AUX Delete getting cancelled by accessory ground

[Tr4ckD4ys]

So I recently stated on here that I’ve finally pulled F42 and isolated the AUX negative cable from the IBS, zip tied away. I did so after one morning I woke up and the car wouldn’t start, and the jump starter wouldn’t power it up either.

However, when working on my vehicle today, I saw the dash lights were on and my driver seat was still electronically movable. That is despite me having fully disconnected and isolated both positive and negative terminals from the main battery. What is going on here?

1. If the AUX has been F42’d, and the main is disconnected, where is the power coming from?
2. If the AUX was dead the other day, and the power in the above case is coming from the AUX, how is this possible? Did it pull charge from the main only by having its positive terminal still connected?
3. Given both batteries are seemingly fine, should I reverse the F42ing of my AUX until it’s actually dead?
4. If I keep my AUX F42’d, how do I properly disconnect the battery at this point? Only pull negative as if AUX is still there? That seemed to have worked in the past when F42 was still there.

I’m genuinely confused at this point.

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[58Willys]

If you still have aux battery installed, there is a negative cable going to a ground inside the engine bay, near the right fender. You need to disconnect this. At least that’s the way my ‘18 is setup.

 

[Cips]

There are others more knowledgeable than me on this, but it sounds like you still have Aux POS connected, correct?

So it's pulling power from the main battery/vice versa. You need to properly disconnect both POS and NEG from the aux battery.

 

[Tr4ckD4ys]

There are others more knowledgeable than me on this, but it sounds like you still have Aux POS connected, correct?

So it's pulling power from the main battery/vice versa. You need to properly disconnect both POS and NEG from the aux battery.

So I thought the point of the simple F42 pull situation was not having to dig into the engine bay to get to the aux and disconnect there. If what you are saying is accurate, then my AUX clearly is still killing my main. I’ve been wondering why the Jeep has been showing 14.5V consistently on my dash when I drive, as if it keeps charging the battery. Now it would make sense if the aux keeps drawing from the main despite the negative cable isolation and F42 pull.

 

[Cips]

So I thought the point of the simple F42 pull situation was not having to dig into the engine bay to get to the aux and disconnect there. If what you are saying is accurate, then my AUX clearly is still killing my main. I’ve been wondering why the Jeep has been showing 14.5V consistently on my dash when I drive, as if it keeps charging the battery. Now it would make sense if the aux keeps drawing from the main despite the negative cable isolation and F42 pull.

There's a POS cable that goes from the Aux to the main, you gotta pull that off and isolate.

 

[THAW]

So I thought the point of the simple F42 pull situation was not having to dig into the engine bay to get to the aux and disconnect there. If what you are saying is accurate, then my AUX clearly is still killing my main. I’ve been wondering why the Jeep has been showing 14.5V consistently on my dash when I drive, as if it keeps charging the battery. Now it would make sense if the aux keeps drawing from the main despite the negative cable isolation and F42 pull.

Disconnecting one cable from a battery breaks the circuit for that battery (i.e. electricity cannot flow to/from the battery). Therefore, you do not need to remove the positive cable from the AUX (ESS) battery.

Removing fuse F42 prevents the JL from breaking the connection between the CR[an}K battery and the system electronics whether the AUX (ESS) battery is connected or not.

That said, based on your description the AUX (ESS) battery appears to still be connected somehow. Maybe post some photos of the connections at your CR[an]K battery.

 

[azwjowner]

You must have removed the wrong cable from the negative battery post. Likely you disconnected the cable that runs to the body ground at the top of the fender nearby instead of to the aux battery. You can verify this by looking at the cable you did disconnect and see where the other end goes.

 

[Tr4ckD4ys]

Disconnecting one cable from a battery breaks the circuit for that battery (i.e. electricity cannot flow to/from the battery). Therefore, you do not need to remove the positive cable from the AUX (ESS) battery.

Removing fuse F42 prevents the JL from breaking the connection between the CR[an}K battery and the system electronics whether the AUX (ESS) battery is connected or not.

That said, based on your description the AUX (ESS) battery appears to still be connected somehow. Maybe post some photos of the connections at your CR[an]K battery.

You must have removed the wrong cable from the negative battery post. Likely you disconnected the cable that runs to the body ground at the top of the fender nearby instead of to the aux battery. You can verify this by looking at the cable you did disconnect and see where the other end goes.

This is the cable I removed (held by a 13 nut), which in the past (when I still had F42), when isolating, would completely cut power from the Jeep (if the whole negative battery terminal harness was also isolated).

 

[AndySpill]

This is the cable I removed (held by a 13 nut), which in the past (when I still had F42), when isolating, would completely cut power from the Jeep (if the whole negative battery terminal harness was also isolated).

Given that you have a 2025, I am inclined to believe you removed the wrong cable: the one you removed being the body ground (which should remain connected) and whose other end connects to the body ground on the passenger's front panel just under the hood. I believe the other cable should have been the one you removed, keep the cable shown as detached, attached.

Please refer here: https://www.jlwranglerforums.com/forum/threads/ess-battery-cable-change.86624/

 

[azwjowner]

This is the cable I removed (held by a 13 nut), which in the past (when I still had F42), when isolating, would completely cut power from the Jeep (if the whole negative battery terminal harness was also isolated).

At least on my 2022 the cable you circled is for the main battery, not the aux. Take a look at the negative cable that's currently connected to your main battery. Trace it with your fingers. Does it go down under the power box or does it go across to a ground on the right side next to the battery and in front of the power box (pictured below)?

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[BRuby]

Here is our 2018 showing the alligator clamp connected to our aux neg.

Pull out your multi and check which leads are providing voltage and go from there. You should be able to get it sorted pretty quick after that.

 

[AndySpill]

Here is our 2018 showing the alligator clamp connected to our aux neg.

Pull out your multi and check which leads are providing voltage and go from there. You should be able to get it sorted pretty quick after that.

To reiterate my post, what is the correct cable for you and me ('Im also a 2018 JL owner) is likely the opposite cable for the @Tr4ckD4ys, a 2025 owner.

It was thought that the cables were swapped some time in 2021:

https://www.jlwranglerforums.com/forum/threads/ess-battery-cable-change.86624/

 

[THAW]

You must have removed the wrong cable from the negative battery post. Likely you disconnected the cable that runs to the body ground at the top of the fender nearby instead of to the aux battery. You can verify this by looking at the cable you did disconnect and see where the other end goes.

Given that you have a 2025, I am inclined to believe you removed the wrong cable: the one you removed being the body ground (which should remain connected) and whose other end connects to the body ground on the passenger's front panel just under the hood. I believe the other cable should have been the one you removed, keep the cable shown as detached, attached.

Please refer here: https://www.jlwranglerforums.com/forum/threads/ess-battery-cable-change.86624/

Removing the wrong cable kills all power to the Jeep. OP stated he has power at the dash and power seat.

 

[Badunit]

I am genuinely confused, too.

• All current should flow through the IBS so it can be monitored. Therefore, all battery grounds should go through the IBS. There should be no grounds from either battery that do not go through the IBS. My 2023 wiring diagrams shows no extra grounds.
• If the battery/body ground was removed and isolated instead of the aux battery negative (which I think you did), you should have no power even with the main battery connected. But somehow you have power.
• If, instead, the aux battery negative was removed and isolated you would have power from the main battery, but not if you then removed the IBS from the main battery negative post. But somehow you have power.

Either way you should not have power but you do. I'd love to hear how this turns out.

 

[Badunit]

Looking at the 2023 wiring diagram, the Power Control Relay has the AUX and Main positives connected unless the relay is energized. If fuse F42 is removed, the relay cannot be energized so the two battery positives will always be connected. That is a good thing because the hot wire from the Aux battery to the power distribution center at N1 runs a lot of stuff (which is not shown in the image below).

My green scribbled line shows the connection between the two positive terminals. I also circled fuse F42, which is in the path that would energize the relay to isolate the batteries.

That's how I interpret it, anyway.

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