AUX battery delete/bypass - Which method is proper?

[AndySpill]

A volt meter may be able to rule in a bad battery, provided a known working charging source fails to bring the battery's voltage high enough, but it won't rule at a bad battery, the latter reserved, IMHO, for load testing.

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[Mguy]

Deleting the aux battery using the F42 fuse method does not change the Jeep system. The stop/start should still work. If it's not working, chances are your main battery is not putting out enough voltage to satisfy the go/no go tests that a JL/JT Jeep does as it cranks up. In my experience, the systems start generating error messages when the battery gets near 12.4v (disconnect the battery, let it sit overnight, and test with a volt meter).

Before doing this open circuit test, make sure the main battery is fully charged by either 4 or more hours of engine operation or use of a 3 stage charger set for AGMs.

 

[andy29847]

Before doing this open circuit test, make sure the main battery is fully charged by either 4 or more hours of engine operation or use of a 3 stage charger set for AGMs.

I considered the poster's statement that he was commuting to work and his drive was 1 hour each way. His batteries should be charged. Still, your advice is good advice. Verification of the charging system is the one thing you can get out of the Jeep voltmeter. Normally, after the first start of the day, the voltmeter will show something north of 14v.

If you take your Jeep to the dealer when you are having battery trouble, one of the things they should do is isolate both batteries, charge them, and then load test them. One of the problems with this procedure is that old batteries will pass the test, but will not support the ESS Stop/Start system. That is one of the reasons that many people having battery trouble end up making multiple visits to a dealer.

It actually took 5 visits to my dealer to get my battery problem solved. Here are the write ups for the first 4. The 5th one was because when they replaced my main battery, they used a filled lead acid battery (not and AGM). On the 5th visit I got 2 new AGM batteries. BTW, my Jeep was a 2020 and this service occurred over a 6 month period near the end of my warranty.

 

[MeefZah]

Just completed the 42 fuse / aux battery neg cable disconnect to hopefully stave off main battery issues on my new 24. I read most of all of the various threads on it before taking action. Thanks to all who contributed information.

On my 24 the aux negative cable is the one on the left, looking down at the battery from the front of the Jeep. It's the one with the larger bracket and threaded bolt attached to the bracket.

I had no issues before and I had no issues after with the weird caveat that my clock forwarded 4 hours and my outside temperature went up 30 degrees. I reset the clock and the temperature went to normal after a few minutes.

 

[Rhinebeck01]

Just completed the 42 fuse / aux battery neg cable disconnect to hopefully stave off main battery issues on my new 24. I read most of all of the various threads on it before taking action. Thanks to all who contributed information

On my 24 the aux negative cable is the one on the left, looking down at the battery from the front of the Jeep. It's the one with the larger bracket and threaded bolt attached to the bracket.

I had no issues before and I had no issues after with the weird caveat that my clock forwarded 4 hours and my outside temperature went up 30 degrees. I reset the clock and the temperature went to normal after a few minutes.

@MeefZah

Suggest you routinely trickle charge (maintain) the bypassed aux battery.

When you take your JL to the dealership service department for work, do hook the Aux back up for that visit. After the visit, bypass the aux once again.

 

[AndySpill]

Just completed the 42 fuse / aux battery neg cable disconnect to hopefully stave off main battery issues on my new 24. I read most of all of the various threads on it before taking action. Thanks to all who contributed information.

On my 24 the aux negative cable is the one on the left, looking down at the battery from the front of the Jeep. It's the one with the larger bracket and threaded bolt attached to the bracket.

I had no issues before and I had no issues after with the weird caveat that my clock forwarded 4 hours and my outside temperature went up 30 degrees. I reset the clock and the temperature went to normal after a few minutes.

As stated, not keeping the Aux battery charged risks, when its reconnected prior to service visits, its cannibalization of the main battery.

Placing your charger on the main battery's postive terminal and the now temporarily exposed dangling cable you detached will charge the Aux battery only.

In fact this very talk makes me wish there was a way I knew of to have owners of dual AGM battery JLs to hook the Aux up only to the alternator's power without interfacing with the main battery..probabaly easier said than done..

 

[MeefZah]

So dumb question just to validate my own understanding here - to jump the JLU with the aux battery deleted we are using the traditional means to do so - the main battery terminals, yes?

And to jump it with the aux battery connected you do not use the positive battery terminal but instead use a specific terminal on the bar to the side of the battery?

 

[AndySpill]

I think that placing your jumper cables on the main battery in both cases is indicated: the charger charging both batteries prior to crank if the Aux batterY is connected as per factory spec.

This approach benefits from a point of understanding.

Prior to cold cranking a dual AGM battery JL with Fuse 42 intact and both batteries connected and no fused jumper between N1 and N2 in the Power Distribution Center (PDC) the vehicle will energize a normally closed relay to separate the batteries and test the Aux battery in isolation.

If adequate power ( a threshold less than that needed for ESS events) in the aux battery is detected the batteries are reconnected and both energize the engine crank..

if the Aux battery fails the test the crank will be aborted but the next attempt to crank will be done solely against the main battery for all but the earliest of 2018s (which won’t crank). If the main battery has adequate power the vehicle turns on the ESS off button in the dash (the “A” icon with a near circle around it and a slash thru it) and will forever more continue all future cold cranks on the first try against only the main battery.

Only the next cold crank, if any, when an energized Aux battery is detected will the light on the dash turn off and normal cranking procedure resume.

all this said, for you , you can throw this 411 in in the trash.

I say this because you’ve pulled fuse 42 so the aux battery can’t be isolated.

You need that fuse to energize the aforementioned normally closed relay.

Your pre crank test is against both batteries or, since you disconnected the Aux, just the main battery, which for the precrank test your JL thinks is the Aux battery.

now, all this aside, if I were stranded with limited jump starting capability I might do this.

id put my jumpers on the positive of the main battery and the detached cable and reinstall fuse 42.

my jump starter’s power would be put solely in the aux battery, which when charged—and it takes less to charge because it’s smaller than the main, can alone crank the JL.

 

[Zjamison]

2018 JLU sport, manual with the 3.6
So I have just replaced my main battery with the larger h7 and tried the aux battery bypass.
Pulled the smaller negative cable from the main battery to the aux and fuse f42. Now I have no power at all. Checked the n3 fuse and all is good there. When I put the aux negative cable back on the main battery, everything works as it should.
I’m worried that the aux is going to kill the new battery which is why I tried the bypass.
Any ideas as to why it would not be working?

 

[Jay Gatsby]

2018 JLU sport, manual with the 3.6
So I have just replaced my main battery with the larger h7 and tried the aux battery bypass.
Pulled the smaller negative cable from the main battery to the aux and fuse f42. Now I have no power at all. Checked the n3 fuse and all is good there. When I put the aux negative cable back on the main battery, everything works as it should.
I’m worried that the aux is going to kill the new battery which is why I tried the bypass.
Any ideas as to why it would not be working?

You most likely pulled the wrong negative cable. This procedure has worked for everyone. Jeep swapped the placement of the two negative cables in 2021 I think. Make sure you are removing the right one.

 

[Zjamison]

2018 JLU sport, manual with the 3.6
So I have just replaced my main battery with the larger h7 and tried the aux battery bypass.
Pulled the smaller negative cable from the main battery to the aux and fuse f42. Now I have no power at all. Checked the n3 fuse and all is good there. When I put the aux negative cable back on the main battery, everything works as it should.
I’m worried that the aux is going to kill the new battery which is why I tried the bypass.
Any ideas as to why it would not be working?

You most likely pulled the wrong negative cable. This procedure has worked for everyone. Jeep swapped the placement of the two negative cables in 2021 I think. Make sure you are removing the right one.

Thanks!
Since mine is the 2018, the negative to the aux battery is supposed to be the one closer to the passenger side fender, not the one with the "L" bracket which is the one that gets pulled on the newer models. Since that wasn't working I switched them and everything seems to be working fine. I'm not sure why the cables are different on my 2018 compared to others. I'm wondering if there was an issue that the previous owner had requiring the harness to be changed.

 

[AndySpill]

2018 JLU sport, manual with the 3.6
So I have just replaced my main battery with the larger h7 and tried the aux battery bypass.
Pulled the smaller negative cable from the main battery to the aux and fuse f42. Now I have no power at all. Checked the n3 fuse and all is good there. When I put the aux negative cable back on the main battery, everything works as it should.
I’m worried that the aux is going to kill the new battery which is why I tried the bypass.
Any ideas as to why it would not be working?

Hi Zach:

Are you certain that the cable which you pulled wasn't the one whose distal end is connected to the vehicle's body ground location on the front passenger's quarter panel, and rather, as correctly effected in this procedure, the factory cable whose distal end is not described by the above, and therefore by process of elimination, the cable whose distal end connects to the negative post of the Aux battery?

Please follow the cable you've disconnected in insure that it's distal end, even if not visible (i.e. the correct cable to pull in this procedure,) is not the body ground.

 

[Zjamison]

Hi Zach:

Are you certain that the cable which you pulled wasn't the one whose distal end is connected to the vehicle's body ground location on the front passenger's quarter panel, and rather, as correctly effected in this procedure, the factory cable whose distal end is not described by the above, and therefore by process of elimination, the cable whose distal end connects to the negative post of the Aux battery?

Please follow the cable you've disconnected in insure that it's distal end, even if not visible (i.e. the correct cable to pull in this procedure,) is not the body ground.

Thanks!
I did figure it out. Even though mine is a 2018, the negative Aux cable was the the same cable as the 2021 and up models. I'm not sure why and I am wondering if there was work done while in the previous owners possession.

 

[AndySpill]

Thanks!
I did figure it out. Even though mine is a 2018, the negative Aux cable was the the same cable as the 2021 and up models. I'm not sure why and I am wondering if there was work done while in the previous owners possession.

Every since this post https://www.jlwranglerforums.com/forum/threads/ess-battery-cable-change.86624/ I've opted to use guidance wording that has the Aux battery deleting owner visualize the distal end of the cable that they are (not) removing from the main battery's negative post (i.e. the body ground cable) as this guidance is generic enough, I hope, to withstand factory cable changes across model years.

 

[Grayhound]

Is there harm in just connecting auxiliary POS and neg to the main without removing fuse 42?

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