INTRODUCTION
I own a 2006 Jeep Wrangler Unlimited (LJ) Hardtop with the 4.0L and 42RLE transmission. I bought it back in 2018 with just 26,500 miles on the odometer. It came from Florida, where it likely spent its first 12 years in someone’s garage as a weekend beach cruiser. Everything was bone stock when I got it, and it made a great daily driver.
The only immediate issues early on were spark plugs and catalytic converters—probably from sitting so long. Over the years, I’ve replaced two radiators, three water pumps, and one harmonic balancer. Typical 4.0L stuff. Fortunately, I’ve always loved working on vehicles, and as an ASE mechanic with an Associate Degree in Automotive Technology, those repairs didn’t faze me.
That said, I honestly can’t believe Chrysler put the 42RLE behind the 4.0L in a Wrangler. In my opinion, it’s basically a minivan transmission—under-oiled, under-cooled, and cursed with a laughable overdrive ratio.
After years of running 33s and later 35s, I eventually cooked the factory 42RLE. At 70,000 miles the fluid was dark cherry red, smelled burnt, and caused solenoid and shift issues. I managed to get it to act nice with a flush, but by 90,000 miles it went into limp mode for good. My scan tool even showed a transmission temperature of 1,025°F—clearly a meltdown. Reverse was starting to fail, so I parked it.
CHOOSING ATS DIESEL
After a lot of research and asking around, I found ATS Diesel. They were the only name-brand company I could find that rebuilt and upgraded the 42RLE using quality aftermarket parts. Many of my diesel friends had great experiences with ATS transmissions in their Cummins and Powerstroke trucks, which helped seal the deal.
I went with their Stage 1 42RLE package, rated for 300 HP and 35s, backed by a 5-year / 500,000-mile warranty that actually covers off-road use.
Highlights include:
• Billet cover torque converter with hardened pump and turbine
• HD composite lock-up clutch
• Lower stall speed (better off-road control)
• Oversized pump gearset
• Modified pressure plates for more clutch surface area
• Heavily modified valve body
• Reinforced planetary gearset
• Internal hydraulic case modifications
• Deep 4.5-quart extra-capacity transmission pan
The Stage 2 version adds a large external cooler and increases the warranty to 400 HP and 35s, but the Stage 1 fit my needs perfectly.
Pricing breakdown:
• Transmission: $3,100
• Freight (two-way): $700
• Core charge: $1,400 (I sent mine in first)
Total: $3,800
Every local shop I contacted wanted $3,500+ for a stock remanufactured unit with a 1-year warranty and no off-roading coverage, or around $4,600 for a "Bullripper" (basic reman with HD converter) with the same 1 year warranty. ATS was the only company offering a fully built, warrantied option for off-road Jeeps.
INSTALLATION EXPERIENCE
I had the transmission shipped to my farm shop but ended up taking it to my local performance shop after injuring my shoulder while installing a rear axle in a Grand Marquis.
Shipping took about two weeks, including my core exchange. The transmission arrived well-packaged, pre-lubed (not filled) with ATF+4, powder-coated purple with new bolts, and with a black finned deep pan. Honestly, it looked too nice to hide under the Jeep.
I supplied a Savvy transfer case cable and 2.5 gallons of Amsoil Signature Series ATF for the installation. I am good friends with the shop owner, and he took care of me. I also had him install a large Hayden transmission cooler.
On the first test drive, it was still in limp mode. I realized the PCM needed to be relearned, so we performed the relearn procedure, and it came out of limp mode. However, once warmed up, I got violent 1–2 and 2–3 shifts—full line pressure at about ¼ throttle (around 275 psi with the ATS mods). It sounded like the transmission was trying to escape the Jeep.
After some research, I discovered the real culprit: the stock 2006 PCM. It was commanding max pressure because it thought the clutches were still slipping. I ordered a Wrangler Fix PCM, and after installing it, every issue disappeared.
DRIVING IMPRESSIONS
With the new PCM installed, the transmission now drives beautifully:
• The lower stall speed matches the 4.0’s torque curve perfectly.
• Shifts are smooth at light throttle and crisp at higher RPMs.
• It no longer hunts for gears or feels sluggish.
• Converter lock-up engages predictably and holds steady.
I’m still running the factory 3.73 gears on 35s (I’ve got 5.13s ready to go in soon), but even as it sits, drivability is drastically improved.
FINAL THOUGHTS
It’s only been a month, but I’m genuinely impressed. The ATS 42RLE feels solid, well-built, and purpose-engineered for real Jeep use—not just pavement duty.
Pros:
• Heavy-duty build quality
• Lower stall converter greatly improves drivability
• 5-year off-road warranty
• Excellent packaging and customer service
Cons:
• Freight and core shipping cost is high
• Needs a healthy PCM (2005–2006 stock units are notorious for issues)
Overall, I’d absolutely recommend ATS Diesel to any Jeeper looking for a serious transmission upgrade. It’s the first time I’ve actually enjoyed driving my LJ with an automatic.
I own a 2006 Jeep Wrangler Unlimited (LJ) Hardtop with the 4.0L and 42RLE transmission. I bought it back in 2018 with just 26,500 miles on the odometer. It came from Florida, where it likely spent its first 12 years in someone’s garage as a weekend beach cruiser. Everything was bone stock when I got it, and it made a great daily driver.
The only immediate issues early on were spark plugs and catalytic converters—probably from sitting so long. Over the years, I’ve replaced two radiators, three water pumps, and one harmonic balancer. Typical 4.0L stuff. Fortunately, I’ve always loved working on vehicles, and as an ASE mechanic with an Associate Degree in Automotive Technology, those repairs didn’t faze me.
That said, I honestly can’t believe Chrysler put the 42RLE behind the 4.0L in a Wrangler. In my opinion, it’s basically a minivan transmission—under-oiled, under-cooled, and cursed with a laughable overdrive ratio.
After years of running 33s and later 35s, I eventually cooked the factory 42RLE. At 70,000 miles the fluid was dark cherry red, smelled burnt, and caused solenoid and shift issues. I managed to get it to act nice with a flush, but by 90,000 miles it went into limp mode for good. My scan tool even showed a transmission temperature of 1,025°F—clearly a meltdown. Reverse was starting to fail, so I parked it.
CHOOSING ATS DIESEL
After a lot of research and asking around, I found ATS Diesel. They were the only name-brand company I could find that rebuilt and upgraded the 42RLE using quality aftermarket parts. Many of my diesel friends had great experiences with ATS transmissions in their Cummins and Powerstroke trucks, which helped seal the deal.
I went with their Stage 1 42RLE package, rated for 300 HP and 35s, backed by a 5-year / 500,000-mile warranty that actually covers off-road use.
Highlights include:
• Billet cover torque converter with hardened pump and turbine
• HD composite lock-up clutch
• Lower stall speed (better off-road control)
• Oversized pump gearset
• Modified pressure plates for more clutch surface area
• Heavily modified valve body
• Reinforced planetary gearset
• Internal hydraulic case modifications
• Deep 4.5-quart extra-capacity transmission pan
The Stage 2 version adds a large external cooler and increases the warranty to 400 HP and 35s, but the Stage 1 fit my needs perfectly.
Pricing breakdown:
• Transmission: $3,100
• Freight (two-way): $700
• Core charge: $1,400 (I sent mine in first)
Total: $3,800
Every local shop I contacted wanted $3,500+ for a stock remanufactured unit with a 1-year warranty and no off-roading coverage, or around $4,600 for a "Bullripper" (basic reman with HD converter) with the same 1 year warranty. ATS was the only company offering a fully built, warrantied option for off-road Jeeps.
INSTALLATION EXPERIENCE
I had the transmission shipped to my farm shop but ended up taking it to my local performance shop after injuring my shoulder while installing a rear axle in a Grand Marquis.
Shipping took about two weeks, including my core exchange. The transmission arrived well-packaged, pre-lubed (not filled) with ATF+4, powder-coated purple with new bolts, and with a black finned deep pan. Honestly, it looked too nice to hide under the Jeep.
I supplied a Savvy transfer case cable and 2.5 gallons of Amsoil Signature Series ATF for the installation. I am good friends with the shop owner, and he took care of me. I also had him install a large Hayden transmission cooler.
On the first test drive, it was still in limp mode. I realized the PCM needed to be relearned, so we performed the relearn procedure, and it came out of limp mode. However, once warmed up, I got violent 1–2 and 2–3 shifts—full line pressure at about ¼ throttle (around 275 psi with the ATS mods). It sounded like the transmission was trying to escape the Jeep.
After some research, I discovered the real culprit: the stock 2006 PCM. It was commanding max pressure because it thought the clutches were still slipping. I ordered a Wrangler Fix PCM, and after installing it, every issue disappeared.
DRIVING IMPRESSIONS
With the new PCM installed, the transmission now drives beautifully:
• The lower stall speed matches the 4.0’s torque curve perfectly.
• Shifts are smooth at light throttle and crisp at higher RPMs.
• It no longer hunts for gears or feels sluggish.
• Converter lock-up engages predictably and holds steady.
I’m still running the factory 3.73 gears on 35s (I’ve got 5.13s ready to go in soon), but even as it sits, drivability is drastically improved.
FINAL THOUGHTS
It’s only been a month, but I’m genuinely impressed. The ATS 42RLE feels solid, well-built, and purpose-engineered for real Jeep use—not just pavement duty.
Pros:
• Heavy-duty build quality
• Lower stall converter greatly improves drivability
• 5-year off-road warranty
• Excellent packaging and customer service
Cons:
• Freight and core shipping cost is high
• Needs a healthy PCM (2005–2006 stock units are notorious for issues)
Overall, I’d absolutely recommend ATS Diesel to any Jeeper looking for a serious transmission upgrade. It’s the first time I’ve actually enjoyed driving my LJ with an automatic.
. What problems have you run into with them?
