No matter what type of coil is used in your Jaguar S Type, signs of ignition coil failure are the same. A problem in the ignition coil will cause a no-start situation or multiple engine misfires. If you don't want your day to be ruined by a vehicle that doesn't start due to a bad ignition coil, you should familiarize yourself with the signs of ignition coil problems and how to troubleshoot them.
A weak coil will lead to low spark intensity, which makes the combustion process inefficient, so the driver will experience a drop in power. This can be diagnosed by disconnecting the coil wire from the distributor cap and inserting a spark plug into the coil wire. The spark plug should be laid on a metal part of the engine, like the valve cover. Try to crank the engine and observe how the spark plug works. A hot blue spark means properly functioning coil; pale yellow spark indicates a weak coil.
The high-tension wire that connects your Jaguar S Type ignition coil to the distributor top pole can have high resistance. It can also become loose or become corroded at the connection points. This can lead to a weak-spark condition, causing your Jaguar to run at lower rpm and completely die out at higher speeds. Burnt or shorted-out high tension wire can also result in a no-start condition. To diagnose the problem, you can check the wire for high resistance using an ohm meter. Any resistance observed indicates high tension wire failure.
This problem is caused by excessive spark gap. Spark plugs with large gaps will need extra voltage in order for the spark to jump from one electrode to another.
As it is well known, a coil that has failed completely will result in a no-spark and no-run condition, to troubleshoot the problem, you need to disconnect one of the spark plug wires from the spark plug. In vehicles with coil-on-plug setting, you need to take off the two 10mm bolts and detach the coil from the plug. Insert a tester spark plug and lay it on a good ground of your engine. You should ask someone to start the engine, so you'd be able to observe the spark produced. No spark means the coil has totally failed and is now in need of replacement.