TRANSMISSION COOLER BYPASS VALVE

THE 10-MINUTE, $60 FIX THAT COULD SAVE YOUR ALLISON

- MCGLOTHLIN

If you work your L5P Duramax for a living or on the weekends and have noticed your Allison running warm, you’re not alone. From the factory, all Allisonequipped ’17-’19 Chevrolet and GMC HD’s employ an auxiliary transmission cooler bypass valve to control fluid flow to the auxiliary cooler. In cold weather, it allows transmission fluid temperature to warm quickly. Unfortunately, the bypass valve is extremely restrictive and doesn’t allow adequate flow to the auxiliary cooler, even when the auxiliary cooler is needed most. The result of the bypass valve not functioning as it should is excessive heat in the transmission—and the problem only gets worse with increased line pressure in the mix, such as is often the case in built transmissions. Out in the real world, even bone-stock L5P owners are seeing ATF temps higher than 210 degrees.

As one of the premier Allison builders in the Midwest, LinCo Diesel Performance noticed the problem shortly after the L5P was released. Thinking on their feet, the guys at LDP came up with an Allison-saving solution that costs $59.99 shipped and installs in less than 10 minutes. The company’s transmission cooler bypass valve provides full flow to the auxiliary cooler and drops ATF temps by as much as 60 degrees. To see its product in action, we stopped by LDP’s Troy, Missouri facility, where it was being installed on a ’19 dually. Following some before and after testing, the bypass valve dropped transmission fluid temp by 49 degrees, and it did so on a 38-degree winter’s day. Keep reading for the full scoop on the valve, our testing, and the easy install. This is one mod every L5P owner should be considering.