Mishimoto Pentastar + Hemi Thermal Bypass Valve Upgrade for 12-18 Jeep Wrangler JK

Automatic transmission heat can become a serious concern when a Jeep Wrangler spends long periods crawling over rocks, climbing steep grades, or working through demanding terrain. Unlike normal highway driving, these conditions often place substantial loads on the drivetrain while limiting the airflow available to remove excess heat. The Mishimoto Full-Flow Transmission Thermal Bypass Valve Kit for the 2012–2018 Jeep Wrangler JK addresses this challenge by eliminating the factory transmission cooler bypass restriction and allowing fluid to circulate through the cooler continuously.

From the factory, the Wrangler JK's automatic transmission uses a thermostatic bypass valve to regulate fluid circulation through the transmission cooler. When the transmission is cold, the valve restricts cooler flow to help the fluid reach its intended operating temperature. As temperatures rise, the thermostat opens to increase cooling. Although this arrangement serves an important purpose in colder environments, a malfunctioning thermostat can restrict fluid circulation and contribute to excessive transmission temperatures. For Jeep owners who regularly operate in hot climates, the factory valve can also represent an unnecessary restriction.

The Mishimoto Full-Flow Thermal Bypass Valve replaces that temperature-controlled arrangement with a continuously open fluid passage. Rather than waiting for a thermostat to open, transmission fluid begins circulating through the cooler as soon as the engine starts. This design eliminates the factory thermostat as a potential failure point while allowing the cooling system to operate without the additional restriction created by the original bypass mechanism. The result is a straightforward way to improve transmission cooling efficiency under demanding driving conditions.

Construction is equally purposeful. Mishimoto machines the replacement valve body from billet aluminum, creating a durable component designed to withstand repeated heating and cooling cycles beneath the Wrangler's hood. Integrated cooling fins increase the valve's external surface area to promote additional heat dissipation, while the anodized finish provides corrosion resistance and a distinctive appearance. Unlike the factory thermostatic assembly, the full-flow valve contains no temperature-sensitive thermostat that could eventually stick or fail.

This configuration is particularly appealing for Wrangler owners who drive in consistently warm climates, tow heavy loads, or regularly encounter demanding off-road conditions. By allowing unrestricted cooler circulation, the system helps address a major contributor to automatic transmission wear: excessive operating heat. However, the full-flow design is not intended for every environment. Because the transmission cooler remains active from startup, the system can prevent fluid from reaching its intended operating temperature in cold weather. Mishimoto specifically advises against using the full-flow configuration in cold climates where the transmission may overcool.

Installation is designed to be straightforward, with a vehicle-specific configuration that connects directly to the Wrangler's existing transmission cooler piping. No modifications to the factory lines are required, and Mishimoto estimates installation can be completed in under an hour. For Jeep owners seeking maximum cooler circulation without adding an aftermarket transmission cooler or modifying the factory plumbing, the Mishimoto Full-Flow Transmission Thermal Bypass Valve Kit offers a compact, durable solution backed by the company's Lifetime Warranty.

Kit Includes: One CNC-machined billet aluminum full-flow transmission thermal bypass valve.

Technical Specifications: Mishimoto part number MMTC-JK-TBVFF. CNC-machined billet aluminum construction with integrated heat-sink fins and an anodized finish. Non-thermostatic full-flow design allows continuous transmission fluid circulation through the factory cooler. Compatible with original transmission cooler piping and backed by Mishimoto's Lifetime Warranty.

Important Cooling Note: This full-flow version does not contain a thermostat and allows transmission fluid to circulate through the cooler immediately after startup. Mishimoto does not recommend this configuration for cold-climate operation because it may prevent the transmission fluid from reaching its intended operating temperature. For owners who require temperature-regulated cooler flow, Mishimoto offers a separate thermostatic bypass valve kit.

Fitment Note: Designed for 2012–2018 Jeep Wrangler JK and JK Unlimited models equipped with the 5-speed automatic transmission. Not compatible with manual transmission models.

Installation Notes

Installation Difficulty:

Level 2Intermediate

Installation Time: Less than an hour

Parts Included

1 Bypass Valve
1 160°F Thermostat (OE Spec)
1 Thermostat Bypass

Warranty

Limited Lifetime Warranty