I’ve never actually done a compression test on my 1992 148k miles XJ40 3.2 Auto that I purchased in 2008. But over the last couple of months I’ve done them on old fashioned coil ignition cars. So just one point to discuss before checking out my Jag’s compressions.
In the Haynes manual it gives the usual instructions - pull the fuel pump relay, jam the throttle wide open etc. As to the spark, they say to ground the king (centre) lead of the distributor securely to the block by making up a lead with a crocodile clip on it to attach to an earthing point. Seems easy enough. But although I completely trust Haynes, I just want to double check that the cars electronic ignition will be happy with this. The alternative is simply to remove the ignition lead and secure it safely away from people or the body etc. Or have the ht “open circuit.” Some electronics can be damaged by running them with no load, others by shorting the load.
Anyone have the authoratative answer? Grounded or open circuit?
I’d just pull the rotor …you aren’t going to be cranking for any length of time so I wouldn’t worry.
The advice to ground the spark is particularly relevant to electronic ignitions. With no rotor, the spark generated by the coil secondary has no where to go. This lack of “load” is reflected into the coil primary. The unloaded primary winding generates a very large “back EMF,” which can become ~600V rather than the nominal ~300V, Output transistors on electronic ignitions are pretty good at absorbing this abuse (caused by disconnected sparking plugs, for example) but not perfect. I’m not an XJ40 person, but the preceding XJ6 saloons had within their AB14 amplifiers a ~600V Zener diode to absorbs excessive back EMF before it could damage the output transistor. Still, it is far safer to avoid the situation by grounding the coil secondary. The coil can’t tell the difference between a direct ground and a jump to ground via a 0.035 inch air gap. IMHO.
I can see from what Robert says why the Haynes manual emphasises how important it is to ensure that the spark is grounded with all the plugs out - the energy would otherwise all be pumped into the coil with unpredictable results. Admittedly cranking periods are relatively short in these tests but no point risking the electronics unnecessarily. So grounded it shall be!
Thanks for all your responses.
I undo the +12 volt lead at the coil or unplug the ignition module to fully remove all risk.