04 Rubicon 42RLE to AX-15 transmission swap

tigman250

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Apr 30, 2019
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Have an 04 Rubicon with the 42rle in it and I HATE the transmission! It’s never in the right gear and refuses to downshift until it’s too late. I have gotten better at manually forcing it to downshift when needed but would much rather have a manual. Before anyone says “just buy a manual jeep” it’s not that easy here in Michigan, clean Jeeps are hard to find and this one is insanely clean (the reason I settled for the auto) so I have to mod this one so the transmission doesn’t suck.😁 I am leaning toward the AX15 but am reading the swaps are terrible especially on the 04’s, not sure if the fact that it’s a Rubicon adds to the complexity or not, partially why I’m here asking.

So what are my options? Manual valve body? AX15? NSG370? NV1550?

Do any of the aftermarket accessories or shift kits help the 42RLE not suck? Really prefer a manual swap over polishing the 42rle turd but the “grey key” and computer issues scare me, anyone done the swap with success? It MUST be reliable and I prefer no CEL but it’s mainly a trail rig so I suppose a CEL isn’t the end of the world but again, I don’t want a cobbled up mess.

Thanks all
 
What size tire are you running and what is your gear ratio?

The first thing is to make sure it's geared right, then get in the habit of always turning off the overdrive as soon as you get in the vehicle and start it up. If you can do that, you'll find the 42RLE is much, much better to live with.

If that still doesn't work for you, then your best bet would be to swap it out for an AX-15.
 
What size tire are you running and what is your gear ratio?

The first thing is to make sure it's geared right, then get in the habit of always turning off the overdrive as soon as you get in the vehicle and start it up. If you can do that, you'll find the 42RLE is much, much better to live with.

If that still doesn't work for you, then your best bet would be to swap it out for an AX-15.
4:10’s with 33’s and I always lock out OD especially off road
 
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X2 exactly. Sometimes a 42RLE problem is nothing more than a mismatch between the tire size and axle ratio. My 42RLE suddenly became well behaved once I regeared my axles to a ratio appropriate to the tire size.

So what tire size are you running and are your axles still at the factory 3.73 ratio?

Edit adding the below update.

4:10’s with 33’s and I always lock out OD especially off road
4.10 is a terrible ratio choice for 33's with the 42RLE and shutting the Overdrive off only affects highway rpms. Doing that wouldn't affect anything when offroad.

I assure you that regearing your axles to 5.13 would change your mind about wanting to convert the transmission type. You'd be loving life again after regearing to 5.13. Really. And 4.10 is not even enough gear for 33's with the NV3550.
 
Keep in mind that 4.10s do much better with a manual transmission than a 42rle, as far as 33s are concerned. People you know may be telling you that the auto is junk because their manual with 4.10s and 33s does so much better. The 0.69:1 overdrive ratio in the 42rle is why such a deep axle ratio is recommended for your application.
 
5.13 gears and keep the overdrive off. Then you’ll like your transmission, I promise.
 
You guys sure 5.13 isn’t too low? Doing the math in 3rd gear (what I climb sand hills in) with the stock 30.4” tires, 4.10 gears and 4:1 low range the wheel speed at 3K rpm is 16.55

33” tires 5.13 gears and 4:1 low range wheel speed is only 14.36 at 3K rpm

Do any of you guys with a auto rubicon on 33’s with 5.13’s climbing sand hills?
 
You guys sure 5.13 isn’t too low? Doing the math in 3rd gear (what I climb sand hills in) with the stock 30.4” tires, 4.10 gears and 4:1 low range the wheel speed at 3K rpm is 16.55

33” tires 5.13 gears and 4:1 low range wheel speed is only 14.36 at 3K rpm

Do any of you guys with a auto rubicon on 33’s with 5.13’s climbing sand hills?

I'm 110% sure.

I had 4.88s in my 2005 Rubicon with a 42RLE and 33s, and that wasn't low enough. In fact, it was a big mistake. I always wished I had gone with 5.13 instead.

And believe me, I took it up plenty of hills, in the sand too!

You have to understand the 42RLE is geared very stupid from the factory, especially with that .69 OD gear.

You NEED to gear low with the 42RLE. This isn't a myth, it's a fact. Do some searching on this forum and you'll very quickly realize this (y)

No one here is giving you bad advice, I promise. Re-gear it and see for yourself.

My last 2005 Rubicon was a 42RLE on 35s and I had 5.38 gears.

On the highway it did 65 MPH and was right around 2600 RPM, where it should be.

5.13 is the correct gear ratio for your 42RLE with 33s. No two ways about that.

Like I said, re-gear to 5.13 and you'll come back singing praise to us, I promise.
 
You guys sure 5.13 isn’t too low? Doing the math in 3rd gear (what I climb sand hills in) with the stock 30.4” tires, 4.10 gears and 4:1 low range the wheel speed at 3K rpm is 16.55

33” tires 5.13 gears and 4:1 low range wheel speed is only 14.36 at 3K rpm

Do any of you guys with a auto rubicon on 33’s with 5.13’s climbing sand hills?
I have an Auto Rubi running 5.38s and sand hills - Much more betterer ;) 14 MPG on the highway too.
 
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You guys sure 5.13 isn’t too low? Doing the math in 3rd gear (what I climb sand hills in) with the stock 30.4” tires, 4.10 gears and 4:1 low range the wheel speed at 3K rpm is 16.55

33” tires 5.13 gears and 4:1 low range wheel speed is only 14.36 at 3K rpm

Do any of you guys with a auto rubicon on 33’s with 5.13’s climbing sand hills?
5.13 is exactly what you want, I'm running 5.38 with 35's and even with that I wish I had more gear. When I regeared from 4.88 to 5.38 2 years ago not only did it perform better on the highway and offroad my mileage even went up nearly 3 mpg from the engine being happier.

YES those ratios seem stupid low and they would be for the 5-speed or 6-speed manual transmissions or the older 32RH 3-speed automatic. But the 4-speed 42RLE automatic has a stupid .69 Overdrive ratio which drops the cruising rpms too much which causes the engine to lug and the transmission to have hunt for a workable ratio which isn't there. Installing extra low axle ratio gearing as we're recommending is the fix.
 
5.13 is exactly what you want, I'm running 5.38 with 35's and even with that I wish I had more gear.

X2 to this. 5.38 is as low as you can gear on the Rubicon Dana 44 axles, but with 5.38 gears and 35s on my 42RLE equipped Rubicon, I still wanted to go deeper. Unfortunately, that's not possible without going to some aftermarket axles.
 
Ok ok, point taken, sounds like a regear to 5.13’s is in order....and if I still hate it I can then do the swap then throw 35’s on it😁

I wonder if it is in fact cheaper to regear or swap, gears with master install kit are $1100 and probably another $500 for install, that has to be getting close to the price of a swap isn’t it? Looking at Yukon gears, any other recommendations?

Thanks guys!
 
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Ok ok, point taken, sounds like a regear to 5.13’s is in order....and if I still hate it I can then do the swap then throw 35’s on it😁

I wonder if it is in fact cheaper to regear or swap, gears with master install kit are $1100 and probably another $500 for install, that has to be getting close to the price of a swap isn’t it? Looking at Yukon gears, any other recommendations?

Thanks guys!
Most people here will recommend Revolution gears. I thought it was more like $650 for a whole kit...
 
5.13 gears and keep the overdrive off. Then you’ll like your transmission, I promise.
Would there be a way to change the overdrive switch from a momentary to a toggle ? I will be the test monkey.
 
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Would there be a way to change the overdrive switch from a momentary to a toggle ? I will be the test monkey.

Anything is possible. Of course you'd have to figure out how the circuit is wired in the FSM wiring diagrams, and then figure out a way to work around it. Would be cool though if you could get it so that the OD was off by default upon starting the vehicle.