Auxiliary Vs Main Battery issue - Puzzled

[Brownpow121]

Hi all. I have a 2021 JL, 20k miles. Bought brand new in 2021. Noticed the last few days the start/stop wasn’t working. Then this morning after sitting for a day it had a weak start. Then battery totally dead in a parking lot but still lights and radio. Just clicked when I hit the start button. Jumped it, threw a new battery in, fired right up and now the check engine light is on.

my confusion lies in
1. Should I go back in and change out the auxiliary battery too?
2. Why is the check engine light now staying on?
3.Do these two work in sync?
4. Is there any harm in driving it like this u til I get the new auxiliary battery?

I am surprised that this already needs replaced. Looking to lear a bit here and get this solved thanks!

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[HK1s]

1. your new battery will die quickly if you do not change in pairs.
2. startup aux battery test fails since it is still dead, or could be some saved error code.
3. they are simply connected, a dead aux will drain and kill a new main battery.

4. yes there is harm
but for now pull fuse 42 to turn off aux battery check, disconnect and wrap the thinner cable from negative terminal of the main battery to isolate the aux. band aid solution until you replace the aux or you can just drive like this without start/stop.

 

[alphawolff]

You need to either replace the aux or delete it entirely. If you wait too long the dead aux will make your nice shiny new battery a doorstop.

 

[Brownpow121]

Thanks all. Going to drive the wife’s Lexus until I get the Aux to replace. Appreciate it

 

[tomk62]

Common issue. Replace both batteries (or aux delete) as referenced in many other threads, and replace your main with a stronger battery ...

 

[Terrymo]

There are some great sticky threads in the Battery Talk subforum.

 

[Leisure Freak]

When I changed my battery the simple loss of power caused it to throw a non related code and the check engine light for Evap. It cleared after a couple hundred miles and multiple starting cycles. I was told by Jeep that it was common for the computer to land on weird codes whenever power is lost and you could be dealing with that. I do agree that if you change one battery, you should change both or bypass the aux battery altogether. I chose to pull the F42 fuse and aux ground cable to run only on the main battery. I never use the auto stop/start.

 

[Brownpow121]

I just went to replace my auxiliary battery (underneath my main battery) but there is no battery there.. am I going nuts?

 

[Brownpow121]

I am reading some other threads and they are saying the eTorque engine doesn’t have an auxiliary battery? I have the eTorque.

my check engine light is still on even with the new battery though. Welcome any thoughts. Thanks

 

[Jay Gatsby]

You have a 48v battery pack mid truck underneath. What DTC is it?

 

[Brownpow121]

DTC?

engine light is off now and start/stop working normal.

 

[HK1s]

DTC?

engine light is off now and start/stop working normal.

lol... you started by asking about auxiliary, pretty confusing

to identify etorque look for 3 separate coolant tanks on the right of the engine bay and no alternator, as the 12v battery is charged by the 48v system. in your case all electricity the etorque MGU generated went to the dying 12v battery so your 48v battery pack is also empty so start/stop don't work. the 48v battery is lithium no need to worry about that.

but the jeep starts on a conventional starter running on 12v for cold weather performance, hence the weak start and eventually unable to start.

 

[rfg9585]

my wifes jeep is behaving similar. cant get the battery to chg and start up. I went to pull F42 but it does not have an f42 fuse. do i just replace the main battery and not be concerned with the aux battery? 2020 jlu shara 2.0.

 

[Rhinebeck01]

my wifes jeep is behaving similar. cant get the battery to chg and start up. I went to pull F42 but it does not have an f42 fuse. do i just replace the main battery and not be concerned with the aux battery? 2020 jlu shara 2.0.

Undoubtedly your JL is an etorque JL...
It has a Main battery and no Aux battery and your JL will not have an F42 fuse.

Install a new H7 battery and you will remedy the issue probably..

Read here: https://www.jlwranglerforums.com/forum/threads/aux-battery-how-do-i-know.120115/post-2501518

and read here:
https://www.jlwranglerforums.com/forum/threads/??-main-battery-replacement-talk-jl-jt-?.100534/

 

[Terrymo]

my wifes jeep is behaving similar. cant get the battery to chg and start up. I went to pull F42 but it does not have an f42 fuse. do i just replace the main battery and not be concerned with the aux battery? 2020 jlu shara 2.0.

You can easily tell if you have the 48 volt etorque battery. It will be here. About the size of a small suitcase.

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