A simple piece with a lot of R&D behind it. LinCo Diesel Performance’s transmission cooler bypass valve is CAD-designed, CNC-machined, and features smooth radius’s for optimum flow. Each unit is also anodized in the company’s signature green. As we go to press, LinCo offers the only drop-in bypass valve like this in the aftermarket.

The 10-Minute, $60 Fix That Could Save Your Allison

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 Allison-equipped ’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.

Meet the test mule for our before and after transmission temperature testing: a ’19 GMC Sierra Denali 3500 HD with 14,000 miles on the clock. It’s been treated to an emissions-friendly tow tune but no TCM tuning, so we’re working with stock line pressure. The owner runs a roofing business and hooks the truck to a trailer every day of the week. When the truck was dropped off at LinCo Diesel Performance’s Troy, Missouri facility, he reported seeing transmission temperature climb higher than 210 degrees on a daily basis.
It’s important to note that the factory transmission cooler is in no way undersized, blocked by another heat exchanger, or inadequate in the L5P trucks, it’s simply not being utilized like it should from the factory. By installing a LinCo Diesel Performance transmission cooler bypass valve you’re forcing more transmission fluid through the cooler, and seeing a drop in ATF temp as a result.
To keep one key variable the same, all of our before and after testing was conducted along the same 10-mile test loop. In addition, all testing was performed in Tow/Haul mode because: 1) the owner always has Tow/Haul on when towing, and 2) more line pressure is commanded in Tow/Haul mode.
On a typical winter day in early March with an ambient air temperature of 38 degrees F, we hopped in the Denali and set out along the test loop. While we expected transmission temp to rise as the truck came up to operating temp, we didn’t expect it to go 118 degrees over ambient (for those that don’t know, 100-degrees over ambient is considered the ideal operating temp for most transmissions). Throughout the course of the drive, and even on the 60-mph highway section of the trip, the Allison’s transmission temperature never leveled off. It only kept climbing.
The factory bypass cooler valve is located under the air intake assembly. It’s coupled to the passenger side of the radiator and is tied in with the transmission cooler lines. A major selling point for LinCo’s bypass valve is that you don’t have to break any transmission lines loose to install it. It’s a 100-percent drop-in installation with zero mess.
To access the factory bypass cooler valve, the air intake has to be removed. After unlocking and disconnecting the mass airflow sensor, the two Christmas tree style retaining clips that secure the air box have to be extracted (one is on the air box itself, while the second is below it on a coolant line). Once free from its mounting fasteners, the air box lifts up and rolls out of place.
Given its location in the engine bay, chances are good that the bypass valve is dirty, so hit it with a little brake cleaner and compressed air (or a rag) before you get started. Pulling the bypass valve begins with the removal of the snap-ring up top, which we made quick work of with retaining ring pliers.
Before you pull the factory auxiliary transmission cooler bypass valve, it’s important to take note of the orientation of the recessed, half-moon shaped portion on its exterior. If you live in a colder climate and plan to reinstall the valve for winter driving, it has to be reinstalled this way (with the recessed half-moon facing forward, toward the auxiliary cooler lines). If it’s reinstalled incorrectly, the auxiliary cooler will see even less flow than it originally did.
Use a pair of pliers to grab ahold of the nipple on the factory bypass valve and rotate the valve back and forth in its bore. This will help break both internal O-rings loose. Then the upper section of the bypass valve assembly can be lifted out of its bore slowly, so as to allow fluid to drain back into the bypass port while you’re removing it.
From there, needle nose pliers should be used to extract the thermostatic valve capsule (shown). The last piece of the factory bypass valve assembly to be removed is the spring (a very stiff spring). An angled pick works best for fishing the spring out.
A simple piece with a lot of R&D behind it. LinCo Diesel Performance’s transmission cooler bypass valve is CAD-designed, CNC-machined, and features smooth radius’s for optimum flow. Each unit is also anodized in the company’s signature green. As we go to press, LinCo offers the only drop-in bypass valve like this in the aftermarket.
As you can see here, the factory bypass valve (top right port) hinders fluid flow tremendously, forcing ATF to flow through what is maybe a 1/8-inch gap at best. With LDP’s drop-in bypass valve in place (top left port), full flow, in and out, is made possible.
Prior to installing the LDP bypass valve, its O-rings need to be properly lubricated. For our install, Trans-gel assembly lube was used, but transmission fluid, silicone lube, or even Vaseline will work just as well. So long as the O-rings are lubed, it reduces the chances of rolling or tearing them during installation.
The comparison view from the outlet side tells a similar story. Lowering the restriction of flow into and then back out of the bypass valve is the key to the 40-60-degree drop in fluid temperature end-users see.
Thanks to its one-piece design, which is void of the factory thermostatic valve capsule and spring, the orientation of the LinCo bypass valve isn’t a concern when installing it. Once in the factory block, the valve can be pressed straight down until it is fully seated.
Next, the factory snap-ring is reinstalled via needle nose pliers to secure the bypass valve. If you hear it snap into place, you know it’s been correctly installed.
Once the snap-ring is in, it’s wise to use a small flat head screw driver to ensure the snap-ring is both fully seated and spread out. Simply put the blade between the two ears of the snap-ring and twist.
Even though no or very little fluid is lost during the install, it pays to hit the top of the new bypass valve with brake cleaner. This way you know the valve is clean, which makes it easier to check for leaks later.
The finished, installed product looks like this. In the amount of time it takes to read this article, the entire job can be performed on your ’17-present L5P Duramax. Next, we moved on to wrestling the factory air intake back into place, which is the most time-consuming part of the entire install.
When it comes to reinstalling the intake system, a retainer on the bottom of the air box installs in the corresponding slot in the metal mounting tray underneath it. Once the retainer is in place, the two legs on the air box can be forced into their respective grommets.
While lowering the factory air box into place, be mindful of where the mass airflow connector is, make sure the coolant bleed-off hose is out of the way, and avoid damaging the charge air temperature sensor. Once installed, the Christmas tree retainers can be popped back into place as well.
With the air intake reinstalled and the clamps at both ends of the intake tube cinched down, the mass airflow sensor can be plugged back in and locked. At this point all labor is complete. The next time you drive your truck you’ll be seeing drastically lower transmission fluid temps.
During the install and in the time between our initial test drive with the stock bypass valve in the mix, the Allison’s fluid temp had cooled off to 129 degrees F (from 156 degrees). Needless to say, this is where we began the second test drive—and it was the highest temperature we would see with LinCo’s bypass valve in place.
Just three minutes into our test drive, ATF temp dropped to 120 degrees. Within five miles of driving, the scan tool was reading 116 degrees, then 114. At the end of the test loop, the Allison was running a cool 107 degrees—a 49-degree drop from where we were with the stock bypass valve in the equation. Instead of watching trans temp climb (as was the case with the stock bypass valve), we watched it continue to drop throughout the drive. At the end of our test, and with an ambient air temperature of 40 degrees, the Allison’s 107-degree reading was just 67 degrees above ambient.

A Simple Fix for a Potentially Fatal Problem


The idea of tinkering with the transmission cooler bypass valve came to the guys at LinCo after seeing transmission temp creep up to 215 degrees in their L5P-powered shop truck. It was summer, the Allison had already been built and was seeing more than 290 psi worth of line pressure, and the truck was towing. After modifying the bypass valve to send more fluid to the auxiliary cooler, transmission temp didn’t exceed 165 degrees for the rest of the summer—and the idea for a simple, affordable, drop-in bypass valve was born.


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