What is a transfer case?

diff fluid
(From HowStuffWorks with modifications.) This diagram illustrates the various components of a four wheel drive system.

If your vehicle is four wheel drive or all wheel drive, you own a transfer case. The transfer case takes the power coming out of your transmission and splits it between the front and rear wheels.

Some transfer cases are selectable, which means you can choose between two and four wheel drive, while others provide full time all wheel drive. Some transfer cases offer a low range, which is an extra gear reduction that increases torque to the wheels, at the expense of vehicle speed. This is shown as “4WD Low” on most vehicles. Some transfer cases can vary the amount of torque that is supplied to the front or rear, while others split the torque evenly all the time.

In some all wheel drive (AWD) vehicles, your transfer case may also be called a power take-off unit, or PTU.

What is transfer case fluid?

Your transfer case is filled with a fluid that provides lubrication and cooling to the internal parts. In some transfer cases, the fluid also provides lubrication to a wet clutch that is used to vary how much power is sent to the front or rear wheels. Most transfer cases are filled with an automatic transmission fluid, which is usually red in colour. Others use a thicker gear oil, and some use a specialized fluid that is specifically made just for that transfer case.

Why does transfer case fluid need to be changed?

airdrie transfer case
(View with fill plug removed.)
This transfer case fluid was neglected far too long, and has become a black slurry inside the case!

Transfer case fluid doesn’t last forever. Besides losing its ability to lubricate properly, transfer case fluid contains many additives that also degrade over time. These include:

  • Extreme-pressure (EP) additives that protect against wear. These additives can eventually react with the water that forms inside the case from condensation, forming acids that may damage seals and other parts.
  • Detergents that capture dirt and debris, suspending it so that the contaminants cannot cause scuffing or wear. Eventually the detergents in a fluid become “full” any cannot trap any more material.
  • Corrosion inhibitors that prevent rust and oxidation inside the case. These additives become less effective as they age as well.
  • Anti-foaming agents that prevent the oil from frothing as the gears spin through it. These chemicals are super important, because foamed oil has air bubbles in it, which weaken the crucial fluid barrier between the parts.
  • Friction modifiers that extend the life of the clutches.

Replacing a transfer case can cost between $1500 and $5000, depending on the type of vehicle. The best way to protect this expensive component is to perform rather inexpensive replacement of the transfer case fluid regularly.

How often should transfer case fluid be changed?

When to replace your transfer case fluid varies greatly from vehicle to vehicle. Some vehicles require a transfer case service every 25,000 km, while others don’t need attention until 100,000 km or more. We usually always recommend following your vehicle manufacturer’s suggested service interval, which can be found in your owner’s manual. We would also be happy to look up this information for you!

There are rare situations where we will recommend replacing your transfer case fluid before the manufacturer’s service interval, such as “lifetime” transfer case fluid. This fluid does not last forever, and “lifetime” usually means the life of the transfer case, and certainly not the life of the vehicle!

Replacing transfer case fluid:

airdrie fluid changes
Checking transfer case fluid level. If fluid is level with the fill plug hole, or just dribbles out, the case is full. If it pours out, the case is over-filled.

Replacing transfer case fluid is a fairly simple task on most vehicles. The fluid is usually drained by removing a drain plug, then the drain plug is re-installed. Then, on most transfer cases, the fluid is filled to a certain level by removing the fill plug. Both of these plugs usually contain a magnet to capture metal filings and other, and they are cleaned during this process.

One some vehicles, the fill plug can be rather hard to access, with no room for a bottle of fluid. In these cases, we use a special pump to fill the transfer case.

Some transfer cases do not have a drain plug, because they are “lubed for life” and not meant to be serviced. This is an idea that hasn’t been working out very well, as many of these units fail before reaching 200,000 km. In order to prolong the life of our customers’ “unserviceable” transfer cases, we have found good success using a special tool to suck out the old fluid instead.

Changing transmission fluid in Airdrie:

If you’re looking for help in properly maintaining your vehicle, including replacing your transfer case fluid, we’re here to help! We take vehicle maintenance seriously, and we stock over 15 different genuine OEM transfer case fluids in order to look after your vehicle properly. If you’d like us to provide an estimate for a service, or even if you just have a question, please get in touch!