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Transmission Cooler Basics

Posted in How To: Transmission Drivetrain on March 1, 2012
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Jeep transmissions love this time of year. Not only do they run cooler in the winter months, but crappy weather along with seasonal trail closures force many Jeeps into a state of hibernation. Sure, interruptions may come in the form of spur of the moment snow runs, or that pesky neighbor who refuses to plow and instead requests your assistance to free his stuck Honda Accord. Like a grizzly bear holed up in a cave, winter downtime can foster laziness from even the most industrious Jeeper. We are here to combat such neglect and inspire you to crank up the shop heater and put in the hours necessary to prep your rig for the imminent thaw of spring. Winter is the perfect time to tackle those little projects you put off during the summer months. Routine maintenance items such as changing out the transmission fluid and filter are a good start. However, if you want to make the most of your vehicle’s hiatus, consider adding an auxiliary transmission cooler. When the ice cycles retreat, and open top weather returns, your Jeep will thank you.

One of the most common mistakes made when plumbing an auxiliary transmission cooler has to do with inlet and outlet orientation. If mounted in a horizontal manner, meaning the fluid flows across the grille opening from driver side to passenger side and back, the proper port configuration is to ensure that the hot ATF enters the cooler on the lower port and exits from the top port. This orientation will reduce the amount of time it takes for ATF to prime the cooling system upon startup—a good way to preserve the life of your transmission.

It’s said that 90 percent of transmission failures are caused by excessive heat. When ATF overheats, it oxidizes and breaks down rendering it ineffective. You can tell when this oxidation has occurred by inspecting the condition of the fluid. Bad fluid will appear dark brown and emit a noticeable burnt aroma. To keep ATF from cooking, the aftermarket has a plethora of transmission coolers available. Generally, they are rated by gross vehicle weight (GVW), typically ranging from 12,000lb GVW all the way up to 30,000lb-plus GVW for heavy-duty applications. For slow-moving Jeeps that don’t get a ton of airflow, we recommend a cooler in the 18,000lb GVW range. This size will provide enough surface area to keep ATF cool at rock crawling pace, yet still fit between the grille and existing cooling stack.

To plumb an auxiliary transmission cooler you need to first identify which of the hard steel lines coming from the transmission is the hot side. The hot side delivers hot ATF to the front of the vehicle. The easiest way to quickly identify the hot side is to scan each with a hand-held laser temperature probe. Of course the vehicle must be driven first to get the ATF up to normal operating temperature. If you are a cheapskate like some members of the Jp staff, you can also drive the vehicle for a short bit and stop to crawl under the rig and physically feel which line is hotter. One should be warm and the other should be cool after a one-minute drive. Once you have established which line is hotter of the two, you can map out your plumbing accordingly. If the vehicle has an existing cooler in the lower portion of the radiator, make your tie-in where the ATF exits the radiator. You want to further cool the fluid after the factory setup. If you are starting from scratch on a vehicle that doesn’t have an existing cooler, it’s best to ask a local transmission shop for advice as line orientation varies significantly from vehicle-to-vehicle. When routing additional lines, it’s a good idea to secure hoses away from anything hot or that might move. High heat from exhaust will counteract the cooler’s effort, and a moving part (like a fan or pitman arm) could potentially damage the lines. A transmission cooler can also be used for cooling power steering fluid, but with steering systems the cooler should be plumbed between the steering box and the fluid reservoir on the return side of the system.


Fife, WA 98424

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