Jumperless Aux Battery Bypass

[Jebiruph]

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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[Htfan]

So, no errors, Aux battery is bypassed and ESS is disabled? If so, this is absolutely awesome

Or does ESS function as it would with the N1/N2 jumper...

 

[Jebiruph]

So, no errors, Aux battery is bypassed and ESS is disabled? If so, this is absolutely awesome

Or does ESS function as it would with the N1/N2 jumper...

Just like with the jumper.

 

[Pismo61]

So, no errors, Aux battery is bypassed and ESS is disabled? If so, this is absolutely awesome

Or does ESS function as it would with the N1/N2 jumper...

So whats the difference between the two?

 

[Htfan]

So whats the difference between the two?

The difference is no need to add the N1/N2 jumper when pulling the fuse described by the OP.

 

[Pismo61]

No need to press the button(full disable) or you press it and it stays were you left it?

 

[Htfan]

No need to press the button(full disable) or you press it and it stays were you left it?

Neither.

ESS is fully functional as designed. Only the Aux battery can be disconnected without errors.

 

[RAO]

So remove the fuse and everything behaves as if the aux battery is still there? Did you remove that battery yet?

 

[Stuckinthesand]

Can confirm this works. My light has been coming on and off over the last few days so I knew it was time to replace the battery. I use the start stop eliminator so I was researching if I could just let the battery be but came across threads that it would drain the main which confirmed my suspicion. I was going to just disconnect the negative cable and live with the icon but eventually that would’ve irritated me. Came across this thread just went out to the Jeep, pulled the fuse, disconnected the cable, taped it off and zip tied it to another cable. Started it up and no icon.

 

[Stuckinthesand]

So remove the fuse and everything behaves as if the aux battery is still there? Did you remove that battery yet?

Remove the fuse and the negative aux battery cable. Behaves as if the aux is still there. Very important to remember though is the main battery will be doing the start after a stop if you forget to disable it or use something that has it remember it’s last state.

 

[RAO]

Remove the fuse and the negative aux battery cable. Behaves as if the aux is still there. Very important to remember though is the main battery will be doing the start after a stop if you forget to disable it or use something that has it remember it’s last state.

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.

 

[Stuckinthesand]

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.

That makes sense. Just remember if your still in warranty you may need to hook that aux back up if it has to go to the dealer. I know most of us don’t go to the dealership unless necessary but just a thought.

 

[RAO]

That makes sense. Just remember if your still in warranty you may need to hook that aux back up if it has to go to the dealer. I know most of us don’t go to the dealership unless necessary but just a thought.

Good point. May be easier to leave in place and just tape off as you suggested for a time.

 

[Fudster]

This is how I am interpreting this thread's info. If someone could agree or disagree that would be great.

The 3.6L has its batteries in parallel as per factory spec at all times but ESS events and an instant at cold crank.

Remove fuse F42 and the 3.6L seems to have its batteries in parallel (provided of course nobody's disconnected their ESS battery) all the time.

 

[Stuckinthesand]

This is how I am interpreting this thread's info. If someone could agree or disagree that would be great.

The 3.6L has its batteries in parallel as per factory spec at all times but ESS events and an instant at cold crank.

Remove fuse F42 and the 3.6L seems to have its batteries in parallel (provided of course nobody's disconnected their ESS battery) all the time.

Exactly. The F42 fuse gives power to the PCR to get the signal from the computer to check the voltage on the aux battery only. By removing the fuse it just sees the main battery. Downside is you have to make sure you disable ESS or you will be starting the Jeep from a stop event with the main battery. Also remember if you have a bad aux battery it will drain off your main battery. Personal opinion I disconnected the aux negative for less of an issue to worry about.

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