I have been chasing perfection for months now trying to make the v12 reliable(lol, my fault) and I’ve found out that when I cold start it fires up fine but has a little stumble, but when I turn it off and it won’t start, I have now found out that I get no spark when the engine is warm(and I turn it off)
got any advice on what part of the ignition system to replace?

 

If you definitely have no spark start with a replacement coil, it is the simplest test. Do you have no spark at all or do you have spark on bank A bur not B - I'm thinking your car would be a Marelli ign. If not a Marelli and is Lucas then heatsoak on the trigger module mounted on the intake manifold

How long do you need to leave it before it will start again ? - you could try using a can of 'air duster' to cool down the coil, next up would be the trigger head in the distributor,

Depends which system you have - I think 89 was the year they changed from Lucas to Marelli.

 

yea, so mine is marrelli and when I try and start it(warm engine) there is no spark on either banks

 

distributor pick up or ignition coil. start with the coil don't overthink this or there are no end of options and permutations on a V12, I seriously doubt that both trigger modules are simultaneously taking a nap. If no spark on either bank it is something they both have in common.

If I remember rightly the rotor arms on Marelli systems are known to burn - might be worth inspecting, don't think it is that but it is easily checked.

There is a write up of the Marelli here about half way down the page (there may be no module in your distributor - wouldn't rule out heatsoak on the ECU but start simple >> http://www.jagweb.com/aj6eng/ignition.php

 

I suspect that either your crankshaft position sensor, or your engine speed sensor is failing. The symptoms you describe are typical. When they get hot, they quit working. When they cool off, they start working again.

 

The 1989 is a Marelli system, and it doesn't have a distributer pickup, but it does have a Crank Position Sensor (at the front) and an RPM VR sensor at the rear. In the vast majority of cases the front CPS goes bad resulting in dodgy, inconsistant, or no spark. That is the most usual issue for a no-spark problem on a Marelli car (or even heat related.. though amps can go bad with heat soak.. but usually not both at once.. same with coils.. but these are far less likely items.) Each coil should have a primary resistance about 0.6 Ohms if I recall correctly.

You can measure the ohm value of the CPS, but even if that's ok it's not conclusive. A bad value would be fairly damning though. Those figures are recorded in the forums here or at Jag-Lovers but I dont' recall what the values are (maybe in the 700 Ohm range?)

The rear RPM sensor *can* go out, but it's rarely reported that it does even though it is exactly the same sensor construction as the front one. I would think though, that as the car warms up, it would eventually die while running, not simply not re-start when hot.

~Paul K.