I have 3 ECUs that I could (in the past) interchange and all worked fine. (They gave me different engine-light warnings, but that's another story...)

ECU #1: from 1995, no updates (LNA 1410AH)
ECU #2: with 1997 EPROM Update (LNA 1410AJ)
ECU #3: with 2002 EPROM Update (LNA 1410AL)

Suddenly, 2 of the ECU's stopped working. Crank, but no start. No signal to RPM gauge. This happened during a very cold streak this winter.
But it runs fine with ECU #3. No ODB codes. Running closed-mode (but with O2 feedback fault...maybe bad catalytic converter)

I can't imagine ECU#1 and ECU#2 died overwinter at the same time...
If I disconnect Crank Position Sensor, then ECU #3 behaves the same way (crank, no start, no RPM gauge).
So I guess it could be a failing CPS. Car has new coils, new plugs, new battery. old injectors (with 120,000 miles).

Only other change to car is a strange engine vibration that happens sometimes, maybe a tiny misfire that comes and goes, sometimes for minutes. Sure, I could have a sporadic bad CPS, coil, or fuel injector.

But what else would cause 2 ECU's to not work at all, all of a sudden? Maybe there is something in the 2002 EPROM upgrade that makes the ECU more resilient to a failing CPS/coil/injector?

thanks for any advice.
XJ6 1995

 

On the non us models there is a starter inhibit that must be satisfied probably by a digital signal from the SLCU

https://www.jaguarclassicparts.com/u...dule-4-0-litre

 

All 3 ECU's have worked great on the car without security resets. Since one of the ECU's not working now is the ORIGINAL ecu, I don't think it is security related...at one point i could swap ECU's at will.
But just in case, when I get a chance I will try playing with the key FOB with different ECU's...

thanks.

 

Did you install the lower aft ECU mounting bolt as it is a case or shell ground .

 

That is a good suggestion...I know that if the 2 ECU bolts aren't tight the same symptoms occur (crank, no start). It's happened to me. I am pretty sure I've tightened them already, but I should verify ground.

Main reason I want to have at least 2 ECU's working again is to verify that the ECU isn't the reason for the random engine vibration/rough idle. And maybe having 2 older non-working ECU's is a clue to the solution. Flaky electrical contacts is a possibility...especially in winter with all the condensation.
thanks.