My wifes Jag occasion while at a stop light turns off and wont restart. will show power at all then after a minute or two it will let you restart

 

Helps us alot to answer your questions if you provide the MY (model Year) and model of your car... for example MY2017 base 6 cylinder, manual transmission, premium.

Your battery may be old and for AGM (absorbent glass mat) batteries, their life span is only 5-6 years, though there are owners here who have gotten more out of them. These cars are very sensitive to insufficient voltage, due to all their electronics. If the car is only driven for short periods of time too frequently, the battery won't be charged sufficiently.

 

I think the older cars had 2 batteries - the second smaller one specifically for the stop/start function. Could be that one or both are near DOA status.

 

Your handbook should list all the criteria that is considered before stop/start will activate, but yours sounds broken if it actually stops and then won't restart. Any codes stored?

 

If you take your seatbelt off while start/stop is enabled, it will completely shut down, requiring pushing the Start button to restart. Maybe you have a bad seatbelt switch?

If you have a 2nd battery (MY14-15, I think), start/stop is disabled if the 2nd battery is dead. I removed mine a few years after it died.

 

No, if second battery is dead, it will cause issues like not restarting. If second battery is removed (mine) then start stop is disabled. Jag says a dead second battery will disable it and this is up for debate, ask me how I know.

 

If the second battery is dead the SS simply stops working. It won't activate the SS. I have had a dead small battery as the positive post corroded all the way off! Did not know it until I was in the trunk either.
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Agreed.
No way a flat secondary battery will allow the system to turn the engine off, it just stops the system from doing anything at all.

 

Maybe it depends on the battery death? If the battery just has no charge, no problem, but if there's an internal failure maybe it upsets the S/S logic.