Service Start/Stop System

[Bob Burd]

It will not work, if you don't have the update from the dealer, that allows the vehicle.to start when the Aux Battery is malfunctioning. I don't recall what it was. I had my JL updated a year or so, after buying it. I know a friend who had to have the update, and that solved several issues, and allowed him to also do the bypass.

This is incorrect. I've bypassed the aux battery on my 2018 JLUR and never had the software update. Works like a champ.

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[DaltonGang]

This is incorrect. I've bypassed the aux battery on my 2018 JLUR and never had the software update. Works like a champ.

Are you sure they didn't update it when it has been in the shop, at some time or another?

 

[Bob Burd]

Hasn't been to a dealer since I bought it.

Are you sure they didn't update it when it has been in the shop, at some time or another?

 

[alphawolff]

This is incorrect. I've bypassed the aux battery on my 2018 JLUR and never had the software update. Works like a champ.

That update wouldn't apply to you anyway because you bypassed the aux battery, lol.

In the original configuration during the engine crank the entire interior vehicle's load is momentarily shifted over to the aux battery. If the aux battery is low voltage this would cause a voltage spike in the interior resulting in the start attempt stalling and the vehicle not starting. At some point they updated this logic so that if the start attempt fails on subsequent starts this check will be bypassed and the PCR is not energized.

This "test" is actually the only time the PCM is able to determine the actual health status of the aux battery. It's what causes those battery "b" state of charge performance DTCs.


Do note that even with the update if your aux battery is *completely* dead there is a solid chance the vehicle still doesn't start on subsequent attempts. I think there needs to be at least some voltage for the BCM to acknowledge the aux battery is in fact dead and to trigger the failsafe mode. This is why when jumping a dead JL you sometimes need to jump the N3 post of the PDC to get to the other side of the PCR and supply both sides with power. Though now that I think about it you could just pull the F42 fuse as well and just jump the main battery like usual.

Hell, someone should put that in an FAQ somewhere. Jeep won't jump start? Pull F42 and try again. If the z-case fuse array isn't blown it'll nullify that entire test and should start. Assuming it's a battery issue, anyway.

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