If you’ve ever pulled your boat up a slippery ramp, run your truck through heavy snow or ice, or trucked right through deep mud on your rural property; then you have your truck’s transfer case to thank for getting you through the rough stuff! Yes, that hunk of metal under the cab of your HD truck houses a mechanism that seamlessly transitions power from the transmission to all 4 wheels of the truck (more on that later) so that you can maintain traction in a variety of challenging situations.

Of course, proper maintenance of your HD truck goes beyond just changing out the oil. The 4 - wheel drive system on these trucks is put under a considerable amount of stress and in order to properly maintain your 4 - wheel drive system, you’ll need to also keep up on changing out your fluid at regular intervals.

At Merchant Automotive, we get a TON of questions on what the best fluid solution is for GM trucks. Trust us, we’ve heard it all! We’ve caught wind of customers using everything from 5W-30 oil to universal ATF.

Sound confusing? Don’t worry, we’re here to set the record straight by covering everything you need to know about this unique component on your GM HD workhorse.

• How does a transfer case work?
• What does good transfer case fluid do?
• What’s the best transfer case fluid?

If you’re interested in learning more about this critical component, we’re ready to pass on the knowledge!

How Does A Transfer Case Work?

The goal of your 4WD system is to allow the driver to operate the vehicle in either 2 wheel drive or 4 wheel drive, depending on the scenario that the driver finds themselves encountering, in a seamless manner. This is accomplished by engaging and disengaging power sent to the front axle of your truck via a clever component known as the transfer case, which you guessed it, transfers engine power to the front axle when called upon.

Engaging 4 wheel drive on your truck will give you optimal traction in slippery conditions but it will also use more fuel due to the power needed to move the additional drivetrain components and the additional wear/tear on those components. For example, if you’re pulling a trailer on a dry - highway for a long period of time, 2 wheel drive is optimal since the extra traction is not needed and fuel economy is the primary goal. Pulling your boat up a slippery ramp though? That’s going to require some extra traction and maybe even some gear reduction.  

The output shaft (from the transmission) powers the components in this assembly. Depending on either driver selection or computer engagement, power is either transferred directly to the rear axle OR shifted between the front and rear axle via a series of gears, chains, and sprockets. This happens seamlessly and within fractions of a second.

The engagement or disengagement of the 4WD system is controlled right from the cab via a selector. This selector could be button-based or dial-based, depending on the year of your truck

2 HI

This setting instructs your truck to ONLY engage the rear axle. Dry roads in the summer would be a good example.

4 HI

This setting engages all 4 wheels where traction may be an issue. For example, on a snow covered road or a muddy field.

4 LO

This setting is for low speed ONLY and engages a separate set of gears that are specifically designed to multiply the torque of your engine when extremely tough situations are present. For example, when going through muddy terrain or when moving a heavy load at low speeds. This is for down-low grunt.

Auto 4WD

This is one of the coolest benefits of modern 4WD trucks. You can simply put your truck into AUTO and it will automatically engage 4WD as needed. It does this via a selection of wheel speed sensors that rotate in sync with the output shaft (towards the rear of the truck). Once there is a difference in rotational speed, the computer seamlessly engages a component that applies force to a series of clutches in order to equalize rotational speed and sending power to the front axle. Boom. Traction!

What Does Good Transfer Case Fluid Do?

So, what’s this all got to do with utilizing the proper lubricant in your HD beast? Well, all these moving parts need proper lubrication to go the distance. A quality product needs to do three things:

1. Lubricate the internal parts
2. Address the specific needs of friction materials in the case
3. Minimize driveline “clunk” or driveline friction.

You’ll hear all sorts of advice when it comes time to what kind of lubricant to put in your 2500 or 3500 HD truck.

Although it may only seem like lubricating oil, it does way more than simply keep things moving along smoothly. In fact, the factory fills the unit with Dexron VI.  

The reason behind this is due to the mechanical elements that are within the unit itself, including the usage of friction wet clutches inside of the unit. These wet clutches utilize a friction material (like all clutches do) and require way more than just engine oil to function properly. Basic engine oil (like a 10W-30) lacks the chemical properties required to properly cool, lubricate and maintain these friction materials, while ATF possesses the ability to properly lubricate these friction materials and lubricate the rest of the mechanical components as well.

Seems like a done deal, right? Not so fast.

What about that driveline clunk? This occurs when the friction on the spline of the main shaft and the slip yoke is sliding over the top of each other without proper lubrication as your truck comes to a stop. These components designed to do this but GM has never really offered a solid solution to stopping this annoying problem. In fact, drivers often come to us with this complaint and indicate an audible “clunk” that can be a bit unnerving. A properly lubricated spline will eliminate this situation.

What’s the Best Transfer Case Fluid?

Well, it’s certainly not engine oil! Hopefully, we’ve established by now that the 4WD system of your HD truck is a complicated piece of machinery that requires a specific type of fluid to operate at peak efficiency for the long haul. So, should we simply use the ATF that’s specified from the factory? For GM, HD trucks,  it doesn’t really address the driveline clunk that is so prominent on the trucks.  Engine oil simply does not have the properties needed to address the wet clutches found in the synchronizer assembly or the clutch pack in the the Auto-4WD transfer cases.  

The solution? Merchant Automotive Performance Transfer Case Fluid.

We’ve designed this product to be a direct substitute for the factory fill and it blended specifically for use in GM trucks but it may be used in Ford, and Chrysler trucks as well. We’ve custom-blended this fully synthetic product to contain the right mixture of additives and modifiers to address the proper lubrication of internal components, address the friction components properly and reduce driveline “clunk” by properly lubricating the spline.

This product is a premium alternative to other fluids specified by GM and is suitable for both push button and floor shift applications. Merchant Automotive has even conveniently sized this container at 2 quarts (the typical factory fill requirement) to prevent overfilling!

How Often Should I Change the Fluid? 

Even if you are using the best fluid available such as Merchant Automotive Transfer Case Fluid, it still needs to be serviced at recommended intervals.  The transfer case has no filter, no temp gauge, and no warning light indicating the fluid may be low.  We recommend changing the fluid every 50,000 miles for best longevity. 

Specs:

Application: Developed for ALL GM trucks that call for ATF or Auto Trak II for the fluid fill. May also be used in Chrysler, Dodge, Ram, and Ford trucks.

Models: GM 1500, 2500HD 3500HD Silverado & Sierra Trucks

Engines: 6.6L V8 Duramax diesel engines (LB7, LLY, LBZ, LMM, LML, L5P)

Years: 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021, 2022, 2023, 2024

Trust The Duramax and Allison Experts

For over 2 decades, the Merchant Automotive team has been the authority in Duramax and Allison trucks. Our absolute expertise, in-house engineering team, and unique specialization allow us to offer products like our Merchant Automotive Performance Transfer Case Fluid that go above and beyond factory requirements for our valued customers. We could just settle for what everyone else recommends or recommend factory spec but that’s not what we do. Our commitment to innovation and our commitment to our customers ensure that our products will keep your truck on the road for the long haul. This is what we do!