How to Install the RT Off-Road Transmission Cooler on a Jeep Wrangler JK

ByWyatt Brooks

First Thing: Get the Instructions Crown Automotive Actually Has

Crown Automotive manufactures this kit. Extreme Terrain resells it. Crown does have a PDF instruction sheet for the RT24006 — it just isn’t in the box, and Extreme Terrain support apparently doesn’t mention it. Before picking up a tool, check Crown Automotive’s website or call them directly to get that document. It includes diagrams of bracket placement and line routing specific to this kit.

The install itself is DIY-friendly. No lift needed. Most people finish it in an afternoon.

What the Kit Includes and What It Does

The RT24006 comes with the cooler core, mounting brackets, rubber hose, and fittings. It’s an auxiliary cooler — it works alongside the factory transmission cooler already built into the bottom of your radiator, not in place of it. The two coolers run in series, so fluid gets two passes of cooling before returning to the transmission. This matters a lot for the 42RLE in the 2007–2011 JK, which runs warm under load and is sensitive to heat over time.

Tools and Supplies

  • Flat-head screwdriver (for the grille clips)
  • Basic socket set
  • Drill with a small metal bit
  • Hose clamps, if the kit doesn’t include enough
  • Drain pan and rags
  • ATF+4 fluid to top off after install

Step 1: Pull the Grille

Six plastic retaining clips run along the top of the grille. On pre-2008 models they’re screw-style — back the center screw out and pry the clip free. On 2008 and later models, slip a flat screwdriver into the center tang to pop it up, then pry the whole clip out.

The bottom of the grille is held by snap clips. Grab it near the bottom and pull forward — they release without tools. Before lifting the grille clear, twist and remove both turn-signal bulbs counterclockwise. It’s a quick step that’s easy to skip, and the wiring gets stressed if you pull the grille with the bulbs still seated.

Step 2: Mount the Auxiliary Cooler

The cooler mounts on the driver’s side, positioned in front of the radiator. That placement isn’t optional — it’s where the factory transmission lines run. Leave roughly an inch of clearance between the aux cooler and the radiator face to keep airflow moving through both.

The brackets in the kit strap around the cooler and bolt to the lower grille support brace. You will almost certainly need to drill one or two holes in that brace. Use the bracket as a template: hold it in position, mark the hole locations with a marker or punch, then drill. Don’t fully tighten the mounting hardware until the lines are routed and the cooler’s final position is confirmed.

Step 3: Route the Supply Lines

Run the supplied rubber hose under the bottom of the radiator on the driver’s side, or between the radiator and the grille support — either path works. Keep lines clear of hot exhaust components and avoid sharp bends. A kinked line restricts flow; a line that rubs against a metal edge long enough will wear through.

Step 4: Splice Into the Factory Transmission Lines

The factory trans cooler lines have a rubber section on the driver’s side, where a steel line transitions to rubber hose near the bottom of the radiator. That’s your splice point. Cut the rubber section at the center.

Before pushing any fittings in, slide hose clamps onto both cut ends. It’s the step people skip, and it means dismantling the whole connection to do it right the second time. Snug the clamps after the fittings are seated but before you run the engine.

Flow direction matters. The line coming from the factory radiator cooler connects to the bottom fitting on the aux cooler. The top fitting of the aux cooler then runs back toward the transmission. Think of it as: radiator cooler first, aux cooler second, then back to the transmission. Reversed connections reduce cooling efficiency and are the most common wiring mistake on this install.

Step 5: Reassemble and Leak Check

Reinstall both turn-signal bulbs. Snap the grille back on with the bottom clips first, seat the top, then press the retaining clips in. Start the engine and let it idle for a few minutes. Slowly shift through all gears, P through L and back, then shut it down and check every hose connection by hand and by eye. ATF shows up clearly.

The new cooler core and lines will hold some fluid, so ATF level may be slightly low after the first run. Check and top off with ATF+4 — that’s the correct spec for the 42RLE; generic ATF is not a drop-in substitute.

One Step Most Guides Skip

After the first few heat cycles, go back and re-snug all four hose clamps. Rubber expands when it warms up, and clamps that were tight at install often back off just enough to produce a slow seep. Two minutes with a screwdriver, and it’s done.

Common Questions

Does the auxiliary cooler replace the factory one in the radiator?

No. The RT24006 runs in series after the factory cooler. Fluid passes through both before returning to the transmission. Removing the factory cooler is not part of this install.

Will ATF spill when I cut the factory lines?

Some will drip. Put a drain pan under the work area before cutting. You won’t lose a significant amount, but don’t skip the pan.

What fluid does the 2007–2011 JK 42RLE take?

ATF+4. Use it when topping off after the install and for any future transmission service.

Sources


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