Differential service, while quite often overlooked, is a routine service that is generally fairly easy, and can be completed in an afternoon. The differential(s) on your vehicle is a crucial part of the running gear and just like the engine or transmission, rely on their fluid for lubrication, cleaning, proper operation, and cooling. They are far more advanced than they used to be, and the fluids are increasingly becoming a more critical part of the differential operation on many of the modern AWD systems. Today, we’re going to cover the following topics: Basic differential construction, fluid types, maintenance intervals, tools, and service procedures. Thank you for looking to ECS Tuning for all of your performance and maintenance needs. We appreciate your business!

Changing your differential fluid is easy, but before you tackle the job make sure you have the correct fluid(s), and the correct tools. Some differentials can have more than one chamber and require two different fluids, some drain and fill plugs can require special tools, and in some cases, special filling equipment is required. Do your research first, and you’ll be able to get it done with no trouble.

Differential Construction:

These diagrams show the basic internal construction of a standard differential. Click on any component in the list below to highlight it on the diagram:

Differential Types and Operation:


The purpose of a differential is to allow the drive wheels to rotate at different speeds when going around a corner. When turning, the inner wheel is traveling around a smaller circumference than the outer wheel. As a result, the inner wheel rotates slower and the outer wheel rotates faster.
Power is transferred into the differential from the driveshaft to the pinion flange which is splined to the pinion gear. It travels through the pinion gear, and from the pinion gear into the ring gear. The ring gear is bolted directly to the differential carrier, which houses the spider and side gear set. The power is therefore transferred through the ring gear and carrier into the spider gears, and then finally from the spider gears through the side gears and into the axles or output flanges. The “differential” function of the spider and side gear set is what allows the wheels to rotate at different speeds.

Open Differential:

An “open” differential is the most basic type. The differential pictured on page 4 is an example of an “open” differential. Very basic and reliable, and only requiring a standard gear oil for lubrication, these are the most economical to produce. The disadvantage to an “open” style of the differential is that if one wheel loses traction, all power will be transferred to that wheel and it will spin freely. This is an extreme disadvantage on slippery roads or for hard acceleration when the traction of both tires is required.

Limited Slip Differential:

A “limited slip” differential is one in which a series of clutch discs and plates are located between the side gears and the differential carrier, and a spring pack keeps the side gears, clutches, and carrier pressed together. There are three keys to operation: 1: The plates are engaged into the carrier and the discs are splined to the axle or output shaft. When the clutches are engaged, the axle and side gears will turn with the carrier, eliminating the spider gears and “differential” function. 2: The spider and side gears are beveled and the more force that is applied to beveled gears, the more they attempt to push away from each other. 3: The spring pack keeps constant pressure on the clutches, keeping them partially engaged.

When turning a corner, speed and torque input is low, allowing the clutches to slip and normal differential action will occur. When one wheel is on a slippery surface, such as an icy road, the tire will slip and begin to spin. This is where the “limited slip” action takes over. Since a spinning wheel has no resistance, the force on the beveled gears on that side is low and the clutches will slip. On the side with traction, since the spring pack keeps the clutches partially engaged, a greater force is applied to the beveled gears on that side and they have pushed apart, applying the clutches and transferring power into the axle or output flange.


A transaxle assembly is a front or rear wheel drive transmission in which the differential is housed in the transmission case along with the transmission shafts and gears. The differential operation is the same, however, instead of the power entering the differential through the driveshaft and pinion gear, it is transferred from a gear on the transmission output shaft directly to the ring gear on the differential carrier.

AWD Differentials:

There are many different types of all-wheel drive systems in today’s cars, they are very sophisticated, and the differentials are an integral part of these systems. Much more than your standard open or limited slip differential, many of these new units will utilize clutch packs and planetary gear sets, the exact same components in an automatic transmission, as well as electric pumps to create the pressure to control these systems.

Transmission/Transaxie Types:

There are many different types of transmission and transaxle configurations in today’s cars. Both front and rear wheel drive vehicles can have either a transverse or longitudinally mounted transmission or transaxle. All wheel drive vehicles can have the same possibilities, with the longitudinal transmission containing an internally housed differential and an additional output in the tail shaft. A transversely mounted transmission on an AWD vehicle will typically have a small transfer case incorporated on the side of the differential to transfer power into a driveshaft. There are too many combinations to list, but the fluid is the most critical part of each one. Identify what your vehicle is equipped with so you do not overlook any fluids.

Differential Fluid:

What is the role of the fluid in your differential? It is much more than you might think. Of course, lubrication is critical, the bearings must be constantly lubricated, as well as the gears and clutches. The fluid carries debris and wear particles away from the critical surfaces, and leaves them deposited on a magnet, generally located in the differential housing or quite often on the end of a drain plug. It also transfers the heat build up from the gears and bearings into the differential case or housing, which is subsequently cooled by the air around it. Some differentials designed for higher performance applications will have additional cooling fins built into the housing which aid in dissipating excess heat from the differential when driving. The correct fluid is critical for proper clutch operation in limited slip differentials, and in modern AWD systems, the fluid is critical to system operation.

How often should you change the differential fluid? Every vehicle manufacturer will have a specified maintenance interval for changing the differential oil. In some cases, it is a very high service interval such as 100,000 miles. Synthetic oils make a lot of this possible, however, keep in mind that differentials do not have any filters or specific cooling systems, and can take a lot of abuse. A good general rule of thumb to follow is that you are not going to damage your differential by changing the fluid. You can only increase its lifespan.

Neglecting the differential fluid can lead to costly repairs. Differentials and transmissions require a number of special tools for proper assembly and set up, and many times you are faced with replacement with a costly used or rebuilt component. The fluid change is relatively easy, and
it can save you a lot down the road. It is a good idea to check the differential fluid level and condition during routine service, especially if any signs of leakage are detected around any of the seals.

Due to the complexity of today’s differentials and AWD systems, always be sure to use the fluid that is specified by the vehicle manufacturer, and be sure the fluid is kept at the proper level.

ECS Tuning offers a wide selection of differential fluids to meet your needs, including fluid kits that have the specified fluid for your vehicle as well as drain and fill plugs for your application.

Always be sure to use only the fluid specified by the vehicle manufacturer for your differential.

When selecting a fluid, keep in mind the following points:

• Be sure to get a fluid with the correct viscosity rating

• Be sure to get a fluid with the correct GL rating

Always be sure to use only the fluid specified by the vehicle manufacturer for your differential.

• Some fluids require an additional additive for limited slip differentials

• ECS Tuning offers a number of service kits specifically designed for your car that contain the fluid as well as new drain and fill plugs.

Differential Service Tools:

Oil Drain/Fill Plug Wrench:

This 8-n-1 drain/fill plug wrench features a swivel head with an assortment of hex and square drive bits that fit many differential and transmission drain plugs. The hex bit sizes are 10mm, 12mm, 14mm, and 17mm. The square drive sizes are 8mm, 10mm, 3/8” and 1/2”. It is available at as ES#2221248.

Drain Pan:

It’s essential to catch all the differential fluid as it drains and there’s nothing better to do the job than this Schwaben 8 liter drain pan. Sturdy construction, an anti-splash lip, and a 1” drain spout make it convenient to use. It is available at as ES#2748892.

Fluid Extractor/Filler System:

Some differentials are not equipped with drain plugs (they only have a fill plug), making this fluid extractor an essential piece of equipment. It also offers the convenience of pumping the new fluid back in. It features a manual pump, extendable foot step, hoses, and adapters. It can be used for manual and automatic transmissions as well. It is available at as ES#2774825.

Pig Mats:

These absorbent mats are excellent to place around your work area when draining fluid. They will catch and absorb any spills or splashes, keeping your garage floor clean. They are available at as ES#2137110.

Fluid Pump:

This Schwaben Multi-Function Fluid Pump Kit is perfect for the differential that is equipped with a drain plug. Simply drain the fluid, then easily refill it with this fluid pump. This kit is also compatible with all automotive fluids and comes with the adapters to use it as a pressure brake bleeder as well. It is available at as ES#2774836.


The standard service procedure for a differential is to drain the old fluid, then refill with new. It’s really that simple, however, some differentials do not have drain plugs, such as the example on the right.

For differentials without drain plugs, in some cases, you can remove the differential cover to drain the fluid, then reinstall it using a new gasket or sealant. On many of these vehicles though, the cover also has a differential mount cast into it making removal more difficult. This is the ideal time to use a fluid extractor to make short work of the job.

Many gear oil containers have a pointed spout, and you can attach a hose to the spout and squeeze the fluid in. The drawback to this method is that it is messy and usually very wasteful.

The transaxle on the lower right, with the differential housed in the case, has both a drain and fill plug.

Refer to the manufacturer’s info for the correct fluid fill level. In some cases it is level with the fill plug, in others, it may be a few mm below the opening for the fill plug.


  • Remove the fill plug BEFORE you remove the drain plug to make sure it comes out. In the event the fill plug is stripped or seized, you can plan the repair before draining the fluid.
  • When the fill level is below the opening for the fill plug, bend a piece of mechanics wire slightly longer than the fill level and use it as a dipstick.