The small AUX battery has nothing to do with how the car runs. The car operates perfect without that battery even being installed. Dozens of cars are running round without that AUX battery. All it does is keep the rest of the car alive when the engine is shut down by the SS system. It has nothing to do with starting the car either.
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Respectfully, I know that you have extensive knowledge and that many times that is correct, but on this one I have to disagree. Not sure why though... :-) Now, it is an unusual 2.0 litre turbocharged XJ, but the car now runs much better. The pick up from low revs is wholly improved. My Range Rover tech predicted this would happen and so it has... Odd indeed :-) :-) Equally odd, the start stop function still does not work... despite the new Exide EK 151 OE battery...

 

QP7, there are many circumstances impacting various sensors that inhibit the the stop-start from functioning on the x351 engine system. Here's the list that can affect it:

Auto Stop/Start System Inhibits



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  • Auto stop/start off switch has been operated by driver
  • Transmission not in neutral
  • Hood open detected
  • Driver door detected open
  • Driver seatbelt not fastened
  • All terrain progress control special programme selected
  • Climate control demand exceeds calibrated threshold
  • Heated windshield operating
  • Trailer connected
  • Brake booster vacuum below threshold
  • Engine coolant temperature below threshold
  • Engine oil temperature below threshold
  • Battery state of charge low
  • Battery cold cranking capability below threshold
  • Ambient temperature above 40°C
  • Ambient temperature below 0 degree C

 

FreddyJ,

In addition, for S/S to work it requires that no CEL conditions are set.... or pending. A P2187 or 9 will stop the S/S dead in it's tracks.

In 2016/7 JLR ditched the Aux battery and when for a capacitor discharge system. Very effective!

wombat

 

Freddy, that is very helpful - thank you.

Given I need to install a new brake vacuum pump, I think the brake booster vacuum being below threshold could well be it.

Ironically it also says battery low, but I have just put in TWO new OE batteries ! We will see how things develop once the new vacuum pump goes in this week...