Automatic Transmission Fluids - 101

By BRETT WINBERG, Senior Editor

The latest generation of automatic transmission fluids have increased oxidation resistance for improved component life. Even so, that doesn’t mean the fluid can be neglected forever... especially if driving conditions push the fluid’s temperature above safe limits. Heat is the number one enemy of ATF.

As the operating temperatures increase, the fluid’s service life goes down. Oxidation, viscosity breakdown and degradation of the friction modifying additives reduce the fluid’s ability to do its job. Eventually the fluid can no longer provide proper lubrication and the transmission can fail. Fluid contamination from normal wear is another concern, so oil sampling becomes a must when determining a filter change or flush. Flushing can cause more damage then leaving the transmission alone and waiting for the oil analysis report.

Why Do Automatic Transmissions Fail?
So what if the fluid is never replaced? Sooner or later the transmission will fail. Ask any transmission shop what the number one cause of transmission failure is, and they’ll tell you it’s fluid neglect.

Not changing the fluid and filter often enough causes more transmission problems and failures than anything else. That’s why most transmission experts still recommend changing the fluid and filter every two to three years or 24,000 to 36,000 miles, even if the transmission does not have miles due to storage of the vehicle, you should change the fluid due to ingestion of dirt, oxygen and degrading of the fluid. Recommended once a year or every 15,000 miles if a vehicle is used for towing or other severe service use.

The Role Of Automatic Transmission Fluid
Automatic transmission fluid performs a number of important functions inside a transmission:

1. Lubrication of the gears, bearings and clutch packs.
2. ATF acts like a fluid coupling inside the torque converter to transmit drive torque from the engine to the transmission input shaft.
3. It also carries hydraulic pressure through the valve body to engage and disengage the clutch packs that change gears.
4. ATF serves as a coolant for the entire transmission, carrying away heat generated by normal operation and friction.

What Occurs As The Fluid Is Used And Abused?
To perform all of these functions well, the fluid must have the correct viscosity and friction characteristics for the transmission application. It should be clean (no contamination, dirt or debris), be in good condition (no oxidation) and be at the proper level.

The wrong type of fluid can cause shifting and engagement problems. Dirty or contaminated fluid can accelerate wear and cause valves to stick. Worn out (oxidized) fluid can also accelerate wear and contribute to shifting and engagement problems. A low fluid level can delay engagement and cause the transmission to slip. A fluid level that’s above the full mark may allow the fluid to become aerated, which can have an adverse effect on shifting, cooling and lubrication.

It is very important to know that using the wrong type of fluid may cause transmission problems and damage! So make sure the replacement fluid meets or exceeds all OEM requirements.

Over the years, there has been a confusing array of different ATF types and specifications. Make sure the replacement fluid meets or exceeds all OEM requirements. Using the wrong type of fluid may cause transmission problems and damage.

This chart below includes most of the automotive applications for ATF. However, always double check your application in your owners manual:

Chart of Types of ATF
Type F — Introduced by Ford in 1967 for their automatics. Also used by Toyota.
Type CJ — Special Ford fluid for C6 transmissions. Similar to Dexron II. Must not be used in automatics that require Type F.
Type H — Another limited Ford spec that differs from both Dexron and Type F. Can be replaced with Mercon.
Mercon — Ford fluid introduced in 1987, very similar to Dexron II. OK for all earlier Fords, except those that require Type F.
Mercon V — Ford’s newest type, introduced in 1997 for Ranger, Explorer V6 and Aerostar, and 1998 & up Windstar, Taurus/Sable and Continental. Must not be used in 1997 or earlier Fords.
Dexron — General Motors original ATF for automatics.
Dexron II — Improved GM formula with better viscosity control and additional oxidation inhibitors. Can be used in place of Dexron.
Dexron IIE — GM fluid for electronic transmissions.
Dexron III — Replaces Dexron IIE and adds improved oxidation and corrosion control in GM electronic automatics.
Dexron III/Saturn — A special fluid spec for Saturns.
Chrysler 7176 — For Chrysler FWD transaxles.
Chrysler 7176D (ATF +2) — Adds improved cold temperature flow and oxidation resistance. Introduced in 1997.
Chrysler 7176E (ATF +3) — Adds improved shear stability and uses a higher quality base oil.
Genuine Honda ATF — Special ATF for Honda automatics.
Toyota Type T — Special formula for Toyota All Trac vehicles and some Lexus models. References from: CTC (Check That Car)

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