XJ8 (308) Mystery failure to start

[peterzoom]

I had a strange occurance yesterday when my XJ8 refused to start. I had just completed a 100 mile motorway run and had parked in the supermarket for about 15 minutes.
To my total surprise when I got back to the car and turned the key, all the usual lights and then........ nothing!
Tried switching off fully, key out and tried again. Still the same, all the lights on the dash as normal but no life in the starter at all, not even a click.
Went through everything I could think of ( re-positiong the gear selector, getting out of the car, locking it, unlocking it (with the remote and then just the key) Checked the fuse boxes and gave the relays a wiggle to make certain they were fully seated, left it for 10 minutes then tried again, nothing!
This time though, remote would not arm or disarm nor would the central locking work when tried with the key alone!
Called the breakdown people and left the car to them and went home by taxi.
Several hours later they called to say they would still be several hours more (just perfect... wishing I had said I was single/lone/pregnant/teenage mum now!)
In frustration, made my way back to the car and as soon as I pressed the remote the car unlocked, tried the ignition and it started first time.
Went to National tyres today to have the (newish-2yrs old, Varta) battery tested.
The battery was perfect, but the fitter felt the charging rate at tickover (13.0 v) was too low and that was likely to be the problem.
I do think that value is low, but is it likely to have caused the problem or what should I be looking for? I feel it is unlikely that the battery, if drained by undercharging on a daylight motorway run would spring back to life when the car was now cold.
I know there is a wealth of genuine knowledge and expertiese here so it would be great to have your valued input.
Thanks, Peter.

[Tame Technician]

Are the battery terminals tight and corrasion free?

Same for the battery earth strap to the body.

13volts at idle is very low, depending on what you had turned on consumer wise.

If thats ok, Get a volt meter and do some checks.

Test (1)
What is the battery voltage with the engine off
(should be 12.7, depending on how much charge in battery, over 12 should still start the car.)

Test (2)
What is it with engine running but all all electrics and A/C switched off.
(should be 13-14 depending on how much charge in battery. A lesser changed battery should give a higher voltage now as the alternator works harder to charge it)

Test (3)
What is it with engine running and everything, lights blower heated screens seats etc, turned on. (will be less than test (2) but should be more than the battery voltage with the engine off,)

Edited by Tame Technician on Tuesday 20th April 00:17

[Simpo Two]

Wow, even my car takes about 5 hours to discharge a battery fully when it gets Random Flat Battery Syndrome!

Edited by Simpo Two on Tuesday 20th April 19:34

[peterzoom]

Thanks Tame Technician for the info. I did as you advised and the results were
Test 1. 12.7v and 120%
Test 2. 14.5v
Test 3. 13v

Since this mystery failure the car has been faultless and starts as it always did before, first spin every time!

A conspiracy theory has been floated past me. It goes like this...
Your car may have been affected by the loop that imobilises the supermarkets trolleys as they reach the car park perimiter or another alarm on a very similar frequency or the aircraft turning beacon that is situated 50 metres away! Take your pick!!

All I seem to know for certain is that the car is pefect now and the battery and charging system seem to be blameless.
Still there is a little niggle in the back of mind as I await a repeat episode

[Tame Technician]

The test results are good.

Keep the multi meter in the car.

If it happens again, check the battery voltage. If the battery voltage is still good, but the car apears to have a flat battery (no cranking no remote functions etc), there will be a poor conection somewhere.

I take it the battery connections were tight?

[peterzoom]

Thanks Tame Technician for getting back to me on the results, I really value your advice.
Yes the battery and all connections are tight and have a thin coat of Vaseline over them to keep out the elements. Not that there is any damp in the battery compartment. Earth straps are the same, the only one that was in way way suspect was the bonnet earth and it wasn`t anything other than a few loose strands showing at the bonnet connection and I replaced it as a matter of course.
I will put up a posting if it happens again, don`t you hate random/intermittent faults? They do have a nasty habit of playing on your mind!

[mikey77]

I'm no technical expert but I used to get a similar problem very occasionally with my Honda and I figured out it was the immobiliser. If I got out, locked the car with the remote and unlocked it again it always started without a problem.
Just a thought.
Oh, I do have a Jaguar as well...

[smallgiant]

I have a similar problem which stared 6 months ago. resulted in car not starting for past two weeks. I don't believe it has anything to do with power as by running live lead to starter motor everything was turning over fine. A jag specialist said it could be gearbox inhibitor related but checked all relays/fuses as best i could. I am now suspicious of the key/ignition switch area as in moving the bottom part of the steering binnacle resulted in a start first time. something must be loose soemwhere. I also saw on another forum that each key for the jag has its own identity which could mean its worn out or corrupted.
I am now using the spare key with no issues however i do not trust the car. So I'm going to get a laptop check and see what the onboard computers say. I reckon its a good old fashioned worn out key barrel or ignition switch. will keep you posted.