Automatic Transmission Service

The first automotive automatic transmissions using hydraulic fluid were developed by General Motors during the 1930s and introduced in the 1940 Oldsmobile as the "Hydra-Matic" transmission.

The hydraulic fluid used in the automatic transmission is a special type of oil, usually of low viscosity and with high viscosity index.

This Automatic Transmission Fluid (ATF) has several functions:

Explaining how automatic transmission works and the details of the above mentioned functions is beyond the scope of this article.

Our objective is to present different design details and how they relate to Automatic Transmission Service.

Specifically following items are considered:

IDEAL Design

In a perfect and ideal world where vehicles are built and designed to last "forever" and are easily and quickly maintained, the IDEAL Design would have these features:

Complete ATF drain is possible without any special tools or equipment.

However once you do drain the ATF, you have to have way to fill up the system with fresh ATF.

This of course requires:

For Automatic Transmission longevity and for reliable operation a good ATF Filter is a must.

The best design is:

For Automatic Transmission longevity and for reliable operation a well designed Dipstick is a must.

The best design is:


Unfortunately, to our knowledge there are no vehicles currently manufactured anywhere in the World that have ALL the features of this IDEAL design !

WORST Design

The absolutely WORST possible design is such that NONE of the points listed in the IDEAL design are present - NONE !

Some current models made by BMW and Daimler (Mercedes-Benz)

Checking ATF is not possible, there is no provision to drain and refill the ATF, no ATF filter is present.


Vehicles so designed are intended to be "disposable", low cost to manufacture (but not necessarily low cost to purchase or own), and to be FREE of any maintenance.

BAD Design

No ATF Filter

The classical AT design originated by GM and latter adopted by FORD always had and still does feature ATF filter in the design.

The ATF flows through this "sandwich" filter before it is pumped into the hydraulic pump, and any large debris are caught to prevent damage to the pump itself and to other components in the AT system.

When Japanese companies like HONDA and TOYOTA designed their own Automatic Transmissions, they both left such filter out of the system


TOYOTA has coarse mesh steel screen that is not intended to be serviced or replaced

No Drain Plug

In the classical GM and FORD Automatic Transmission design, one had to remove the AT Pan to access the AFT Filter, and thus fluid that was contained in the pan (about 1/3 to 1/2 of the total ATF volume in the system) was replaced every time the ATF Filter was replaced or serviced.

Only few Heavy Duty vehicles also had a drain plug on the AT Pan, so fluid could be also changed without removing the AT Pan.

No Dipstick

More and more new vehicles with Automatic Transmissions are leaving the assembly line with absolutely no dipstick !

This is not "oversight" or poor workmanship of the assembly worker, the dipstick was simply LEFT OUT in the design stage

The excuse is that the vehicle is built so good that there NEVER will be any leaks, and thus checking the ATF level is not necessary - EVER !

Some AT without dipstick still have some provision for checking the proper ATF level, but not all of them, and the few that do have such a feature require a very specialized procedure and/or special equipment or tools just to perform this check.

But in general these "No dipstick" Automatic Transmission designs are referred to as "sealed" ATs.


Here are a few examples just to show you how involved it is just checking the ATF level in some vehicle models.

GM 4T40-E and 4T45-E

This transaxle is used in the 1995 and above small GM front-wheel drive vehicles.

It has a two-page long set of instructions just to check the fluid level properly.

This could take several hours dependent upon the current fluid temperature.

Procedure requires to place the vehicle on a level hoist, so nothing that owner can do.


These six-speed transmissions are used in the 2007 and above Tundra Trucks and Sequoia SUVs.

They have a 14-page long set of instructions just to check and adjust the fluid level properly and a special tools are required.


Methods for inspecting and adjusting the ATF levels on "sealed" transmissions will differ from one make to another, even one model to another.

Always reference the vehicle-specific service information and be sure you understand the procedure before attempting to do so.

Failure to do these inspections properly may result in a transmission that is over- or under-filled, and could result in expensive transmission damage

No ability to FULLY Drain

In the old days when ATF service was both required and specified in the service or owner manual, most GM and FORD AT design had provisions for draining the Torque Converter, some vehicles usually trucks also had a drain cock for the ATF cooler.

Today such designs are obsolete, and no light duty vehicle currently has any provision to 100% drain and replace all the ATF.

Torque Converters have no drain plugs.

Cooler does not have drain cock, and the Cooler Lines are not designed to be disconnected for servicing.

The drain and flush services that some service outlets will try to sell you for hundreds of dollars are totally ineffective and it is absolutely impossible to drain ALL of the fluid completely, irrespective of what their advertisements will claim.

If one intents to keep such new vehicle "forever" then multiple and frequent partial ATF drains are required and they must be done long before the ATF deteriorates to such a point where it is no longer serviceable.

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Updated: 2013-08-30