Jeep Won't Start - Aux Battery?

Cory

Cory

VA4WDA
2021 JLU Diesel -- Original batteries.

I can unlock the Jeep, radio, lights, etc, all come on, but if I hit the button to start it... nothing. No crank, starter noise. Eventually it turns itself off and goes into start protection mode or after a few attempts at hitting the button with nothing. Engine light and battery light illuminated. Battery is at 12v on dash.

I'm thinking it's the aux battery issue. I'm over 36k miles, so thinking I'm going to go buy a decent battery and aux and replace them myself.... Diesel supposedly has a trap door under main, so I don't need to go through wheel well or start pulling out fuse boxes to swap it out....
 
Cory said:
2021 JLU Diesel -- Original batteries.

I can unlock the Jeep, radio, lights, etc, all come on, but if I hit the button to start it... nothing. No crank, starter noise. Eventually it turns itself off and goes into start protection mode or after a few attempts at hitting the button with nothing. Engine light and battery light illuminated. Battery is at 12v on dash.

I'm thinking it's the aux battery issue. I'm over 36k miles, so thinking I'm going to go buy a decent battery and aux and replace them myself.... Diesel supposedly has a trap door under main, so I don't need to go through wheel well or start pulling out fuse boxes to swap it out....
Click to expand...
Sounds like the aux battery. The aux battery is used to start most JLs. The "issue" will continue to happen, the aux battery will need to be replaced every couple years (thankfully mine died in my garage). You should bypass it or a more expensive option, buy a Genesis Dual Battery setup. Both of those options correct the issue. There is information regarding the bypass on the JL Forum (www.jlwranglerforums.com) and I believe Kevin and Sarah bypassed their aux battery @Galactica @WNDR-XJ I went with the Genesis Dual Battery setup.
 
The Aux battery is not used to start the Jeep. It's only a 12 amp hour battery and would smoke the first time you tried to use it to start the Jeep. The auxiliary battery runs the Start/Stop system electronics. The main battery actually powers the starter.

If the Jeep won't start and everything hard shuts off when you hit the start button (almost like a system crash), then there is a high likelihood that both batteries are smoked.
 
There's a relay that opens and closes that allows the alternator to charge both batteries when needed. It's is a known issue when one battery starts to go it'll drain the other one, doesn't matter which. There is also an issue when that drain starts to happen it'll burn up the relay and computer for that system. This annoying issue is covered under warranty if you are still covered. I've actually had both of my batteries replaced at no cost with my warranty.

Ummn
 
Spaz said:
The Aux battery is not used to start the Jeep. It's only a 12 amp hour battery and would smoke the first time you tried to use it to start the Jeep. The auxiliary battery runs the Start/Stop system electronics. The main battery actually powers the starter.

If the Jeep won't start and everything hard shuts off when you hit the start button (almost like a system crash), then there is a high likelihood that both batteries are smoked.
Click to expand...
Let me change my statement, when your AUX battery goes dead, you cannot start your rig unless you know how to bypass it. When my aux battery died, my main battery was fine.
 
That makes sense.

My Aux battery died with zero warning. Left me stuck in the middle of the road at a red light. Needless to say, when it turned green, my fellow commuters were not happy with me.

I had my wife come get me with our Ram 1500 and towed the Gladiator home. I took the aux battery out, ran the positive up to the main battery, and then wire-tied the ground cable out of the way. I put an 850cca main battery in and never looked back.
 
swapped out the batteries today... same state, Possible it "burn[ed] up the relay and computer for that system." the relay is in the fuse box? any idea which one? I tried swapping F02, which looked possibly bad (its cloudy and I cannot see the wire inside to see if it burned out) which the spare in the box, but nothing happened there either
 
Here are some potentially relevant diagrams for the 2018 JL. To include the relay diagram. Most of these diagrams are for the 3.6 so your mileage may vary given your diesel.

When you hit start, do you hear anything? Any clicking? Flashing of the dash, any difference at all?
FUSES-RELAY - ASD - 3.6L.png
 

Attachments

My n1-n7 is good. Did a continuity check on it. All fuses related seem to be ok too. I am worried that some of the four prong relay fuses (8474s) might have gone back in backwards or turned. I forgot to take a picture before. Anyone have a ‘21 JL or JT diesel you could snap a picture of the fuse box for me?

All the ones on the web don’t seem to have a pattern… maybe it doesn’t matter?

I am calling it quits though. Tow to dealer at 0700 on Monday.
 
Cory said:
2021 JLU Diesel -- Original batteries.

I can unlock the Jeep, radio, lights, etc, all come on, but if I hit the button to start it... nothing. No crank, starter noise. Eventually it turns itself off and goes into start protection mode or after a few attempts at hitting the button with nothing. Engine light and battery light illuminated. Battery is at 12v on dash.

I'm thinking it's the aux battery issue. I'm over 36k miles, so thinking I'm going to go buy a decent battery and aux and replace them myself.... Diesel supposedly has a trap door under main, so I don't need to go through wheel well or start pulling out fuse boxes to swap it out....
Click to expand...

Pull fuse 42, disconnect the auxiliary battery negative and tape over the end. Here is a video and post that will help:

Jeep Wrangler JL/Gladiator Aux Battery Delete-Odyssey Battery Install (youtube.com)

Help. Time sensitive! Calling Jeep Gladiator Battery Experts. Need help and I'm in Peru. | Page 2 | Jeep Gladiator (JT) News, Forum, Community - JeepGladiatorForum.com
 
Spaz said:
That makes sense.

My Aux battery died with zero warning. Left me stuck in the middle of the road at a red light. Needless to say, when it turned green, my fellow commuters were not happy with me.

I had my wife come get me with our Ram 1500 and towed the Gladiator home. I took the aux battery out, ran the positive up to the main battery, and then wire-tied the ground cable out of the way. I put an 850cca main battery in and never looked back.
Click to expand...
That sucks! Thankfully mine died in my garage!
 
They swapped out the PCM (warranty), no change. They're going to swap out power distro control unit now... waiting on part.
 
Cory,

Out of curiosity, do you have any additional accessories connected to you batteries?

Joe
 
Cory said:
2021 JLU Diesel -- Original batteries.

I can unlock the Jeep, radio, lights, etc, all come on, but if I hit the button to start it... nothing. No crank, starter noise. Eventually it turns itself off and goes into start protection mode or after a few attempts at hitting the button with nothing. Engine light and battery light illuminated. Battery is at 12v on dash.

I'm thinking it's the aux battery issue. I'm over 36k miles, so thinking I'm going to go buy a decent battery and aux and replace them myself.... Diesel supposedly has a trap door under main, so I don't need to go through wheel well or start pulling out fuse boxes to swap it out....
Click to expand...
Just went thru this with a brand new 2024 and it was the aux battery..At least it was under warranty..
 
Warn Winch, connected to the battery. Bull Run connected it. It's been on there for ~10 months, hasn't been used in 6 months. I was surprised they didn't try to claim it was that....
 
Cory said:
Warn Winch, connected to the battery. Bull Run connected it. It's been on there for ~10 months, hasn't been used in 6 months. I was surprised they didn't try to claim it was that....
Click to expand...
I recently went through something similar after a recall and they tried to blame it on the winch. After additional investigation, they found a loose ground and pinched wire (grounded). :bang
 
Btw, same thing happened to my previous 2019 JLUR and they replaced both batteries.
 
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