. And anything made by Lucas is well worth getting rid of IME!
 

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Discussion Starter · #6 ·
Thanks Lightfoot, I have an update, I managed to repair the Lucas ignition box, which if you open it up appears to be just an old Chevy style igniter with a ballast and some non-weather sealed connections which were the reason is wasn't working. The leads coming out of the ballast provide the RPM signal to the ECU for the fuel injection, and testing with my multi-meter I believe it's just an AC signal that increases with RPM but I didn't actually hook it up to a scope to see what the signal looks like. I don't believe that an MSD box would have a provision for the type of signal needed for the ECU to fire the fuel injectors, but I'm not really sure. Another reason the system wasn't working is that the "phantom" parallel ignition coil mounted to the front bumper is shorted, and kills the whole system when it's connected up to the main coil. I have spark and fuel injector pulse now, so since this is a budget operation I'm going to stick with the stock stuff for now. Also I've tested all the injectors and run some Techron cleaner through them. It's pretty close to running again, apparently for the first time in years. My only real dilemma left to figure out is what to do about the rusty fuel tank.
 

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Discussion Starter · #7 ·
Sooooo, I have another update if anyone's interested, and I'd love to hear from anyone knowledgeable on the fuel and EFI systems. For the first time in many years, the old beast ran and drove today! Only drove from one side of the shop to the other, but hey it's a start. Started up and ran great, then died because the Bank A fuel pressure regulator closed off. Bypassed it, ran pretty well again, then decided to go super rich and flooded out. Think it may have poor connection at ECT sensor, installed a 200 ohm resistor, then ran okay at idle, but apparently this was only because the fuel pump failed and it ran okay with 10 psi. fuel pressure. Ordered an ECT and connector, and of course a pump. Does anyone know a way to test the ECU's internal manifold pressure sensor? My line to the intake is good, and the ECU side holds pressure and vacuum. Also, does the Digital "P" system have a fault code output function?
 

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Discussion Starter · #8 ·
Not sure if anyone cares, but I've made some progress. Changed the fuel pump, pressure is good now. ECT sensor was dead, changed the sensor and connector. Still had some pretty severe running problems, and it would lose ignition and die when it warmed up. Converted it to a Ford "E" core coil, that helped a lot. Now my only real problem seems to be that it stills runs horribly rich. Injector pulse varies from 12-40 ms at idle, (should be about 3 ms.). Today I started checking all the circuits and sensor inputs from the ECU connector. So far so good, but I still have to check the circuits to the injector resistor. The connector at the resistor had some pretty bad oxidation, so I soldered in a Ford weather sealed connector to the internal resistor wiring and the harness side. If that doesn't resolve the problem, (didn't have time to finish my circuit checks and run it), I'll be leaning toward an ECU failure. Still would like to hear from any experts out there, or anyone with any experience with ECU failures, thanks
 

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I have a 1995 V12 XJS - is it carbureted or fuel injected?
It's fuel injected - but a much more modern sensor-driven arrangement than on the 5.3 litre HE.
 
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