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Jeep’s 42RLE automatic transmission doesn’t have the best reputation. Our transmission seemed to be shifting poorly so we visited our local transmission rebuilder. “What were you doing on the way over here?!” they asked after plugging in the scanner to our Jeep. “The transmission fluid is 220 degrees!” We weren’t out hot rodding, but the catalytic converter sits right next to the transmission pan on our Wrangler, and the steel skidplate across the entire frame traps the heat inside. A transmission cooler beyond the factory cooler in the radiator was the obvious fix for a problem we didn’t even realize that we had.
This Jeep gets used off road regularly, including rockcrawling and water crossings, so we knew wherever we mounted the cooler it would have to be safe from harm. Skidplates and a lack of air flow are issues that caused our transmission to overheat in the first place, so we had to be careful not to make the same mistakes with the new cooler. We considered mounting the transmission cooler behind the grille in front of the radiator, using the existing lines that currently go to the factory cooler inside the radiator. There is plenty of room here, the cooler would get lots of air flow, it would be easy to mount, and it would be protected from harm. In fact, if you have a 2007-2018 Wrangler JK, B&M offers a kit (PN 70291) to easily mount a transmission cooler behind the grille.
We have a 2006 Jeep Wrangler LJ though, so there was no bolt-on solution. We worked with fabricator Nick Maher from Martec Engineering in Carson City, Nevada, to come up with custom mounting for our transmission cooler. Nick specified a compact Earl’s UllraPRO fluid cooler with an integrated fan and mounted the cooler under the chassis next to the rear driveline. This utilizes available space, makes for easy plumbing, and is protected from harm. Air flow is admittedly not as good as in the grille, particularly at freeway speeds, but the included SPAL electric fan flows 342 cfm and works just as well at crawling speeds as it does at freeway speeds. Additionally, we didn’t have to be concerned about the transmission cooler heat soaking the radiator with this mounting location.
Since space was an issue, we wanted the most efficient fluid cooler available since we couldn’t just mount up a huge, overkill transmission cooler. Martec recommended an Earl’s UltraPRO cooler for the rugged stacked plate architecture, aluminum construction that efficiently transfers heat, and state of the art brazing that can handle the rigors of off-roading. Finned tube coolers are a good option if you have more space or are on a budget, but they are less efficient than stacked plate coolers.
One option to keep your JK's transmission cooler is B&M's Hi-Tek Direct-Fit Automatic Transmission Supercooler (p/n 70291), which fits beautifully behind the grille shell.
The end result is lower transmission temperatures, with translates into less slippage, longer fluid change intervals, and ultimately a longer transmission life. For a Jeep that travels over the Rubicon on a regular basis, we will take all the help we can get.
Mounting the cooler behind the grille, in front of the radiator, places the cooler in the path of maximum air flow.
Nick Maher started by removing the old tranny cooler lines, which appeared to be blocked. This likely had something to do with our cooling issues. All of the work was done at Martec on their two-post lift. You could do this at home in the driveway, it just isn’t as easy or convenient as using a lift in a fully equipped shop.
All the fittings used are sold separately from the Earl’s fluid cooler to allow you to customize your installation. These special fittings going into the cooler look like normal ORB (O-Ring Boss) fittings, but have shorter threads so that they don’t bottom out in the cooler. We used -6 fittings (PN AT585106ERL).
Martec built a small bracket to hold the Earl’s UltraPRO cooler and used rivnuts into the tub to mount it with rubber isolators to reduce vibration. The cooler is located under the body inside the frame where it is protected from harm but still has a decent amount of airflow. The included SPAL fan makes airflow a non-issue, and the short plumbing to the transmission means that we don’t have to worry about cutting or rubbing through a line when off-roading.
Earls UltraPRO fluid coolers are not only efficient but also incredibly durable. This was a priority in our application, where the cooler is mounted under the vehicle. While every effort was made to protect the transmission cooler from damage and we certainly don’t expect to use the cooler as a rock slider, the stacked plate construction is much more robust than traditional finned tube coolers.
We plumbed the transmission cooler with Earl’s Super Stock synthetic rubber hose that uses a reinforced interior braided fabric sheath to withstand up to 250 psi. Earl's has an interactive plumbing selector on their website that is super helpful for projects like this. Just input the type of fluid and type of fittings you want to use (crimp, push together, etc) and the plumbing selector takes care of the rest.
Earl’s makes just about every fitting you could ever imagine for any application in any size and configuration. Martec used Earl’s Super Stock hose ends in a -6 AN size (PN 709167ERL) that match the 3/8-inch Super Stock hose. They are machined from aluminum and available in a blue or black anodized finish.
Nick Maher showed us a trick where he sprays window cleaner on the push-lock fittings prior to installing them to make them slide on more easily. We had seen this trick before on vinyl graphics but never hose fittings. The premise is the same either way, the window cleaner reduces friction initially but also evaporates rather quickly.
Earl’s UltraPRO fluid coolers are available with or without SPAL electric fans. 4.8-inch diameter fan on the cooler we chose has an integrated shroud and pulls 342 cfm, regardless of vehicle speed. This makes it perfect for our rockcrawling Jeep where the transmission is often taxed at low speeds.
We needed adapters (PN AT981666ERL) to convert the NPT fittings out of the transmission to the factory transmission cooler over to AN -6 for the Earl’s Super Stock hose and fittings. Earl’s carries adapters for nearly every transmission on the market, including our 42RLE.
Earl’s Flame Guard offers an extra layer of protection to ensure that the hoses don’t melt if they are subjected to heat. We sized the Flame Guard a little too tightly (PN 571008ERL) and it was a challenge to install, but the end product looks great. While most commonly found in their distinctive brick color, Flame Guard is also available in black for a stealth appearance.
There are a few different ways to wire the fan on the UltraPRO cooler. Our Jeep has a mechanical engine fan so we did not have the option of splicing into that wiring, but a thermostat switch could be added inline with the hose. We opted to just have the fan turn on with the ignition for the sake of simplicity.
Our transmission fluid temperature dropped by over 25 degrees at the pan after installing the Earl’s UltraPRO cooler. Many of the shifting issues that motivated us to visit a transmission shop in the first place have been resolved by the cooler fluid, saving us money and providing peace of mind on the road and on the trail.
Now we can run our Jeep down the freeway or the trail without concern about overheating our transmission fluid. This translates into less frequent fluid changes and a longer life for the automatic transmission in our Jeep Wrangler, more than offsetting the money we spent to purchase and install the Earl’s UltraPRO fluid cooler.
