Guarantor of reliable car operation: changing the oil in the transfer case

What is a transfer case?

The transfer case is an integral part of an all-wheel drive vehicle. It is designed to distribute torque along the axles of the vehicle, as well as to increase torque when driving off-road and on bad roads.

Depending on which all-wheel drive system is installed on the vehicle, the design of the transfer case differs. However, each of them has common elements:

• Front and rear axle drive shafts
• Chain/gear and reduction gears
• Center differential with locking mechanism
• Drive shaft

The drive shaft serves to transmit torque from the gearbox to the transfer case. The center differential distributes torque between the axles and allows them to rotate at different angular speeds. Such a node can be symmetrical (distributes the moment equally) and asymmetrical (distributes it in a certain ratio).

In cars where all-wheel drive is activated automatically or manually, a center differential is usually not installed.

The center differential lock allows you to fully realize all-wheel drive capabilities. It means partial or complete disengagement of the differential, which ensures a rigid connection between the front and rear axles. Blocking can be done either manually or automatically.

Manual shutdown is carried out using a hydraulic, mechanical, electric or pneumatic drive. Today, the automatic locking mechanisms used are viscous coupling, Torsen limited-slip differential and multi-plate friction clutch. There are designs that provide both of these options.

The chain drive consists of a drive and driven sprocket, as well as a chain. It serves to transmit torque from the rear axle to the front. Instead of a chain drive, the transfer case can also use a spur gear. A bevel gearbox is usually installed if the all-wheel drive system is automatically connected.

A reduction gear increases torque when driving off-road and on bad roads. It is installed on individual transfer case structures of off-road vehicles. Modern vehicles are equipped with a planetary gearbox.

When should you change the oil in the transfer case?

Not all car owners know, but axles and transfer cases need periodic replacement of lubricant. During operation, the oil becomes contaminated with mechanical wear products, which over time leads to abrasive wear of surfaces and failure of the transfer case.

By periodically checking the condition of the lubricating fluid, you can quite accurately determine what condition it is in and whether it needs to be replaced with a more recent material. Also, during a normal visual inspection, oil leaks can be detected.

Changing the oil in the transfer case must be carried out strictly according to the regulations established by the manufacturer. Usually it is about 20-40 thousand kilometers. With regular checking and replacement of the lubricant, the transfer case can last quite a long time.

Some automakers claim that the transfer case is filled with oil for its entire service life. This statement is true only if you plan to scrap the vehicle in 3-5 years and buy a new one. Therefore, even in such cars the oil should be changed, otherwise you may end up with expensive repairs or an even more expensive replacement of the transfer case.

DIY replacement tips

• Changing the oil in the transfer case should be carried out within 10-15 minutes after a trip of at least 5-10 km. During operation, the oil heats up and becomes thinner, which makes it drain better.
• On most cars, the drain and filler holes are made “square”, so to unscrew you only need a ratchet or a wrench of the appropriate size. Of course, this is provided that the bottom is not covered with anther, which will need to be dismantled before replacement.
• The accompanying tool will most likely require a hydraulic syringe for technical fluids. In some cars, using a container with a “spout” allows you to replace it without a syringe.
• On most transfer cases, it is considered normal when the level is slightly below (5-10 mm) the filler hole. You can determine the actual amount of oil poured with your finger or a piece of bent wire. To speed up the process of self-replacement, you can add oil until it flows out of the level control hole - this will not harm the transfer case or rear axle at all.

How to check the oil level in the transfer case?

There are no inspection windows for checking the level and condition of the transfer case oil, so control and replacement can only be done through the filler hole.

Before checking, the car must be placed on a flat surface and the filler bolt, or, if present, the control bolt must be removed. Usually they are unscrewed with four or hex keys, or regular wrenches.

The normal oil level is just below the inspection or filler hole. The need for replacement is determined visually. To do this, take a small amount of oil with a syringe with a flexible tube. If the oil is cloudy, black and contains traces of wear, it should be replaced.

How to choose oil?

Transmission oil is poured into the transfer case. It is selected based on the climatic conditions in which the car is operated.

There are two main classifications for such fluids: SAE J306 and API GL.

SAE J306 is a classification of lubricating fluids for drive axles, transfer cases and manual transmissions based on viscosity. It shows the dependence of oil viscosity on temperature changes. Thanks to modern production technologies, you can safely use all-season oils. If the name of a liquid contains the letter W, this means that it is all-season. The most common oils are 75W-90, 80W-90, 75W-85. The first number indicates viscosity at low temperatures, and the second number at high temperatures.

The API GL classification divides oils into quality classes, which depend on the basic performance properties of the formulations. These classes are determined by how anti-foam, anti-wear, anti-oxidation additives and corrosion inhibitors work. The minimum class of fluids that can be used in transfer cases is API GL-3. API GL-4 oils are suitable for distributors of most foreign cars and all domestic cars. API GL-5 fluids are intended for hypoid gears, drive axles and transfer cases of modern passenger vehicles.

If in winter the temperature drops below -30 °C, it is recommended to use 70W oils. Liquids with a low temperature rating of 75W can also be poured, but at very low temperatures it will be too thick. In temperate latitudes, 75W and 80W oils can be used. Considering the high-temperature indicator, oils of 80 and 90, for example 75W-90, are suitable for civilian SUVs and cars.

What and how much to fill

The table below will allow you to select oil by car brand and find out how much is required.