Jumperless Aux Battery Bypass

[CarbonSteel]

Voltages in the 13-14 range are measurements of charging voltage.

let you Jeep sit overnight. In the morning, press the start button twice without putting your foot on the brake to put you ignition in the run condition with the engine off. Scroll through your dash gauges to you voltmeter and the voltage you see is the main (aux still disconnected) minus the system load (.3-.4 tenths on my Jeep). Leave the voltmeter up and crank your Jeep. Note the voltage as it cranks. Mine goes to 11.7 which is OK but maybe not good. Leave the engine running and watch the voltmeter. You should reach charging voltage in 15 seconds or so. My Jeep charges in the 13.9 - 14.4 range using just the main battery. I’ve been working on my Jeep the past 24 hours and made these test and discussed them here on pages 27 and 28. I started out think all was well but with the help of Rhinebeck, I know that my dealer installed a flooded lead acid battery last JAN instead of the required AGM battery (2 batteries connected together need to be the same type). If it is easy, tell us which batteries you have.

I have two Diehard Platinum AGMs:

H7 Main (2 years old in March)
Aux 14 (1 month old)

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[CarbonSteel]

With the Aux disconnected, the voltage you see displayed when vehicle is not running is in regard to the Main. When vehicle is running that voltage displaying is alternator.

Give it a few days and see if you have another event..

So far so good. Initially, the voltage was 14.x, but now I'm seeing 13.7 to 13.8. Fingers crossed it's sorted.

 

[CarbonSteel]

@Rhinebeck01 - any idea how much parasitic draw happens on the main with F42 pulled and auxiliary negative disconnected?

 

[Rhinebeck01]

@Rhinebeck01 - any idea how much parasitic draw happens on the main with F42 pulled and auxiliary negative disconnected?

Would vary .... Amount of normal draw on a parked, stock JL is tied to how that particular stock JL is factory equipped.

 

[CarbonSteel]

Would vary .... Amount of normal draw on a parked, stock JL is tied to how that particular stock JL is factory equipped.

I am seeing 14.x volts at startup, but now it drops to 13.8-13.7 almost immediately (would take miles to do that right after I disconnected it). I would guess the IBS has had time to relearn since the last disconnect that I did and the alternator is not having to constantly put out higher voltages attempting to charge the aux.

Very happy with this so far--hopefully, it is the solution, and it sounds like the auxiliary has failed again. Will give it a while like this and will pull the aux and go have it replaced under warranty. I will probably sell it at that point and not reinstall it.

Thanks for all of the help!

 

[Swagger]

Would bad batteries cause the engine to misfire? I just had the cel and ess lights come on halfway through a 3 hr drive. Obvious misfires during driving so I stopped and did the aux bypass, but did not help with the misfires. In this case, the cel comes on first, then the ess, which I've heard that the ess light will come on if the engine is misfiring. I've read that a valve cover leak on the o2 sensor can cause the same symptoms, so I'm trying to diagnose which issue it may be. I think the batteries are iffy because ess was not engaging much in the past months, so I'm leaning in that direction, but since the bypass didnt help, it may be pointing to the other issue. Is there a easy way to determine which problem it might be? 2018 3.6 JLUR. Thx in advance for any help!

 

[acheron800]

I understand that the main battery then does everything which is like most other vehicles with the stop start. I plan to push the button. But I would then remove that aux battery as don’t want a non functioning battery in the Jeep.

Question regarding this.

Wouldn't it be beneficial to leave the battery in place but the negative cable taped and tied off?

That way just in case you kill your main battery, there is a slight chance you can get started up by reconnecting the negative cable and reinserting the fuse?

Almost like having a backup battery?

 

[Sting Gray Rubi]

I did the aux negative disconnect and pulled F42. I also have the Tazer with auto start/stop disabled. Since doing this jm getting an aux switch notification in start up but goes away within 20 seconds but my aux switches aren't available until around 30-45 seconds. The other day I tried to aut start with the fob and it wouldn't start. Any advice is appreciated.

 

[CarbonSteel]

Would bad batteries cause the engine to misfire? I just had the cel and ess lights come on halfway through a 3 hr drive. Obvious misfires during driving so I stopped and did the aux bypass, but did not help with the misfires. In this case, the cel comes on first, then the ess, which I've heard that the ess light will come on if the engine is misfiring. I've read that a valve cover leak on the o2 sensor can cause the same symptoms, so I'm trying to diagnose which issue it may be. I think the batteries are iffy because ess was not engaging much in the past months, so I'm leaning in that direction, but since the bypass didnt help, it may be pointing to the other issue. Is there a easy way to determine which problem it might be? 2018 3.6 JLUR. Thx in advance for any help!

Absolutely (at least it did in my case and threw codes + check engine light).

Did you also disconnect the auxiliary negative battery cable from the main battery and insulate it?
Did you clear the codes?

 

[CarbonSteel]

I did the aux negative disconnect and pulled F42. I also have the Tazer with auto start/stop disabled. Since doing this jm getting an aux switch notification in start up but goes away within 20 seconds but my aux switches aren't available until around 30-45 seconds. The other day I tried to aut start with the fob and it wouldn't start. Any advice is appreciated.

How healthy is the main battery?

 

[Sting Gray Rubi]

How healthy is the main battery?

I place it on trickle from time to time and seems ok. Although it is OE battery on a 2020 with 50k

 

[CarbonSteel]

I place it on trickle from time to time and seems ok. Although it is OE battery on a 2020 with 50k

If the auxiliary was bad, it likely took out the main with it. I had this happen to me and it did not take very long for it to happen. My OE died within 3 years and 50K miles (sounds like a familiar timeframe, right?).

I would wonder about the actual health of the main battery, though most auto parts stores cannot properly test it. The other thing to check is the N3 fuse in the high amp fuse array especially if you worked on the aux battery.

 

[RAO]

Question regarding this.

Wouldn't it be beneficial to leave the battery in place but the negative cable taped and tied off?

That way just in case you kill your main battery, there is a slight chance you can get started up by reconnecting the negative cable and reinserting the fuse?

Almost like having a backup battery?

My concern is that the aux battery will need to be charged from time to time in order to be useful, otherwise you will think you have a good battery when all you have is another dead battery.

 

[acheron800]

My concern is that the aux battery will need to be charged from time to time in order to be useful, otherwise you will think you have a good battery when all you have is another dead battery.

Yeah but better than nothing, and if you put a trickle charger on it once every 3-6 months it should be fine. Seems like a good option to have a backup.

 

[Rhinebeck01]

I place it on trickle from time to time and seems ok. Although it is OE battery on a 2020 with 50k

You pulled F42 and you disconnected the Aux Neg cable ..... if you are randomly seeing the Aux switches not available EVIC notice, and remote start fails ....it's because your Main battery is iffy.

Replace the Main with an H7 as soon as possible.

https://www.jlwranglerforums.com/forum/threads/-main-battery-replacement-talk-jl-jt-🪫.100534/

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