Ya, I know... but I just wanted to make sure.
To disable Stop-Start operation without otherwise effecting the car I should disconnect the negative cable from the battery.
Discuss...
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My karma just ran over your dogma
Current Hers: '13 Lexus ES350
Current Mine: '15 Jaguar XF (its aubergine)

 

Two ways to do it on a car with the secondary battery - disconnect either the negative lead OR the earth lead (from the secondary battery not the main battery!)
At least that's how it works/worked on my F-Type, I disconnected the earth lead which runs from the negative terminal from where it bolts to the floor of the boot/trunk over three years ago now and the only effect it has ever had has been to permanently disable the damnable stop/start, without a single problem in all that time.
It should work the same on the earlier X250 with the dual battery set up, although can't see an earth lead running from the negative terminal of the secondary battery in the above pic. If it doesn't have an earth lead then just disconnect the negative cable and tie it up in such a way that it cannot contact any metal part(s).

 

>can't see an earth lead running from the negative terminal
My hunch is that its the black cable that comes out from behind the battery and connects to the bracket at the top.
>It should work the same on the earlier X250 with the dual battery set up
Thanks I'll try that.
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Current Hers: '13 Lexus ES350
Current Mine: '15 XF (X250) Portfolio AWD
Jaguar - making mechanics out of customers since 1935

 

If you disconnect the auxiliary battery in an X250 you will get an error symbol on the dash, it's not quite the same as the other models.

 

Just hit the warning triangle to remove the message/symbol, although you will have to do it every time you start the car.
A small price to pay to banish the 'orrible stop/start system forever!

 

>If you disconnect the auxiliary battery in an X250 you will get an error symbol on the dash
Drat, but thanks Cambo.
But hey are the fires still burning near Sydney? I find it amazing that California and Oz are burning at the same time..
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Jaguar: Grace, Pace, and Space - Sir William Lyons
Current: '15 XF (X250) Portfolio AWD 3.0 (it's aubergine)
Past B: '08 S-Type 4.2 "Satin Edition" (250.06 whp / 259.67 torque)
Past A: '05 X-Type 3.0/auto Jaguar Racing Green

 

From what my local jag dealer told me last week....the auto stop/start feature will soon be on all their cars with no way to deactivate it. Enjoy the freedom now....

 

I would hesitate to trust any statement from them about the future!