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Keep Your Cool, Allison: LinCo Diesel Performance’s $60 Transmission Saver

Since 2001, the Allison 1000 has been available behind the 6.6L Duramax diesel V-8 in GM’s heavy-duty trucks, a hot-selling and highly sought after powertrain combination. Throughout the years, they’ve been treated to various upgrades at the OE level in order to keep pace with the ever increasing horsepower and torque ratings of the Duramax. However, beginning on ’17 model year trucks (and coinciding with the all-new, 445hp and 910 lb-ft L5P Duramax) the Allison left the factory with a highly restrictive auxiliary transmission cooler thermal bypass valve.

As a company that spends a lot of time rebuilding and reinforcing Allison transmissions, the folks at LinCo Diesel Performance started to notice a troubling trend with their own L5P tow-mule. On hot summer days, they noticed that transmission temperature continued to climb, even though the truck was cruising 75-mph down the highway and the cooler was seeing plenty of airflow. With a trailer in tow, things worsened, with trans temp creeping as high at 215 degrees. Upon closer inspection, they diagnosed the factory bypass valve as the culprit and promptly set to work making a drop-in replacement.

Insufficient Flow

Stock Allison Cooler Bypass Valve

This is the offender behind the warm-running Allison condition in your ’17-’19 GM HD: the factory transmission cooler thermal bypass valve. This little valve unnecessarily restricts ATF flow to the transmission cooler. No, your factory auxiliary cooler isn’t undersized or being starved for airflow—it’s just not being used like it should. The insufficient fluid flow supplied to the cooler leads to higher ATF temps and the problem only gets amplified further on trucks with aftermarket tuning or a built transmission that sees increased line pressure.

LDP’s Transmission Cooler Bypass Valve

Linco Diesel Performance Transmission Cooler Bypass Valve

LinCo Diesel Performance’s transmission cooler bypass valve is a direct replacement for the factory valve. The company’s patent-pending, one-piece valve (the factory piece is three) is simpler in design than other aftermarket alternatives, is CNC-machined with smooth radius’ to maximize fluid flow and drops directly in place of the stock assembly.

Accessing the Factory Valve

Transmission Cooler Bypass Valve

Installing LinCo’s bypass valve begins with the removal of the factory one, but first you have to unplug the mass airflow sensor and pull the air intake. Though the bypass valve ties into the transmission cooler lines on the left side of the radiator, none of them have to be broken free during the install (this means no loss of ATF and no mess to clean up). What does have to be broken free is the snap ring that holds the bypass valve in place. Prior to removing the valve, the position of the half-moon on the top of the valve should be noted. If you plan to run the stock valve in winter (for quicker transmission warm up) you’ll need to reinstall it the same way.

LinCo Valve Vs. Stock Valve

Allison Bypass Cooler Valve Comparison

In this cutaway comparison, you can see how much more freely fluid flows with the LinCo bypass valve installed (right) vs. what the stock one offers (left). Increased flow from the radiator (top left port) to the auxiliary transmission cooler (top right port) directly relates to lower transmission operating temperatures. Customers with stock trucks report seeing ATF temps 30 to 50 degrees lower and LinCo has seen as much as a 60-degree drop on trucks with higher line pressure.

Outlet Port Comparison

Bypass Valve Outlet Comparison Allison Transmission

This view shows the outlet port of two separate transmission cooler bypass valve blocks, one with the stock valve installed (left), the other with LinCo’s valve in place (right). With more transmission fluid flowing into and then returning from the factory cooler mounted behind the front bumper, it doesn’t take a rocket scientist to figure out why LinCo’s valve works so much better than stock.

Direct Drop-in

L5P Duramax Transmission Cooler Bypass Valve

To keep from rolling or tearing an O-ring, dab some assembly lube on the new bypass valve before installing it. Then simply drop the valve into place and push down until it is fully seated. Thanks to its one-piece design and no thermostatic valve capsule or spring being present, it doesn’t need to be aligned or oriented any certain way.

Wrapping Up the 7-Minute Install

LDP Transmission Cooler Bypass Valve

After reinstalling the bypass valve snap-ring, ensure it has been properly spread out (and seated) using a flat head screwdriver. To make it easier to detect a leak (which isn’t likely, but this covers all the bases), hit the top of the bypass valve with brake cleaner before reinstalling the air intake and mass airflow sensor. From start to finish, the entire install takes roughly seven minutes.

A Cheap, Quick and Cool Fix

ATF Temperature Allison 1000 Duramax

With full fluid flow making it to the auxiliary transmission cooler, don’t be surprised if your transmission temp fails to exceed 165 degrees after the install—even on the hottest of summer days. For $59.99 and seven minutes of your time, you can finally take full advantage of the stout L5P Duramax platform without having to worry about the Allison transmission seeing excessive heat.

Curious about what other parts are available for the L5P Duramax? You can find some of them here, along with the reasons why they’re necessary.

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