8L90 Transmission Cooler 2015-2018

This part is for the cooler circuit on a 2015-2018 Chevrolet Silverado 1500 or GMC Sierra 1500 with the 8L90 eight-speed, and it is worth being clear about what it is: not a radiator-style cooler core, but a billet bypass valve block that bolts in where the factory thermal bypass valve sits, so fluid is sent through the cooler you already have. It is CNC-machined 6061 aluminum with a black anodized finish and comes with mounting hardware. If your truck is running hot, shuddering when warm, or you tow and want the cooler working the whole time, this is the piece people are usually searching for under the name 8L90 transmission cooler.

What the bypass block changes in an 8L90 cooler circuit

From the factory, fluid leaving the 8L90 goes to an external thermal bypass valve before it ever reaches the cooler. Per GM bulletin 22-NA-182, that valve does not open until the fluid reaches 194 F (90 C). Below that, fluid loops back to the transmission without being cooled. The trade-off is quick warm-up in cold weather against a valve that has to work every time; if its thermostatic element sticks, the cooler sits idle while the fluid keeps heating. This block is sold as an upgrade for that valve, built to keep fluid moving through the cooler and lower operating temperature, which is why truck owners who tow or plow fit it.

Signs your 2015-2018 8L90 needs cooler flow

• Gauge or scan-tool fluid temperature that runs well above normal on a highway pull or with a trailer
• Overheat messages in stop-and-go traffic in summer
• Torque converter shudder at a steady 25-80 mph cruise that gets worse as the truck warms up
• Harsh or flared shifts after a long hot drive that clear up after the truck cools down
• Fluid that smells burnt or has darkened early

Check these first

Before adding parts, rule out the free fix. GM's 22-NA-182 lists a twisted rubber cooler line as a common cause of overheat, shudder, surge and flare complaints on low-mileage trucks, and it can set DTC P27EC on some units. Our 8L90 cooler line replacement guide shows how to spot and correct it. Shudder by itself is often fluid, not heat; GM bulletin 18-NA-355 calls for a full exchange to Mobil 1 Synthetic LV ATF HP, explained in 8L90 common problems and fixes. If heat persists with straight lines and fresh fluid, the bypass valve is the next suspect.

Fitment

Listed for 2015-2018 Chevy and GMC 1500 pickups with the 8L90. The same truck years were also built with the 6L80 six-speed, and the 2019-and-newer trucks changed the valve, so do not assume it carries over. Look at the RPO code on the glovebox or door label, compare the shape and port layout with the valve on your truck, or send us your year and VIN and we will check it. Tahoe, Suburban, Yukon and Escalade owners should confirm with us before ordering. Other parts for this unit are on the 8L90 / 8L45 parts page.

Install notes

• Install cold. Disconnect the battery if the work area is near wiring, set a drain pan under the valve and disconnect the cooler lines.
• Unbolt the factory valve, mount the block with the supplied hardware, and reconnect the lines with clean, fluid-lubricated O-rings. Check that neither rubber line is twisted after reassembly.
• Refill with DEXRON HP only, the fluid GM specifies for the 8L90. Expect to lose some fluid from the lines and cooler.
• With more fluid going to the cooler, warm-up in winter will take longer. Set the level at the check plug only once the fluid is at 35-45 C (95-113 F) with the engine running.
• If you are doing a fluid exchange at the same time, add the 8L90 filter and pan gasket kit. A unit that still shifts harshly once the heat is under control may need the 8L90 valve body solenoid kit.

Is this a transmission cooler core?

No. It is a billet bypass valve block that replaces the factory thermal bypass valve so fluid flows through the existing cooler. It does not replace the cooler in the radiator or the auxiliary cooler.

Which trucks does it fit?

It is listed for 2015-2018 Chevrolet Silverado 1500 and GMC Sierra 1500 with the 8L90. Trucks from the same years with a 6L80, and 2019-up trucks, may need a different part, so send your year and VIN if unsure.

When does the factory 8L90 bypass valve open?

GM bulletin 22-NA-182 lists the 8L90 valve as external to the transmission and opening at 194 F (90 C). Below that, fluid skips the cooler.

Will my transmission take longer to warm up?

It can. With more fluid going to the cooler, the fluid heats more slowly in cold weather, so wait until it reaches 35-45 C before checking the level.

What fluid should I add after installing it?

DEXRON HP only. GM's 8-speed shudder bulletin uses Mobil 1 Synthetic LV ATF HP, which meets that specification.

• 8L90 Cooler Line Replacement: Diagnosis, Procedure and the Fluid Reset
• 8L90 Common Problems and Fixes
• 8L90 Shift Solenoid Symptoms and Replacement
• 6L80 vs 8L90: Differences, Interchange, and Which Is Stronger