Jumperless Aux Battery Bypass

[AndySpill]

Perhaps written somewhere in the 57 pages, but I'll restate if it helps others. My 19 was reporting to service start/stop but kept it disabled, which was nice. I did the disconnect here and the service message continued. I disconnected the main battery for 10 minutes, reconnected and the ESS message ceased and the functionality resumed. I then purchased the eliminator from https://www.autostopeliminator.com/ and all is good in the world

I am surprised, based on this description, that it was necessary to disconnect the main battery as you did, with nothing else to get the message to go away.

I'm glad that you're good, but after you disconnected the Aux battery and presumably bypassed it with a Fuse 42 pull, the pre engine crank test of the Aux battery should have been diverted, unknown to your vehicle, to the main battery, which should have provided (unless it's on its way out) ample power to convince the vehicle that your Aux battery had ample power.

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[Y2KFirehawk]

I am surprised, based on this description, that it was necessary to disconnect the main battery as you did, with nothing else to get the message to go away.

I'm glad that you're good, but after you disconnected the Aux battery and presumably bypassed it with a Fuse 42 pull, the pre engine crank test of the Aux battery should have been diverted, unknown to your vehicle, to the main battery, which should have provided (unless it's on its way out) ample power to convince the vehicle that your Aux battery had ample power.

Dead on. Cannot agree more!

 

[Austin23]

This might be one of the most helpful threads on the forum. I went this route a couple years ago and it's been working flawlessly. However I recently had to disconnect the batter and ever since I reconnected it, I'm now getting the ESS Warning Light on my dash. It's been 3 weeks and it won't cycle off. I'm sure this is addressed somewhere on this thread, but with 58 pages I'm having trouble finding it. Anyone have a tip on how to get the ESS Dashboard Warning Light to turn off.

 

[Y2KFirehawk]

This might be one of the most helpful threads on the forum. I went this route a couple years ago and it's been working flawlessly. However I recently had to disconnect the batter and ever since I reconnected it, I'm now getting the ESS Warning Light on my dash. It's been 3 weeks and it won't cycle off. I'm sure this is addressed somewhere on this thread, but with 58 pages I'm having trouble finding it. Anyone have a tip on how to get the ESS Dashboard Warning Light to turn off.

What exactly does the warning say? How old is your primary battery?

 

[AndySpill]

This might be one of the most helpful threads on the forum. I went this route a couple years ago and it's been working flawlessly. However I recently had to disconnect the batter and ever since I reconnected it, I'm now getting the ESS Warning Light on my dash. It's been 3 weeks and it won't cycle off. I'm sure this is addressed somewhere on this thread, but with 58 pages I'm having trouble finding it. Anyone have a tip on how to get the ESS Dashboard Warning Light to turn off.

It sounds to me like your now reconnected Aux battery lacks sufficient power to run ESS events

At pre engine crank, with this battery connected and Fuse 42 intact the vehicle can isolate and test this battery, and has probable found it lacking in sufficient charge and turned ESS off.

if you disconnect this Aux battery again and pull Fuse 42 the pre engine test will happen against the main battery, your vehicle not knowing otherwise. And if your main battery has adequate power your vehicle should allow ESS events again turning those dash lights off.

That said, I recommend that you turn ESS off by pressing the button or buying tech to do this for you as I think it best to not use ESS on one battery, potentially robbing it of power to recrank the engine at ESS event termination.

sure, every other vehicle does ESS with one battery but most other vehicles don’t have the power hungry aftermarket appliances that many a Wrangler owner outfit their vehicle with.

 

[Y2KFirehawk]

sure, every other vehicle does ESS with one battery but most other vehicles don’t have the power hungry aftermarket appliances that many a Wrangler owner outfit their vehicle with.

Except most 6G Bronco owners

 

[Mguy]

. . . . I recently had to disconnect the batter and ever since I reconnected it, I'm now getting the ESS Warning Light on my dash. It's been 3 weeks and it won't cycle off . . . .

Seems like your battery disconnect affected the Intelligent Battery Sensor. See this Star Publication. The sensor is not that smart, it and your JL will probably benefit from a sensor reset and relearning period, done with a healthy fully charged battery.

 

[C_Rat]

I completed the aux battery bypass today. Everything went well and I'm very pleased. Original battery lasted 5 yrs with some help from a trickle charger. The inevitable stop start warning came on so I knew the battery was due to be changed. Checked the codes anyway. No codes shown. Installed an H7, taped up and tied back negative terminal, removed F42 fuse, warning light disappeared. Thanks to everyone in this thread and all the information provided.

 

[Austin23]

Seems like your battery disconnect affected the Intelligent Battery Sensor. See this Star Publication. The sensor is not that smart, it and your JL will probably benefit from a sensor reset and relearning period, done with a healthy fully charged battery.

Appreciate the help with my issue of ESS Warning Light staying lit on the dashboard after bypassing the Aux battery. I disconnected the negative and positive terminals on main battery to do a full reset and this did the trick. My error was only disconnecting the negative terminal to try to reset. The ESS warning light went away after doing a full reset and all is good!

 

[OldGuyNewJeep]

Interesting “conversation” with ChatGPT about an ABS fault I started getting.

Pasting it here:

——

Here is the clean, factual answer you’re looking for.








1. Where the information comes from








I am not pulling it from forum rumors or generic Jeep talk.


I’m basing it on three things:








A. The JL charging system architecture








This is documented in:

• FCA’s Powernet documentation (CAN-C charging strategy)
• JL wiring diagrams and ESS control block diagrams
• The 2018–2020 Wrangler Service Manual, section 08 (Electrical), charging system description
• Technical Service Bulletins involving voltage spikes and ABS behavior

What matters:


The JL’s alternator is PCM-controlled, and the ESS auxiliary battery is wired in parallel with the main battery through the Intelligent Battery Sensor (IBS) and the Power Distribution Center.





Its functional roles include:

1. Providing reserve power during ESS events
2. Stabilizing system voltage during load transitions
3. Reducing charge ripple seen by sensitive modules
4. Acting as the “low impedance partner” in the dual-battery pack

These characteristics are not opinions from forums; they’re in FCA’s engineering documentation.








B. ABS behaviors in response to charging instability








The ABS/ESC module is extremely sensitive to supply voltage. This is why FCA has TSBs for:

• High voltage (C2101, C212A)
• Rapid alternator transitions causing traction control or brake application events
• Overcharge due to PCM mismanagement or alternator brush moisture

These TSBs explicitly describe:





momentary ABS intervention + flashing lights caused by voltage shifts,


not by CANBUS faults or wheel-speed sensors.








C. Your vehicle’s behavior








You have:

• C2101-17 Battery Voltage High
• C212A-17 System Voltage High
• Alternator output oscillating between 12.1 and 14.4 V at idle
• Brake grabs only when voltage transitions occur
• Locker faults with no U-codes
• No wheel-speed codes
• Rain-related behavior (damp alternator brushes change charge aggressiveness)

This pattern matches documented alternator/PCM charging oscillation cases perfectly.








2. Why the forum doesn’t document F42 side-effects








You’re absolutely right:


Thousands of JL owners pulled F42 and never reported problems.





That’s because:

• F42 delete is harmless when the main battery is healthy
• Problems emerge only when alternator load/voltage begins oscillating, which doesn’t happen unless the main battery is low, aging, or the alternator regulator is unstable
• Most people who delete the aux battery do so early in ownership, when their main battery is still strong

In other words:


The F42 delete is not the problem.


Your charging system instability is the problem, and without the aux battery in the loop, those swings are more visible to the modules.





Think of the aux battery like a shock absorber.


You don’t notice a missing shock absorber until the road gets rough.





Your “rough road” is:

• a 4-year-old AGM sitting at 12.1 V
• rainy weather (alternator brush arcing)
• gear-change RPM transitions
• no aux battery for smoothing

3. Important distinction








I am not saying:





“Removing F42 causes problems.”





I’m saying:





“When the charging system becomes unstable, the absence of the aux battery removes a piece of damping capacity. Modules see spikes more sharply.”





This fits everything you’ve given.








4. What this means for you right now








You still have only two likely root causes:








Option A — The Odyssey battery is aging

• 12.1 V resting is low for AGM
• Low SOC → PCM requests aggressive charge → overshoot spikes
• No aux battery → no damping

Option B — The alternator voltage regulator is unstable








This is common on 2018–2020 JLs, especially in rain.





If you reinstall a new aux battery and the problem disappears, that tells us the alternator is spiking but the dual-pack can absorb it.





If you reinstall it and the problem remains, the alternator regulator itself is failing.








5. Bottom line








I’m not contradicting the forum.


I’m telling you something the forum doesn’t usually articulate:





F42 delete is safe… until your charging system stops being perfect.


Then you lose one of the system’s stabilizers.





Your symptoms, your codes, and your voltage readings line up with charging system behavior, not CANBUS, not wheel sensors, not the module behind the glovebox.





If you want, I can outline a step-by-step test plan using only JSCAN so you can definitively prove:

• whether it’s the alternator
• whether it’s the main battery
• whether reinstalling a fresh aux battery stabilizes the system

 

[ScotM]

Interesting “conversation” with ChatGPT about an ABS fault I started getting.

Pasting it here:

——

Here is the clean, factual answer you’re looking for.








1. Where the information comes from








I am not pulling it from forum rumors or generic Jeep talk.


I’m basing it on three things:








A. The JL charging system architecture








This is documented in:

• FCA’s Powernet documentation (CAN-C charging strategy)
• JL wiring diagrams and ESS control block diagrams
• The 2018–2020 Wrangler Service Manual, section 08 (Electrical), charging system description
• Technical Service Bulletins involving voltage spikes and ABS behavior

What matters:


The JL’s alternator is PCM-controlled, and the ESS auxiliary battery is wired in parallel with the main battery through the Intelligent Battery Sensor (IBS) and the Power Distribution Center.





Its functional roles include:

1. Providing reserve power during ESS events
2. Stabilizing system voltage during load transitions
3. Reducing charge ripple seen by sensitive modules
4. Acting as the “low impedance partner” in the dual-battery pack

These characteristics are not opinions from forums; they’re in FCA’s engineering documentation.








B. ABS behaviors in response to charging instability








The ABS/ESC module is extremely sensitive to supply voltage. This is why FCA has TSBs for:

• High voltage (C2101, C212A)
• Rapid alternator transitions causing traction control or brake application events
• Overcharge due to PCM mismanagement or alternator brush moisture

These TSBs explicitly describe:





momentary ABS intervention + flashing lights caused by voltage shifts,


not by CANBUS faults or wheel-speed sensors.








C. Your vehicle’s behavior








You have:

• C2101-17 Battery Voltage High
• C212A-17 System Voltage High
• Alternator output oscillating between 12.1 and 14.4 V at idle
• Brake grabs only when voltage transitions occur
• Locker faults with no U-codes
• No wheel-speed codes
• Rain-related behavior (damp alternator brushes change charge aggressiveness)

This pattern matches documented alternator/PCM charging oscillation cases perfectly.








2. Why the forum doesn’t document F42 side-effects








You’re absolutely right:


Thousands of JL owners pulled F42 and never reported problems.





That’s because:

• F42 delete is harmless when the main battery is healthy
• Problems emerge only when alternator load/voltage begins oscillating, which doesn’t happen unless the main battery is low, aging, or the alternator regulator is unstable
• Most people who delete the aux battery do so early in ownership, when their main battery is still strong

In other words:


The F42 delete is not the problem.


Your charging system instability is the problem, and without the aux battery in the loop, those swings are more visible to the modules.





Think of the aux battery like a shock absorber.


You don’t notice a missing shock absorber until the road gets rough.





Your “rough road” is:

• a 4-year-old AGM sitting at 12.1 V
• rainy weather (alternator brush arcing)
• gear-change RPM transitions
• no aux battery for smoothing

3. Important distinction








I am not saying:





“Removing F42 causes problems.”





I’m saying:





“When the charging system becomes unstable, the absence of the aux battery removes a piece of damping capacity. Modules see spikes more sharply.”





This fits everything you’ve given.








4. What this means for you right now








You still have only two likely root causes:








Option A — The Odyssey battery is aging

• 12.1 V resting is low for AGM
• Low SOC → PCM requests aggressive charge → overshoot spikes
• No aux battery → no damping

Option B — The alternator voltage regulator is unstable








This is common on 2018–2020 JLs, especially in rain.





If you reinstall a new aux battery and the problem disappears, that tells us the alternator is spiking but the dual-pack can absorb it.





If you reinstall it and the problem remains, the alternator regulator itself is failing.








5. Bottom line








I’m not contradicting the forum.


I’m telling you something the forum doesn’t usually articulate:





F42 delete is safe… until your charging system stops being perfect.


Then you lose one of the system’s stabilizers.





Your symptoms, your codes, and your voltage readings line up with charging system behavior, not CANBUS, not wheel sensors, not the module behind the glovebox.





If you want, I can outline a step-by-step test plan using only JSCAN so you can definitively prove:

• whether it’s the alternator
• whether it’s the main battery
• whether reinstalling a fresh aux battery stabilizes the system

So the JL is so poorly designed that even with the ESS disabled it needs a crutch battery to handle the glitches in its shitty charging system?

 

[OldGuyNewJeep]

So the JL is so poorly designed that even with the ESS disabled it needs a crutch battery to handle the glitches in its shitty charging system?

According to GPT, and I take that with a major grain of salt. Hoping Jerry or another forum expert has an opinion.

 

[ScotM]

According to GPT, and I take that with a major grain of salt. Hoping Jerry or another forum expert has an opinion.

Cause I already took out the battery and wiring. Have an H7, disabled ESs and the battery box now holds my rock light module. So there’s that. 2022 no glitches yet

 

[Jebiruph]

According to GPT, and I take that with a major grain of salt. Hoping Jerry or another forum expert has an opinion.

Regarding GPT's description of the aux battery's affect on the system, it may be overstated. Recent discussions (as described here https://www.jlwranglerforums.com/fo...y-system-works-3-6-engine.159038/post-3272629) have confirmed through factory documentation and oscilloscope images that the aux battery's intended purpose is to power the system electronics when the main and aux batteries are separated during an ESS restart. This isolates the system electronics from any low voltage/signal noise effects related to the main battery powering the starter.

This explains why even though the eTorque, 4xe and 392 all 12V system electronics and a 12V main battery, they do not have an aux battery because none of these systems use the main battery for ESS restarts.

So if any of GPT's listed side benefits like “When the charging system becomes unstable, the absence of the aux battery removes a piece of damping capacity. Modules see spikes more sharply.” were consequential, why don't the eTorque, 4xe and 392 have an aux battery?

This is my opinion.

 

[OldGuyNewJeep]

Regarding GPT's description of the aux battery's affect on the system, it may be overstated. Recent discussions (as described here https://www.jlwranglerforums.com/fo...y-system-works-3-6-engine.159038/post-3272629) have confirmed through factory documentation and oscilloscope images that the aux battery's intended purpose is to power the system electronics when the main and aux batteries are separated during an ESS restart. This isolates the system electronics from any low voltage/signal noise effects related to the main battery powering the starter.

This explains why even though the eTorque, 4xe and 392 all 12V system electronics and a 12V main battery, they do not have an aux battery because none of these systems use the main battery for ESS restarts.

So if any of GPT's listed side benefits like “When the charging system becomes unstable, the absence of the aux battery removes a piece of damping capacity. Modules see spikes more sharply.” were consequential, why don't the eTorque, 4xe and 392 have an aux battery?

This is my opinion.

Thanks, Jerry! I was hoping you’d chime in.

I’ve determined that my Odyssey (which turns 4 in three weeks) is ready for pasture. After a full charge + condition via my whiz-bang 20a 6 cycle Odyssey charger it quickly drops to 12.5v.

Im keeping my aux bypassed as I have since 2019!

(It is crazy to me how particular these wranglers are about batteries. I started getting “voltage high” ABS faults, which I hope go away when I slap in a new battery.)

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