The rule of faulty earths is if operating one circuit causes another to go ON or OFF its an earth fault.

There are two basic methods.
You need (for a jag) a 20 ft length of wire with a crock clip on each end .You clip one end on the battery negative as that is the only 100% earth point and you put the other end on what should be earthed . If that puts the fault right then the earth you touched is faulty.
Now with most cars the earths are black so spiking black wires with a probe will find you the fault . Jags are not most cars so consult the diagram .The best probe is a double-ended pin chuck with a sewing needle in it . For the inexperienced, it may be an idea to put a fuse in the 20 ft line as you may proble a live wire by mistake ! ( or your finger)
The other way is to have a 20ft lead on your voltmeter set to 20v . Clip on to the battery neg and probe with the other meter probe . When both ends are earthed the meter will read zero ( well maybe up to 1v) but for a broken earth it will show battery volts. A bad earth rather than broken 6v or more


Pin Chuck with Needle or hypodermic syringe needle for those of you addicted ( to jaguars )

 

Good Stuff... I have had my XJS for a couple few years now and have never (except when cranking) seen the speedometer move. I've suspected and tried different things for checking ground, this will be the next tool in the bag O tricks I'll set up before hand, when I dive in again, come spring. Thanks!

 

Send me a link to the speedo /dash wiring for your XJS and I will advise .. You should use the technique with the voltmeter as whatever you touch will not do damage due to the high resistance of the meter.