Then the relay isn’t being triggered. Or, it is being triggered but is faulty and not making the internal contact between 30 and 87
You could. At least you’d know you have a good ground
Don’t do this
(Assuming you mean “+” battery voltage) this is obviously wrong. Not sure I understand.
Battery “+” voltage on a ground bolt would mean smoke and fried wires.
Cheers
DD
Doug, a question: being as the other voltages you referenced at 12.7 or battery voltage (if not fully charged to 12.7), I am presuming that you are measuring to “ground” (battery negative and/or chassis connection).
With that presumption of measuring to ground … I’m not following your “Terminal 85 is the ground. If tested with a voltmeter it will show “-” battery voltage”? If one is measuring “ground” to “ground”, there should not be any voltage.
Unless, of course, there is a poor ground connection, or high amp draw, such as when the starter is cranking the engine over, as a high amp draw will create a minor voltage drop across a “conductor” (such as a copper wire or the steel chassis) as the “conductor” is also a “resistor” due to the limited resistance of the conductor (that old E=IR stuff).
Or are you measuring “Terminal 85 is the ground” to a 12V positive source? I read through the posts, but I may have missed the reference to what that voltage measurement was being made to, if not measuring to ground.
That’s it.
Sorry for the confusion. Not the best way to express things. I think of ground as “-'” voltage and a voltmeter will show it that way depending on how you arrange your leads.
In reality a 12v test light can be easier to use in such situations where you’re simply looking for the presence or absence of voltage. Sort of like a go/no go thing. Then a volt/ohm meter for more nuanced checking
Cheers
DD
#86: 12v
#85: direct path to ground when energised, better checked with test light or continuity tester
#30 and #87 is a NO switch, ON when energised
#30 and #87a is a NC switch, OFF when energised
better checked with test light or continuity tester.
Doug
The ground voltage numbers were (-) I used a Dremel round the grounds to get bare steel I then placed the four ground wires back where they were, two to each bolt. Next I found why I was nor getting a voltage reading from main relay terminal 87. The relay has to be pushed a bit deeper to make contact but then there is no space to get a reading. Had a helper turn on ignition while i lightly held the relay, success, it clicked and pulsed as the ign key was turned on, the fuel pump motored for the requisite few seconds.
Next I did a continuity check on the wire from terminal 85 to the inertia switch, (disconnected at each end), Next I bypassed the inertia switch with a loop.
Finally fired the engine up and ran the car up and down the drive for 20 minutes. Drove back into the workshop, turned engine off and restarted it. Will wonders never cease, it did not go into the dreaded no start mode. Left it for 15 minutes, unbelievingly all the dash lights lit up and engine started again.
As no fault has been found I am not convinced the nightmare NO START issue is eradicated. But the above successful restarts were encouraging.
Roger
Good work.
I’m very familiar with the uncertainty that comes of not finding a conclusive fault. I think we all are!
Loose/dirty/corroded grounds and connections are a common issue on older cars…and the main root source of the infamous Lucas problems. 
Cheers
DD
Good work and results.
Is it possible that the relay socket is worn and making unreliable connections (i.e., relay socket is bad)?
You can get relay sockets at NAPA (when I was in Daytona Beach, Florida, none of the other auto parts places had them in stock or could/would even order them). The sockets come with leads attach.
Or you could use spade lug connectors on the wires and just plug onto the relay (I’ve also done that).
For using a test ligth or meter to read down into connectors, or where a much smaller test lead is needed, straighten out a paperclip, place the paperclip on your test lead, clamp a straight section to the lead with pliers, then make a few tight wraps of the paperclip aroud the test lead, leaving about half of the paperclip sticking out as your test lead.
Allows you to get that test lead down into small tight places.
Doug
I had brain wave as to the possible cause. FAULTY IGNITION SWITCH. Every no start occurrence is preceded by the line of dash lights being out. Looking at the relevant wiring diagram the second position of the ign switch seems to power the dash lights and the starter relay. If the dash lights are out the starter relay is also off. Hence turning the key to the start position nothing happens. However how on earth does it take a main relay swap to get it going.???
Ignition switch failures are not unusual on these cars, as I mentioned a few postings back.
Also not unusual to have multiple failures or partial failures at the same time, confusing us!
Cheers
DD
I was away for a few days and came back to check in on this one-
An amazing story, and one that gives just another scenario leading to no-start situations. These can be dreamt up almost without end with these cars!
Good work!
A good test of ignition switch, is to wiggle the key a bit while engine is running.
It’s not too hard to pull ignition switch and take it apart. I checked mine out and it had some corroded connections worth cleaning. Inside uses copper. So can wear out.
Doug
Here is one for the books re the ignition switch. A used switch arrived a fortnight back. I removed the switch off my V12 to compare with the replacement. Rotating the switches with a small screw driver showed that the replacement was rubbish but the one off the car appeared to be perfect. Nonplussed I examined the five pin male/female connector.
The plain white 12g power wire connection showed signs of over heating. Multi meter showed considerable resistance. This caused the 12v to the main relay to be low or periodically AWOL which of course was the reason for the exasperating no start and cut-out failures.
I spliced a 10g loop across the faulty white terminal.
Completely cured the problem, car no starts and runs as the blokes at Coventry intended. I am like a dog with a dozen tails.
More to this story.
I have here three used ignition switches, ALL three have the same fault. The terminal of the white power wire all have erosion of the male pin and the female receiver. SEE PHOTO.
This suggests the problem is endemic. I intend to replace the Lucas 6 pin connector with a bus bar thus curing this problem for good.
Mine was similar, i just cleaned it up and put it back.
There are tons of issues like this on the XJS. While other car manufacturers were using more modern sealed connectors by the 80s/90s, Jaguar was stuck in the 1960s using simple spade connectors and unsealed connectors etc. that allow plenty of moisture to creep in.
Same goes for paper gaskets. Ford sorted that out in the 90s, and luckily they can be fit to earlier XJSs.
I cleaned mine up but it was too thin to make contact. This switch corrosion was a major problem that took a long and expensive session to sort out. The only cure was a loop wire past the corroded terminal.
It was truly looking for a needle in the hay stack situation
