Q&A How-to for JK Wrangler, JK Rubicon and JK Unlimited!
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Sent: Thursday, March 15, 2012 9:46 PM
To: 4WD Mechanix
Subject: 2010 Jeep JK Transmission
I have a 2010 JK with the 42RLE transmission. I would like to build the tranny stronger. I
have a Ripp Mods Supercharger, Rubicrawler, 37 Tires, full headers, 4:88 gears etc
I've looked everywhere on the West Coast (I live in Oregon) and cannot find anyone really
knowledgeable about the 42RLE. I've heard of Level 10 or Ten out east but that's really
What upgrades are available for the 42RLE like friction plates, torque converter
Reply from Moses below—
Mike, the upgrade parts from Sonnax always point to the weak areas in an automatic
transmission. I looked up the Sonnax upgrades for the 42RLE. Pore through them for details
on failure-prone OEM parts:
By identifying the weak areas in your transmission, you can consider upgrades. I rely on
Sonnax for fixes and have been very satisfied with the quality of the products. As for frictions and bands,
the aftermarket offers some products, check with B&M and TCI.
One area emphasized these days is fluid. Mopar ATF+4 would seem sufficient for most
applications, although I stumbled onto this reference while addressing your
I use Mopar ATF+4 and have not used this aftermarket ATF. However, I can say that late model
transmissions are highly dependent upon formulated ATF for specific friction materials, seals and the heat
anticipated. An add-on transmission cooler would be a good idea with your powertrain…We can discuss the details
of a cooler installation.
I support your concern. The 42RLE is considered a “medium strength” automatic, not in the
same category as the A580 that has replaced it in the 2012 JK Wrangler. The A580 has survived behind modern
hemi V-8s in heftier vehicles.
The JK Wrangler, especially as you have equipped yours, is certainly no “lightweight”. I
am pleased that you went to 4.88 gears, this will help preserve the 42RLE. I’m doing the same with our Ram
truck, going from 3.73 to 4.56s after a recent move to 35” diameter tires plus an additional 1,100 pounds in
accessories and added fuel capacity. Mileage fell off dramatically with the diesel, and the load on the
powertrain and U-joints is unacceptable…4.88s sound right in your case.
Diesel JK Rubicon Unlimited Tow
Chrysler/Jeep installs a 3.0 diesel in the JK Unlimited, I would consider trading in my Tahoe for one of them. The
JK would be a daily driver and tow our trail Jeep. Would this be a good tow vehicle for a 4,500‐pound load? Is the
JK Unlimited beefy enough to withstand the rigors of being used as a daily driver and weekend tow truck for my
trail Jeep?—Bill S.
Bill, the towing
capability has more to do with torque ratings of front and rear axles, transmission and transfer case. Brakes would
be an issue that applies to the combined vehicle weight and tow load (the gross combination weight or GCW). A
weightier diesel engine with typical diesel torque would require a beefy front and rear axle, transmission and
transfer case to handle the torque. The final consideration is frame modular section stamina. All of this applies
to the 3.8L gas V‐6, too, including the brake capacity for a given GCW.
In my view, the
biggest concern for towing is wheelbase length and track width. The JK has a very rugged frame, and Rubicon models
use Dana 44 axles. All of these parts are impressive. The JK has a 5” wider track than a TJ, a big step in the
right direction for towing. If the vehicle’s GCW rating would cover the heft of your trail Jeep on a lightweight
aluminum trailer, this might be a way to go—but currently, maximum towing capacity for a JK is officially 2,000
pounds. Optional Trailer Sway Control or TSC is recommended for that tow
weight. (TSC works in conjunction with Electronic Stability Control or ESC.)
preferred tow package is our Dodge Cummins diesel-powered 3500 Quad-Cab 4x4 and a 7,500-pound capacity trailer. The
car hauling trailer has electric brakes at each of its four wheels.
For more serious
towing, I opt for the Cummins 5.9L or 6.7L diesel in a Ram 2500 or 3500 Dodge truck. Our ’05 Ram 3500 Quad‐Cab 4WD
Cummins truck consistently averages 23.8 mpg without a trailer and 18 mpg at legal speeds while towing our XJ
Cherokee (approximately 3,800 pounds) on a car hauling trailer (additional 1,500 pounds). The 140.5” wheelbase is
very stable with a trailer in tow.
you’d like to know how I get that kind of fuel mileage from a stone stock Ram diesel truck that has a 7,800‐pound
curb weight, I’d be pleased to share. Hint: It’s all about driving technique!
Much to its
credit, the current WUL is the very best tow profile of any CJ/Wrangler class vehicle to date. The WUL has
a 116” wheelbase, similar to traditional ½‐ton SWB pickups, so that certainly does qualify for lighter towing. That
said, I personally will not tow anything of size with less than a 110”‐142” wheelbase. The 110” figure is from the
Grand Wagoneer days, when the Grand Wagoneer boasted a hefty chassis and a wide, J‐truck track
As for the 3.0L
diesel, I’d buy a JK 2‐door in a heartbeat if Jeep offers the diesel in that chassis—for a trail runner, not tow vehicle. My guess? Chrysler would use the current Grand Cherokee diesel and
not a Cummins. In the Grand Cherokee, the V‐6 turbo‐diesel delivers 376 lb‐ft of torque at 1600‐2800 rpm!
This is true diesel power at a slightly higher speed than the Cummins inline sixes, yet still capable of some
serious fuel efficiency—especially in a JK chassis.
Our car hauling
trailer has tandem axles with electric brakes at each of four wheels. For toting the XJ Cherokee to events, I use a
load distributing equalizer hitch. The heavy trailer coupler has a 2‐5/16” ball size. This equipment adds an extra
margin of safety within the factory tow rating.
As a tower,
though, compare a turbo V‐6 JK WUL prospect to the Dodge Ram 2500 or 3500. Our Ram 3500 has Dana 60/70 axles, a
600‐plus lb‐ft rated transmission and a massive transfer case. At a 142” wheelbase, this truck has huge, four‐wheel
disc brakes and a full‐floating rear axle design. Wife Donna and I towed our XJ Cherokee to Moab for a Jeep Safari
and Four‐Wheeler TV shoot. I used the car‐hauling trailer and pulled at any speed we desired—governed strictly by
the amount of fuel we were willing to burn. The tow was safe in crosswinds, up and down steep grades and in urban
traffic through Salt Lake City. Our GCW was around 13,500 pounds on
that trip. Best of all, Donna really likes the visibility, stance and performance of the Ram
truck. She is very comfortable driving and riding in the Dodge, a big point with us since we put up to 18,000
miles per year on the vehicle. We each drive this truck, primarily for business
My pure guess
would have the trailer tow limit of a JK Unlimited diesel in the range of 3,500 to 5,000 pounds. Track width would
likely be the same as non‐WUL JKs. In my view, the WUL wheelbase length and track dimensions do not make this the
ultimate trailer puller any more than an XJ or Grand Cherokee has been suited for heavy towing. Despite the
7,400‐pound towing limit of the Grand Cherokee 3.0L V‐6 diesel model, I would still opt for a longer wheelbase
Dodge Ram 2500 or 3500 4WD truck.
Fuel mileage with
a 3.0L diesel would offset current fuel cost extremes. For that reason, I would target the JK Wrangler 2‐door
diesel (if offered) as the ultimate multi‐purpose, trail‐running Jeep. With the current wave of 4.7L V‐8 and 5.7L
hemi swaps into the JK, the 3.0L turbo diesel deserves some serious consideration. I’d be game for a