3.6L ESS Aux Battery Bypass

Jebiruph

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I've seen quite a few posts related to failed Aux batteries causing flaky or dead electronics, stalling and failing to start/auto restart. I've come up with a simple way to temporarily bypass a bad Aux battery (also blown PCR fuse or bad PCR) and get back on the road. Warning - this post contains a lot of informed assumptions, so post your related experiences to help validate or invalidate any information I've provided.

Here's a picture of my bypass kit. The jumper is used to connect the N1 terminal to the N2 terminal. I wanted to be able to install the jumper securely, but still be able to quickly disconnect it. A fused jumper seemed to be the best and cheapest way to go. Removing the fuse is a simple way to disconnect the jumper and it protects against overloading the wire. I don't know how much current will pass through the wire, so I used the largest I could find.
bypass kit 2.jpg


Here's how I tested the bypass. I disconnected the Aux battery at N1 and the ground wire at the Main battery negative terminal to simulate a bad aux battery. The voltmeter confirms the disconnect with low voltage at N1.
aux bat disconnected.jpg


Here's a shot of the dead instrument panel due to the disconnected Aux battery.
dash off.jpg


Here's the jumper installed between N1 and N2, voltmeter shows the Main battery voltage at N1.
bypass installed.jpg


Here's the live instrument panel.
dash on.jpg


Here's the instrument panel after starting, only error indicated is open hood.
dash running.jpg



For those interested, following are some diagrams to illustrate how I understand it works. (See this thread for additional information on the dual battery system https://www.jlwranglerforums.com/forum/threads/3-6l-ess-battery-diagram.14401/)

Here's the battery voltages on a working system, the PCR is connecting the batteries in parallel, both batteries providing power to everything. Normal operation is verified with 0 volts between N1 and N2.
ess normal operation.PNG


Here's the effect of a failed Aux battery, no power to critical electronics. This should be verifiable by a voltage difference between N1 and N2.
ess bad aux.PNG


Bypassing is accomplished by jumping N1 to N2. This diagram illustrates the jumper getting power back to the critical electronics.
ess bypass.PNG



(updated 04/13/2019)
Additional 3.6L ESS system information is available here.
3.6L ESS Dual Battery Consolidated Information
https://www.jlwranglerforums.com/forum/threads/3-6l-ess-dual-battery-consolidated-information.25377/



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Eloib

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I haven't dug deep into getting knowledgeable of the electrical system of the wrangler, but from what I have seen and observed, it looks about right - missing alternator. Regarding the cable, the fuse you selected determines its minimum diameter.
 

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What would be awesome is if someone figured out a way to isolate the batteries in such a way that you can run accessories on one battery with no fear of killing the other, so you can easily start your rig if one dies...without having to run an aftermarket dual battery setup. It doesn't seem there is a way, currently to do this. If you run a fridge without starting the rig, it will eventually kill both batteries it seems, would be nice to make that NOT happen.
 

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What would be awesome is if someone figured out a way to isolate the batteries in such a way that you can run accessories on one battery with no fear of killing the other, so you can easily start your rig if one dies...without having to run an aftermarket dual battery setup. It doesn't seem there is a way, currently to do this. If you run a fridge without starting the rig, it will eventually kill both batteries it seems, would be nice to make that NOT happen.
Full jump boxed are cheap enough now. I carry one often. Not the little sandwich size ones but the full size lunchbox ones. Mine also has an inverter and usb power a light and air compressor.
 

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Full jump boxed are cheap enough now. I carry one often. Not the little sandwich size ones but the full size lunchbox ones. Mine also has an inverter and usb power a light and air compressor.
@Hgrace

I found such, not to have enough ooomph lets say to do the deed.. IF you ever have the occasion to use it on your discharged JL, let us know how you make out.
 

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NFRs2000NYC

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Full jump boxed are cheap enough now. I carry one often. Not the little sandwich size ones but the full size lunchbox ones. Mine also has an inverter and usb power a light and air compressor.
True but again, we already have a dual battery system in the Jeep, would be nice not to have to carry an extra piece of gear/weight and utilize what we already have efficiently.
 
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Jebiruph

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Iv jumped V10 motorhome as well as my v8 2590HD truck no problem
The difference with the JL is the existence of two batteries. Not to say that your system wouldn't be up to the task, but the additional power needed to start a larger engine is probably less than the additional power needed to over come an additional drained battery. Both drained batteries will be competing with the starter for power up to a point. Eventually they will be charged enough for most of the power to go to the starter.
 
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Jebiruph

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True but again, we already have a dual battery system in the Jeep, would be nice not to have to carry an extra piece of gear/weight and utilize what we already have efficiently.
FCA's original ESS in the Cherokee runs off of one battery and I suspect problems with that system lead to implementing the supplemental (meaning small additional) Aux battery. I believe the full size and mini battery were always intended to behave like a single battery system, not a dual battery system (except when auto stopped).
 
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NFRs2000NYC

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The small battery is the starting battery in the JL. Members have successfully started the Jeep with the large battery completely removed from the vehicle.
 
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Jebiruph

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The small battery is the starting battery in the JL. Members have successfully started the Jeep with the large battery completely removed from the vehicle.
It will start with just the small battery, but the starter is wired to the main battery. If you haven't yet, watch this video
In it, he explains that the Aux battery is used to run the interior accessories when auto stopped.
 
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NFRs2000NYC

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What does that have to do with my point? And for the record, I wouldn't put too much trust into his videos, a lot of what he does/says is INCORRECT, for instance, the steering issue. His fix is EXPRESSLY against the factory service manual which specifically states NOT to tighten the allen head bolt because it will cause the steering box to wear prematurely.
 
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The small battery is the starting battery in the JL. Members have successfully started the Jeep with the large battery completely removed from the vehicle.
In it, he explains that the Aux battery is used to run the interior accessories when auto stopped.
What does that have to do with my point?
I thought your point was that the Aux battery was the starting battery, so I addressed that point. Here's some additional information I found on a Pacifica forum.

This is from the factory shop manual:
- When the engine stops, a smaller secondary battery keeps accessory features powered, like the fan, wipers, headlights, audio system, etc.
 
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NFRs2000NYC

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I meant to say that the Jeep can start solely off the aux battery, meaning, it provides enough juice to BE a start battery in an emergency...why is why I'd love someone to figure out a way to isolate the drain, so accessories like fridges, LED aux lights, whatever will ONLY drain the big battery, saving the small one for an emergency start.
 

                           
























































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