Bypassing the Norm – Thermal Bypass Valve R&D, Part 1: Design to Delivery

The transmission is the backbone of your 2013–2018 Dodge Ram 6.7L Cummins. It’s responsible for transmitting up to 930 lb-ft of torque from the engine to the wheels. That much power generates a lot of heat and if that heat isn’t dissipated, it can destroy your transmission. The transmission cooler does a decent job of keeping transmission fluid temperatures in check, but only if the fluid can reach it.

The stock cooling systems of both the 68-RFE and Aisin AS69RC utilize an in-line thermal bypass valve that controls the flow of fluid to the transmission cooler. The purpose of this valve is to prevent transmission fluid from flowing to the cooler until it reaches operating temperature. In cold environments, this operation is important to prevent overcooling, but there’s an inherent problem with the stock bypass valve.

The thermostat inside the stock thermal bypass valve is prone to failure. When the thermostat fails, it prevents flow to the cooler and the transmission can overheat. The stock bypass valve isn’t serviceable either, so the only option is to replace the entire valve. Proper thermostatic operation of this valve is vital, especially in cold climates, but given the failure rate of the stock valves, a factory replacement isn’t the ideal choice. So, we decided to come up with a better solution for your 4th Gen.  

Our engineer, Jason, started development by inspecting the stock bypass valve and finding ways to improve it. The stock valve’s function is about as simple as its appearance. The valve has four ports, two connecting the transmission to the valve and two connecting the valve to the transmission cooler. The passage of the valve that carries fluid from the transmission to the cooler contains a thermostat just like the one found in engine’s cooling system.

When the transmission fluid is cold, the pin in the thermostat is compressed by a spring, which forces the thermostat up and closes the passage to the cooler. The fluid then flows down to the return passage and back into the transmission. As the fluid warms up, wax inside the thermostat expands and pushes the pin out. This forces the thermostat down, closing off the bypass and opening the passage to the cooler.

Aside from the fact that the thermostat is prone to failure and there’s no way to service it, the stock thermal bypass valve has another problem—flow. The passage that carries fluid from the transmission to the cooler is exceedingly small and necks down even more around the thermostat. This limits flow to the cooler and how effective the cooler can be.

To begin solving these issues, Jason created a 3D model of our thermal bypass valve and 3D printed a prototype. Our replacement valve will utilize the thermostat components from our 2003–2009 Dodge Ram transmission cooler. Where the stock valve has a pressed cap preventing access to the thermostat, our valve will be accessible by way of a threaded plug. For our customers who live in hot climates and aren’t concerned with overcooling, we’ll also include a bypass pin that allows full flow to the cooler at all times. To solve the flow issue, Jason enlarged the trans fluid passageways. For a final aesthetic touch and more cooling, we will also be machining cooling fins into the valve body.

Our 3D printed prototype let us verify our design, but the plastic valve wouldn’t stand up to the heat and pressure of the transmission cooling system. To finish R&D, we needed an aluminum production sample. Jason finalized his design and sent the drawings off to production. A few weeks later, we had our CNC-machined aluminum thermal bypass valve in hand.

All that was left to do was test fit the valve on the truck and make sure the thermostat opened as it should. So, we brought in our volunteer Ram and swapped the valves.

Our thermal bypass valve not only looks better than the stock unit, but it performs better too. The thermostat we chose for our bypass valve opens about 10°F earlier than the stock thermostat. This allows fluid to flow to the cooler sooner while maintaining fast warm-ups. To verify that the thermostat was working properly and that we had no leaks, we let the transmission get up to temperature.

The Mishimoto bypass valve fit and functioned flawlessly. With a successful test fit in the books, we wrapped up our R&D on this 2013–2018 Dodge Ram 6.7L Cummins thermal bypass valve kit. If you’re interested in improving the reliability and performance of your Ram’s transmission cooling system, head over to our website to purchase this kit and, as always, let us know what you think.

Thanks for reading,
-Steve

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