What Is Transfer Case Fluid and How Often Should You Replace It?

What is a transfer case? Like the engine and transmission, the transfer case consists of interacting components that produce considerable heat and friction. There are bearings, a chain, some sprockets, a planetary gearset, and most transfer cases even have an integral oil pump.

Now that you know the answer to the question of “What is a transfer case?”, it’s time to talk about its lubricant. Transfer case fluid cools and greases these parts to extend their lifespan. Read on to learn more about this critical lubricant, and find out when you need more fluid for the transfer case.

What Is Transfer Case Fluid?

Transfer case fluid is the lubricant for the transfer case of four-wheel drive (4WD) vehicles.

Transfer case fluid comes in either conventional or synthetic form. Conventional fluid is one of the products of crude oil refining, while synthetic fluid comes from a laboratory.

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Pro Tip: Most transfer cases are filled with an automatic transmission fluid, which is usually red. Others use a thicker gear oil, while some use a specialized fluid specifically made just for that particular transfer case.

Most modern multi-axle vehicles use specially formulated transfer case fluid. However, older vehicles lubricate their transfer case with gear oil. The latter lubricant also goes by the alternative name of transfer case oil to prevent confusion with typical fluid.

What Does Transfer Case Oil Do?

Transfer case fluid performs the same tasks as engine oil and transmission fluid. It provides lubrication and cooling for the transfer case’s parts.

First, transfer case fluid coats various components in a protective layer. The layer prevents metal-on-metal contact, reducing the wear and tear experienced by the parts. It also decreases friction produced by movement and physical contact.

In addition, transfer case fluid also regulates the temperature of the transfer case. It absorbs the heat produced by moving parts that come into contact.

Eventually, transfer case fluid leaves the part, taking the heat with it. A new batch of lubricant then enters the transfer case.

Some transfer case fluids can clean the components. They prevent contaminants like dirt and moisture from reaching the parts.

Lubricant For Wet Clutches

In some manual vehicles, transfer case fluid also lubricates the wet clutch.

The clutch can connect and disconnect the driving shaft to the driven shaft. It enables the manual transmission to shift gears, changing the power delivered to the drive axles.

Most clutches are dry systems. While they receive lubrication, they don’t soak themselves in lubricating oil.

In comparison, a wet clutch sits in a literal oil bath. The extra lubrication makes the part last longer and keeps it free of contaminants.

There are some differences between typical transfer case fluids and the products that grease wet clutches. The wet clutch’s lubricant is usually thicker, and its lubricating agent includes sulfur-based compounds with a distinct smell.

How Often Do You Have To Change the Transfer Case Fluid?

Transfer case fluid doesn’t last forever. It can get contaminated by carbon buildup on parts after cooldown. Degraded fluid is less effective at cooling and greasing the components. If the transfer case fails to get sufficient lubricant, it will develop issues like strange noises and reduced performance.

Generally, follow the car manufacturer’s recommended maintenance schedule for changing the transfer case fluid. You can usually find the timetable in your owner’s manual. Barring that, contact the dealership or look up a reliable source, such as a trusted mechanic.

Some experts recommend changing the transfer case fluid every 20,000-30,000 miles. While some vehicles can go up to 100,000 miles between changes, those are exceptions.

Do you regularly use your 4WD or all-wheel drive (AWD) for off-road adventures? Or do you hitch up tow trailers to your vehicle? In either case, you must replace the transfer case fluid more often, usually around 20,000 miles.

The transfer case goes into action whenever you use 4WD or AWD mode or use your vehicle for towing loads. The more often it handles additional torque, the more heat it generates and needs to shed. Transfer case fluid will cycle at a higher rate, which degrades the lubricant faster and increases carbon contamination. You’ll have to replace the fluid more often.

Transfer Case Fluid Additives

Transfer case fluid contains additives that also degrade over time. “Extreme-pressure” additives can mix with water condensation, which can result in acids that damage seals and even some metal parts.

Transfer case fluid contains additives that also degrade over time. “Extreme-pressure” additives can mix with water condensation, which can result in acids that damage seals and even some metal parts.

–Richard McCuistian, ASE Certified Master Automobile Technician

Detergent additives are included to capture all manner debris and suspend it in the fluid, reducing the wear that might be caused by this debris. But eventually, detergent additives become saturated with debris so that they can’t trap any more.

Corrosion inhibiting additives prevent rust and oxidation but lose their effectiveness over time.

Anti-foaming additives keep the oil from turning into “calf slobber” while the gears are whipping through the oil.

What Vehicles Need Transfer Case Fluid?

Any vehicle with a multi-axle drivetrain has a transfer case. These include all cars, SUVs, and trucks with 4WD or AWD modes. These vehicles will need transfer case fluid.

Four-wheel-drive vehicles rely on the transfer case to deliver torque to the differential and the axles that drive the front wheels. The drivetrain takes engine torque from the transmission and distributes the force between the front and rear axles using the transfer case to deliver the torque.

The axles are connected to the hubs when necessary using moveable splined collars that marry the inner shaft to the outer shaft driving the wheel. They may also have a locking hub that is either manually engaged, automatically engaged mechanically, or engaged using a vacuum piston assembly in the hub area.

The only AWD vehicles that don’t need transfer case fluid are the electric ones. Instead of a transfer case,  electric vehicles (EVs)  have electric motors that directly drive their front and rear wheels.

Can You Put Oil in a Bad Transfer Case?

It’s entirely possible for your transfer case to end up damaged if it runs low on transfer case fluid for too long. It’s also possible to damage the transfer case if you use motor oil or other incompatible fluids to top it up. 

Simply refilling a failing transfer case with oil is not enough to fix its issues. You need to bring your vehicle to a trusted auto repair shop so they can identify and repair any underlying problems.

If your transfer case is inoperative or malfunctioning, you’ll notice the following symptoms:

• Gear shifting issues
• Puddles forming under the transfer case location
• 4WD issues or 4WD warning light engaging
• Grinding, growling, or humming noises

A bad transfer case can cause serious issues while driving. Make sure to bring your vehicle to an auto repair shop immediately if any of the above symptoms manifest.

Transfer Case Fluid vs. Transmission Fluid

You might have noticed the similarities between transfer case fluid and transmission fluid. Both fluids serve as cooling lubricants for their respective systems.

However, the two lubricants are different. While some transfer cases use automatic transmission fluid instead of dedicated transfer case fluid, the two are generally not interchangeable. Using one in a system not designed to handle it will cause problems.

In particular, you can’t use transmission fluid in older vehicles that use gear oil to lubricate their transfer case. Conversely, gear oil isn’t usable in the transmission.

You can differentiate between transfer case fluid and transmission fluid by looking for the following:

Color

Transfer case fluid is usually blue, brown, green, or yellow. Meanwhile, transmission fluid designed for use in the transfer case is often a clear fluid with hues of dark red.

In comparison, fresh transmission oil is usually red in color. If the lubricant becomes contaminated, its color changes to black or brown.

Replacement

Transfer case fluid and transmission fluid need regular replacement. However, the two lubricants have different lifespans. It’s rare to replace both fluids at the same time.

Transmission fluid generally has longer lifespans than transfer case fluid. As mentioned earlier, transfer case fluid lasts for 20,000 to 30,000 miles. In comparison, manual transmission fluid requires replacement every 30,000 to 60,000 miles. Automatic transmission fluid lasts even longer. However, heavy use will shorten the lifespan of these lubricants.