Derale 13225 Frame Rail Transmission Fluid Cooler

Transmission fluid that runs hot is transmission fluid that is on its way out. Once ATF lives above about 220F the additive package starts to cook off, the friction material on the clutches glazes, and the unit starts flaring on the 1-2 and 2-3 shifts long before anything actually breaks. The Derale 13225 is an add-on air-to-fluid cooler you plumb inline with the transmission cooler lines to keep those temperatures in range when you tow, haul, or sit in traffic in July.

What makes this one different from the usual square core that hangs in front of the radiator is the shape. It is a long, narrow 24-inch cooler built to bolt along the frame rail, behind the bumper, or anywhere else there is length but no height. That matters on trucks and swapped cars where the radiator support is already packed with an A/C condenser, an intercooler, or a power steering cooler and there is simply no real estate left up front. Plumb it low, out of the way, and you still get a full-size cooling surface.

The core is 1/2-inch copper tubing mechanically expanded into aluminum cooling fins, which transfers heat faster than an all-aluminum tube-and-fin core of the same size. Each tube has embossed turbulators pressed inside it that stir the fluid as it moves through instead of letting it slide straight down the center of the tube, so more of the ATF actually touches the tube wall. It is a 2-pass design, and the inlet and outlet are -6AN so you can run braided line or adapt to 3/8-inch hose. Extruded plastic fin guards along the edges keep the fins from getting flattened by road debris or by a wrench during install. The whole assembly weighs 1.5 pounds.

Run it in series after the factory radiator-mounted cooler, not instead of it, so the fluid still gets brought up to operating temperature on a cold start and then gets the extra drop before it goes back to the pan. It is universal, so it works behind any automatic regardless of model, and owners also use it as a power steering or engine oil cooler. Add the small amount of fluid the new lines hold and check the level hot.