3.6L ESS Aux Battery Bypass

  1. Jebiruph

    Jebiruph Well-Known Member

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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
     
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  2. Rhinebeck01

    Rhinebeck01 Well-Known Member

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    @Jebiruph

    It's really obvious you have spent lots of time doing the diagrams, getting to the bottom of this issue, taking the time to post this info, etc..

    I for one am greatly appreciative! Thank you!
     
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  3. Eloib

    Eloib Well-Known Member

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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.
     
  4. NFRs2000NYC

    NFRs2000NYC Well-Known Member

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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.
     
  5. Hgrace

    Hgrace Well-Known Member

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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.
     
  6. Rhinebeck01

    Rhinebeck01 Well-Known Member

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    @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.
     
  7. Hgrace

    Hgrace Well-Known Member

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    Iv jumped V10 motorhome as well as my v8 2590HD truck no problem

    Even drove th HD2500 a few miles with it to a shop for a new alternator
    https://www.amazon.com/gp/aw/d/B07892JQ1Q/ref=dp_ob_neva_mobile
     
  8. NFRs2000NYC

    NFRs2000NYC Well-Known Member

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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.
     
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    Jebiruph

    Jebiruph Well-Known Member

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    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

    Jebiruph Well-Known Member

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    #10 Oct 27, 2018
    Last edited: Oct 28, 2018
    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).
     
  11. NFRs2000NYC

    NFRs2000NYC Well-Known Member

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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

    Jebiruph Well-Known Member

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    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.
     
  13. NFRs2000NYC

    NFRs2000NYC Well-Known Member

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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.
     
  14. OP
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    Jebiruph

    Jebiruph Well-Known Member

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    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.
     
  15. NFRs2000NYC

    NFRs2000NYC Well-Known Member

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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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