6L80 Transmission Cooler 1999-2013, new and ready to install. Fits the 6L80. Ships fast from the USA.
If you tow with a 1999-2013 Chevrolet Silverado or GMC Sierra and the dash keeps warning that the transmission is hot, or you have already cooked one set of clutches pulling a camper up grades, a bigger cooler is the cheapest insurance you can buy. This is the Mishimoto MMTC-GMT-99 performance transmission cooler, a stacked-plate aluminum unit made as a direct fit for GMT800 and GMT900 gas trucks with either a 4L-series four-speed or the 6L80 six-speed. Mishimoto rates the core at 521 percent more volume and 278 percent more frontal area than the stock cooler.
Every automatic turns some engine power into heat, mostly in the torque converter and in clutches as they slip into apply. In a 6L80 that heat leaves through the fluid lines, first through the cooler section in the radiator and then through the external cooler at the front of the truck. When the external cooler is small for the work being done, fluid temperature climbs, the fluid oxidizes faster, and the friction additives that keep the converter clutch smooth break down. Converter shudder from fluid breakdown is one of the best-known 6L80 failures, and heat is what drives it. A larger core with internal fins and louvered outer fins pulls more heat out at highway speed and at the low speeds where towing and trail driving are hardest on the fluid.
Watch transmission fluid temperature on a scan tool or the gauge cluster during the kind of driving that causes trouble. If temperature climbs steadily while towing and settles only after you stop, the system needs more capacity. Also check for a stuck thermal bypass or a kinked line, since a restriction mimics a weak cooler. Our 6L80 common problems and fixes guide explains how heat ties into shudder and clutch wear, and the 6L80 Escalade common problems guide covers the same drivetrain in GM's heavier SUV.
Mishimoto lists this cooler as a direct fit for 1999-2013 Chevrolet Silverado and GMC Sierra gas trucks with 4LXX or 6L80 transmissions. That window is wider than the 6L80 itself: the earlier GMT800 trucks use four-speed 4L-series units, and the 6L80 arrived in the GMT900 trucks from the 2007 model year on the larger V8s. The cooler fits either. Diesel Duramax trucks are not covered, and the heavy-duty 6L90 is not named by the manufacturer, so check before ordering for a 2500 or 3500. Fitment can vary with cab, trim and towing package, so send us your year and VIN if you are unsure. For other parts for this transmission see our 6L80 transmission parts page.
The kit includes the cooler, two precision-machined -6 ORB quick-disconnect fittings, a mounting bracket and a relocation bracket for the ambient air temperature sensor so the outside temperature reading still works. The quick-disconnects connect to the stock cooler lines, and they can be removed if you want to run -6 AN lines. Mishimoto rates the install as medium difficulty. Catch fluid when you open the lines, keep dirt out of the fittings, and top off afterward with Dexron VI such as our Valvoline Dexron VI full synthetic. Check the level with the engine running at operating temperature and inspect each fitting for seepage after the first drive. If the fluid is already dark, change it and the filter at the same time; a 6L80 pan kit with drain plug and filter makes the next service a lot cleaner.
| Fits | 6L80 1999-2013 |
| Part type | Transmission Cooler |
| Condition | New |
| Brand | OEM |
| Ships | From the USA |
| Warranty | 1-year |
Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
The manufacturer lists it for 1999-2013 Silverado and Sierra gas trucks with 4L-series or 6L80 transmissions, so a 2005 gas truck with a 4L60E is in that range. Send us your VIN if you want a fitment check.
Diesel trucks are not covered, and the 6L90 is not named in the manufacturer's application, so do not order it for those trucks without confirming fitment first.
No. The included -6 ORB quick-disconnect fittings connect to the stock lines. You can remove them and run -6 AN lines if you are building a custom setup.
Yes. The kit includes a relocation bracket for the ambient air temperature sensor so it keeps working after the cooler is installed.
A 6L80 takes Dexron VI. Check your owner manual for a 4L-series truck. Set the level hot with the engine running after the install.
For light commuting the stock cooler is usually enough. It pays off for towing, plowing, hauling, hot climates and stop-and-go driving where fluid temperatures run high.
