1990 Jaguar XJS



October, 7, 2011 AT 10:26 PM

My 'A' bank coil and circuit back to the ICU is fine but the 'B' bank circuit causes the output of the coil to quit after ~1second.

When it does fire initially, it seems timed correctly wrt to bank 'A'.

I've swapped signal leads to the coils and the problem follows the 'B' bank circuit. I've swapped power amps and again, the problem follows the 'B' circuit.

It seems to be a problem with the Ignition ECU behind the kick board on the right side but I don't know what would cause it to work fine for a brief period and then stop.

I've checked and compared the signals are pin 10 and 14 of the Ignition ECU, 'A' bank and 'B' bank respectively and the 'A' signal is consistent in its waverings while the 'B' signal looks the same while spark is present but then seems to go away at the same time spark stops.

The engine won't start as there are other problems so I can't tell if this is only at idle or not but I would like to solve the spark issue first in any event.

I've never heard of an Ignition ECU failing in that way so I am thinking it might be an electrical connection issue of some sort but I'm not sure what connection issue would cause the signals at the Ignition ECU to be similar initially and then not.



9 Answers



October, 7, 2011 AT 10:39 PM

Check and clean all grounds. Make sure all connections are free of contaminants and that the connections are solid. Check the primary side of the coil for impedance. It should be between.6 and.8 Ohms.



October, 7, 2011 AT 10:53 PM

"Check and clean all grounds", not done yet but good idea.

One of the coils are new and it exhibited the same problem when swapping inputs to the coils during fault elimination so that is an unlikely source of the problem, although I am replacing the other anyway.

I'm just glad you didn't say, 'Replace the Ignition ECU'! You've earned my donation! Thank you!



October, 7, 2011 AT 11:39 PM

My pleasure. And thank YOU for the donation!

Using a multimeter, also verify the integrity of the wiring harness. The ECU is far too expensive an item for me to recommend replacing without at least trying to verify that IT is the problem. Right now, it's a suspect. But, the wiring, grounds, and connectors are easy to "interrogate" and rule out before we take that big jump.



October, 8, 2011 AT 4:29 AM

I'm working on the grounds, I've only got those on the right 'A' Frame to do yet but I need a special wrench to get to them so that will have to wait until tomorrow.

I found that the grounds in the engine compartment just forward of the bulkhead on the right side, under a plastic cover which is also the location of the starter relay, weren't even tightened and spun on the stud. Not any more. ; -)

Continuity to ground from the power amps are much better but I don't plan on testing the spark again until I get the grounds on the right side 'A' frame which is where I think the ECU gets its ground cleaned up as well. After I get it cleaned, I'll also know for sure if it is the ground point for the ECU.

I'm also working on ringing the ignition portion of the wiring harness out but there again, I need some multimeter test lead extensions which I'll have tomorrow as well.

As for the Ignition ECU, it's still under warranty so if it does turn out to be bad, it ain't my problem. ; -) Still though, I'd rather not have to replace it if not absolutely necessary.

I'll update this again after I finish getting the grounds cleaned up, the wiring rung out and try the spark test again.

Thanks again for your help!



October, 8, 2011 AT 4:37 AM

My pleasure. "Ringing out". : ) Sounds like someone has some extra experience and knowledge with electrical stuff. That makes what you're doing so much easier. : )



October, 11, 2011 AT 12:37 AM

Someone does. Seemingly centuries ago, I worked for GTE ringing out huge PBX wiring harnesses. Now though, I think I have to draw on another area of experience, football, namely "Drop back ten yards and punt." ; -)

I went over ALL of the ground points on the car, disconnecting all ring lugs at each point, sanding the ring lugs with medium coarse sand paper to provide a better mechanical connection, cleaning with contact cleaner, and then after reconnecting each to their respective ground points and giving the retaining bolts a good tightening, covering each with a dielectric grease so I hopefully would never have to do that again.

The result is all ground points at the various parts of the involved circuits are no more than 0.1ohms to the engine block.

Most of the circuit involved in the Ignition ECU I could test at the connector to the ECU itself. The resistance at the connector to the Crank Position Sensor and the Engine Speed Sensor measured around 680ohms each. The Coolant Temp Sensor measured 2.2k at the ECU connector, which is about right for the ambient temperature.

The connections between the ECU and the power amps ring out as it seems they should, 0.1 - 0.2ohms as do the connections between them and the coils.

I also tested supply voltage at the amps and coils as well as the ECU itself to make sure each was getting full battery voltage with the ignition switch in the run position.

Everything tested out as it should, statically.

All bets are off though as to what may be happening when the engine it trying to crank over.

I say that because the 'B' bank circuit still stops sparking after about a second or two.

I'm not ready to give up yet though. For one thing, the Crank Position Sensor has been a problem for some time in that the connector on the ECU side which mates to the connector molded to the sensor cable is pretty well shot as is the temp sensor and the crank sensor has had to be jiggled now and again so that the car would start. Both circuits test good statically, which I've mentioned but although neither should cause only one bank's spark to fail, they need fixing anyway and I've parts on the way.

Another thing I think I should check are the connections at the various switched ignition feed points because if any of those are loose or otherwise showing a 'high' resistance, I'm thinking they could test as good statically but cause a voltage drop when the system is under load.

What do you think, worth a shot?

Worse case scenario is that I try and don't find anything and have it towed to a garage and let them deal with it but I'll have learned a LOT more about my car in any event and some of the things I've done would have cost an arm and a leg for them to do.



October, 11, 2011 AT 4:31 AM

Yes, I think it's worth a shot to go forward as you mentioned. Basically, once you've ruled out everything you've ruled out so far, and the connections you've mentioned, it basically points to just the ECU.

I installed voice and data cabling systems for 6 years after I decided working on cars was too messy. Small world.



October, 12, 2011 AT 8:02 AM

Houston, we have lift off!

It WAS the Crank Position Sensor connector even though it ohm'ed out to the ECU and even though I saw a signal, although evidently not the right signal! ; -)

I replaced it and the coolant temp sensor which feeds the same ECU as well as it is near the 'Valley of Hell', the 'V' between the banks which gets hotter than Hades normally and so the temp sensor connector, the same type as the CPS sensor connector, a Bosch injector connector was essentially crumbling.

It threw me for a loop though because one coil was working well. Usually it is an all or nothing issue with the CPS. I changed the CPS connector and got good spark on both coils.

I'm guessing that the signal lead from the CPS was making 'good' contact but that the shield side was not so much so the signal wouldn't have had a reliable reference and was essentially floating, albeit with a flutter of true signal making it through to at least drive one of the banks' coils correctly.

Sure would have been nice to have schematics for the ECU and I could have probably figured it out without all the guesswork but with it being essentially a black box, all bets are off.

Anywhoo, now I get to drive it to the garage for full plugs, fluids, filters, lube and belt changes tomorrow instead of having it towed!

I know I probably should do those things myself considering what I already have been through but changing plugs is a real nightmare on the HE 5.3L V12s. That, I'll pay someone to do. ; -)

Thank you VERY much for your help and moral support, it is greatly appreciated!



October, 12, 2011 AT 9:09 PM

My pleasure. Glad to be of some assistance and happy it's working again.

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