What is Transmission Fluid?

Misty Amber Brighton

Transmission fluid is a car fluid used to lubricate and cool automatic and manual transmissions. It is put into transmissions of cars, trucks, boats, recreational vehicles (RVs), and motorcycles, and other vehicles. The weight or thickness of this lubricant depends largely on the type of vehicle and make of transmission. It is added by pouring the fluid from the bottle directly into an opening in the top of the transmission.

A woman shifting gears with a manual transmission.
A woman shifting gears with a manual transmission.

Automatic transmission fluid is usually made up of a variety of substances including detergents, rust-preventatives, and lubricants. It normally contains a pink dye that will turn black or brown when it becomes dirty, signaling the need to have it changed. Most bottles hold around one quart (.95 l) of liquid.

Cutaway view of a transmission.
Cutaway view of a transmission.

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Manual transmissions vary greatly in the type of fluid required. Some may actually use motor oil, and others need to have automatic transmission fluid added. People who have a manual transmission in their vehicle should check the owner's manual to see what type the manufacturer recommends.

A transmission without the housing.
A transmission without the housing.

In order to check the level of transmission fluid in a vehicle, the engine should be warm and running. A driver should make sure her car is parked on level ground, and then allow the motor to idle for one to two minutes. After this, she can remove the dipstick found on the top of the transmission and wipe it clean with a paper towel. She should then insert the dipstick into the designated slot. The fluid should reach the line marked "full"; if levels are lower than the full line, enough fluid should be added to fill the transmission to that point.

A clutch, part of a transmission.
A clutch, part of a transmission.

Automobile manufacturers recommend changing the transmission fluid periodically. This is because sand, grit, or even metal shavings can get into the lubricant and pass through the transmission, potentially causing harm. The interval for changing this fluid varies based upon the type of vehicle and the way it is normally driven. Most of the time, this service is recommended every 30,000 to 60,000 miles (48,300 to 96,600 km), but should be done more often if a driver notices discoloration or particles in the fluid.

Manual transmission fluid must be changed more frequently than in an automatic.
Manual transmission fluid must be changed more frequently than in an automatic.

Transmission fluid is generally very inexpensive to purchase. Failure to keep this lubricant at proper levels can cause a great deal of damage, which may be costly to repair. For this reason, drivers should know how to check the amount of lubricant in their transmissions and do so on a regular basis.

Manual transmissions require more frequent fluid changing.
Manual transmissions require more frequent fluid changing.
Some manual transmissions use motor oil as transmission fluid.
Some manual transmissions use motor oil as transmission fluid.

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