Jumperless aux battery/PCR bypass

[ShadowsPapa]

SO i suppose that a malfunctioning aux battery causes the battery to go lower, even though the voltage reads 14.2 or whatever, and states battery charging... @jebiruph as well. Sorry I dont get on here all the time.

What you see on the display, either off-road pages gauges, or the vehicle information page in the cluster is the current system voltage - as it's measured at the crank battery.
You could have a crank battery doing fine, but the aux battery doing not so fine.

If I ever tear into mine, I have a strong urge to wire in a way to display the voltage as it's measured directly from the posts of the aux battery.
That would tell me a whoooole lot of things, and give the ability to prove or disprove some stuff because I could measure - accurately - voltage drops across various connections, wire runs and more.

I'd love to be able to use my phone or tablet to gather more information, sort of like a custom display. I may try to find a cheap digital voltage display to stick into that unused cubby ahead of the shifter where the Rubicon switches sit and be able to display those other voltages.

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[TroutFishingInAmerica]

@jebiruph fantastic! This is the easiest method yet. I do weeks at a time solo and this is absolutely simplicity, great for a backup starter battery like you described long ago in the "other" forum. I do have a super capacitor for jumping but this would be my go to first. Very nice. Thanks.

 

[Skull]

If this eliminates the aux battery how will it start at all as it supposed to provide initial starting either first thing in morning or every stoplight…forgive me for not knowing but do not understand without reading a full wiring diagram.

 

[Mr._Bill]

If this eliminates the aux battery how will it start at all as it supposed to provide initial starting either first thing in morning or every stoplight…forgive me for not knowing but do not understand without reading a full wiring diagram.

The ESS battery is there to keep the computers stable during ESS events. The Main and ESS batteries are only separated from each other during ESS events. The rest of the time they are paralleled together. The Main battery is the one that drives the starter.

 

[ShadowsPapa]

The ESS battery is there to keep the computers stable during ESS events. The Main and ESS batteries are only separated from each other during ESS events. The rest of the time they are paralleled together. The Main battery is the one that drives the starter.

And that's why it's called the crank battery in the literature.

Yes. The aux will die and drain the main battery, leaving you high and dry.

Opinion

If you change the "will" to "may" - you'd have it.

I've proven multiple times with our own Jeeps - that even if that does happen, a jump is very possible.
Even using a jump pack. I've done it.

Aux battery died - the main battery was too low to even activate anything or crank the vehicle. It appeared totally dead. Get in, push button, nothing at all happens. Nothing.
Connect booster, get in - it starts fine.
I've even used my own JT to start a dead Jeep with bad aux battery, and I've used a charger over-night to get things to the point it would start.
Diagnosis from dealer - aux battery bad, killed main battery, both were replaced.
However, the vehicle could be jumped and driven without issue. (just be wary of shutting it down LOL)

The few that get stranded yell and holler loud enough and insist THIS WILL ABSOLUTELY HAPPEN TO YOU BECAUSE IT DID HAPPEN TO ME! - and that makes it so. (interesting to note, also, that in most cases these Jeeps are older, batteries have age on them, and they've been ridden hard and put away wet, so to speak, so all bets are off as far as connection integrity, voltage drops across cables and connections and how their accessories are hooked up, how they've charged batteries in the past - my bet - the owner/operator had as much to do with the problem as the batteries did)

It may happen - but it's not likely.
Batteries can die in many ways, and I've seen most of those ways over my 50 years of auto electric work.
There is no one single thing that causes their death, and when they die, the effects can be a multitude of things depending on the cause of death or way they died.

 

[Rocksalt]

from what I read about this on the JL forum, it is not uncommon for the sux battery to drain the system over time until one day it no longer starts. Jebiruph's solution is a simple one to avoid the situation.

 

[Rocksalt]

I was looking at the fuse descriptions on the underside of the PDC cover and saw this - F42 PCR CTRL FEED (ESS)*. Removing this fuse defeats the cold start aux battery test and allows the aux battery to be disconnected without generating 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, both actions prevent the PCR from activating and separating the batteries.

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.

does one need to disconnect the neg aux as well? this was brought up on JL forum

 

[ShadowsPapa]

from what I read about this on the JL forum, it is not uncommon for the sux battery to drain the system over time

Be careful what you read as often things are posted by those who have no clue what REALLY happened so the wrong thing gets blamed.
There's only two ways the aux battery can "drain" the crank battery:
The aux battery is bad in a way that it's shorted internally,
or
there is a drain on the truck - and since the aux battery is first in line to the system electronics, and it's sooo much smaller, it's going to get drawn down and the crank battery is next.
In a normal system, if the N3 fuse is good and the aux battery isn't shorted, it won't drain anything, but the TRUCK can with time.

 

[Fastwake]

Be careful what you read as often things are posted by those who have no clue what REALLY happened so the wrong thing gets blamed.
There's only two ways the aux battery can "drain" the crank battery:
The aux battery is bad in a way that it's shorted internally,
or
there is a drain on the truck - and since the aux battery is first in line to the system electronics, and it's sooo much smaller, it's going to get drawn down and the crank battery is next.
In a normal system, if the N3 fuse is good and the aux battery isn't shorted, it won't drain anything, but the TRUCK can with time.

What does removing FUSE F42 do?

 

[Mojave chad]

So removing this fuse takes place of removing the battery. In saying this, I mean once the aux. battery dies, will it cause affect on the main battery since the fuse is pulled?

 

[Lunentucker]

Jumperless Aux Battery Bypass | Jeep Wrangler Forums (JL / JLU) - Rubicon, Sahara, Sport, 4xe, 392 - JLwranglerforums.com

 

[TroutFishingInAmerica]

So removing this fuse takes place of removing the battery. In saying this, I mean once the aux. battery dies, will it cause affect on the main battery since the fuse is pulled?

There's a lot of information on this here and the Wrangler forum. A search will give you hours of reading.
If your aux is dead and you choose not to remove it (but you want to isolate it) you will need to remove the aux ground, and yes you should at that time remove the fuse.

 

[ShadowsPapa]

There's a lot of information on this here and the Wrangler forum. A search will give you hours of reading.
If your aux is dead and you choose not to remove it (but you want to isolate it) you will need to remove the aux ground, and yes you should at that time remove the fuse.

And removing the aux ground is simple as it's one of the cables at the top of the IBS on the crank battery. The other goes to the body ground between battery and fender (at least on the gas versions)

 

[Geoarch]

And removing the aux ground is simple as it's one of the cables at the top of the IBS on the crank battery. The other goes to the body ground between battery and fender (at least on the gas versions)

So removing the ground and fuse eliminates the aux battery from the circuit. Does the light come on and stay on? It seems like it wouldn't.

 

[jebiruph]

So removing the ground and fuse eliminates the aux battery from the circuit. Does the light come on and stay on? It seems like it wouldn't.

Removing the ground is what eliminates the aux battery from the circuit, removing the fuse is what prevents the system from detecting that the aux battery has been removed, keeping the light off.

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