Jumperless Aux Battery Bypass

[Livin4Today]

While working on another project, I was looking at the fuse descriptions on the underside of the PDC cover and saw this - F42 PCR CTRL FEED (ESS)*. Could it really be this easy? Yes it is. I pulled the fuse, disconnected the aux negative cable from the main battery negative post and it started right up without any errors.

Unplugging the PCR has always been an alternative to using an N1 N2 fused jumper, both methods defeat the aux battery pass/fail test that occurs prior to a cold start. Unlike using a jumper, unplugging the PCR doesn't require any additional parts, it's just not easy to get to the PCR plug. Removing the F42 fuse accomplishes the same thing as unplugging the PCR and the fuse is easy to get to.

Other than a couple starts, I haven't had the chance for real world testing, but I did look at the schematic and verified that pulling the fuse accomplishes the same thing as unplugging the PCR, both prevent the relay from activating.

Here's the underside of the PDC cover showing the description and location of the fuse.

Here's the fuse location with the fuse removed.

Here's the schematic showing the fuse in the circuit.

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[Livin4Today]

Quick question (Newb JLU guy - it's the wife's), so I was looking into this very issue and thought viola the answer to my problem - Aux switches temporarily unavailable while charging aux battery.

I followed the steps outlined, and everything worked except that I still received the aux switches temporarily unavailable... message in the evic. Then I also noticed a loose neg term and tightened that up, then fully charged main battery and the message continued. Feeling I hadn't solved my issue I decided to put it all back to OEM,

1. So I can schedule a firmware upgrade,
2. I figured the loose terminal might have been the issue of bad charge

My question is shouldn't that message disappear when the Aux is taken out of the equation?

 

[GrayWolf.Overland]

Quick question (Newb JLU guy - it's the wife's), so I was looking into this very issue and thought viola the answer to my problem - Aux switches temporarily unavailable while charging aux battery.

I followed the steps outlined, and everything worked except that I still received the aux switches temporarily unavailable... message in the evic. Then I also noticed a loose neg term and tightened that up, then fully charged main battery and the message continued. Feeling I hadn't solved my issue I decided to put it all back to OEM,

1. So I can schedule a firmware upgrade,
2. I figured the loose terminal might have been the issue of bad charge

My question is shouldn't that message disappear when the Aux is taken out of the equation?

Does your 2018 JLU have the TSB update fix on the ECU/Software that fixed a problem related to ESS and starting the jeep without aux battery? See previous replies for more info on the TSB

 

[Jebiruph]

Quick question (Newb JLU guy - it's the wife's), so I was looking into this very issue and thought viola the answer to my problem - Aux switches temporarily unavailable while charging aux battery.

I followed the steps outlined, and everything worked except that I still received the aux switches temporarily unavailable... message in the evic. Then I also noticed a loose neg term and tightened that up, then fully charged main battery and the message continued. Feeling I hadn't solved my issue I decided to put it all back to OEM,

1. So I can schedule a firmware upgrade,
2. I figured the loose terminal might have been the issue of bad charge

My question is shouldn't that message disappear when the Aux is taken out of the equation?

That message doesn't have anything to do with the aux battery, it's based on the battery sensor's analysis of the main battery. It means that either the main battery is low on charge or the battery sensor needs reset.

 

[Livin4Today]

Does your 2018 JLU have the TSB update fix on the ECU/Software that fixed a problem related to ESS and starting the jeep without aux battery? See previous replies for more info on the TSB

Not yet - Hence Bullet #1

 

[Livin4Today]

That message doesn't have anything to do with the aux battery, it's based on the battery sensor's analysis of the main battery. It means that either the main battery is low on charge or the battery sensor needs reset.

How is the sensor reset?

 

[rmartino99]

so, what is the difference with this or disconnecting the start stop pin under the hood

 

[Jebiruph]

How is the sensor reset?

By unplugging the 2 wire connector for a short time. It's best to have both batteries fully charged before you reset it, that allows it to resync faster.

 

[miketomp1972]

I’m having a dilemma, I did the pull fuse and disconnect of the aux negative terminal, I spent the week at OCMD for Jeep Week and on the way home the icon for the ESS came on saying service it soon ?

 

[miketomp1972]

Just disconnect the negative if you leave the aux battery in place. If you remove the aux you need to run it to the main. Tape off the negative.

Hey bud, mine has been working flawless, but yesterday my avengers light came on saying service start stop. F42, aux negative and a new AGM battery since aug 12. Light keeps coming on any ideas ?

 

[Stuckinthesand]

Hey bud, mine has been working flawless, but yesterday my avengers light came on saying service start stop. F42, aux negative and a new AGM battery since aug 12. Light keeps coming on any ideas ?

I would double check that fuse F42 is in fact pulled and verify the negative aux cable does not have any part of the clamp touching metal giving it a ground. I am not sure how if F42 is pulled that the system is seeing anything with the aux battery because it should stop the relay from operating therefore nothing should be checking the state of the aux battery.

 

[Al G]

How does the Jeep react to a dead main battery after doing this disconnect? Does it act like other vehicles or does it go into total failure mode like some of us have experienced?

 

[Stuckinthesand]

How does the Jeep react to a dead main battery after doing this disconnect? Does it act like other vehicles or does it go into total failure mode like some of us have experienced?

When my main went bad it acted like any other car would. I tried to start it and it just clicked. I jumped it and it started. I will say the only difference I found was I had to keep the rpm’s up when I was at a stop on my way to getting the new battery. If I didn’t keep the rpm’s around 1500 it would stall out and I’d have to jump it again.

 

[correfe]

How does the Jeep react to a dead main battery after doing this disconnect? Does it act like other vehicles or does it go into total failure mode like some of us have experienced?

When my main battery died, it was like any regular vehicle.
You might be able to see the dash light up but the Jeep won't start, and if you boost it it will start, but die because it doesn't have enough juice.

 

[hilldweller11]

I left my '19 JL Rubicon parked for a couple of months, not on a tender (I know), and although it started up ok, have had some of the messages for ESS others have mentioned. Despite being on an Optima 1200 charger for a while charging both aux and main batteries, it never moved off of 75%.

So I decided to do the jumper F42 pull / remove aux negative cable trick, and buy a new sealed AGM H7 / Group 94R battery.

I installed a Batteries+ X2 Power Group 94R SLI94RAGMDP and got the F42 fuse pulled, and everything works great so far. I'm pretty happy I don't need to buy a new Aux battery, and really amped to have a sealed AGM in the Wrangler. I have had an X2 Power Group 31 sealed AGM in my Space Trailer (camping trailer) powering a Dometic cooler for about a year, and it has been really great keeping the cooler cold on 7 day+ trips. I use a Noco 10 charger mounted in the trailer to maintain it wherever I go.

I think I might replace the Group 31 in the trailer with another H7/Group 94R so that I can do swapping if needed in a tight spot.

Thanks to jebiruph and rhinebeck for all the info. Happy with my setup now, and don't feel the need to buy the Genisis dual battery kit, although it looks like a nice solution.

Cheers,
Ray

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