auto stop can only last about 10 seconds and it automatic turns on.
battery shows 14.8-14.9 when driving.
i drove 150 miles and doing the same.

 

Lots of factors can affect the auto stop/start and not just battery state. If you look in the handbook there is a list of exceptions and pre-conditions but there will be others ie the DPF recirculating on the diesel. Vehicle also needs to be at operating temperature.

Are you getting the yellow 'A' symbol signifying the stop/start is not engaged? If so this means it's operating normally. As I like you thought mine was faulty.

There is no indication when mine will or will not function. Some days I can do a 100 mile continuous high speed motorway/freeway drive and when I come to a stop it wont operate, other times it will. Short journeys similar inconsistency.

 

But my battery charging voltage alway 14.8-14.9. Never go down below that. Is that normal?

 

How fast does the voltage drop when the it's in the Auto Stop mode?

I know mine would work much better if I had it on the charger the night before so it's really a matter of how healthy the battery is and not the charge voltage alone.

 

Drops to 12.8v in 10 secs

 

That's about right, it's where mine gets to and doesn't drop below this.

Take tonight for example I'd just done 100 miles 70mph driving along the motorway and come off onto town roads into traffic. Stop/start activated appox 6 times over the next 10 miles of traffic with natural stops. Thereafter I get the yellow light and no more stop/start for the remaining 5 miles home in rush hour.

Having monitored how stop/start works I cant decide when it will and will not activate but summed up this is normal given there are no fault codes