(Updated on July 28, 2022)
A transmission is a complex package of gears, shafts, and sometimes pulleys that takes torque from the engine and either multiplies or divides it before that torque is sent to the wheels.
Every transmission has some sort of fluid in it. Fluid may be used for lubrication, cooling, hydraulic actuation, or all of the above inside a transmission.
When Do I Need to Add Fluid?
Many transmissions come with a dipstick to help you know when you need to top up your transmission fluid. Some transmissions are sealed units and do not have a dipstick.
Most transmissions won’t need to be filled outside of their normal fluid replacement interval unless you have a visible leak. Leaks can happen due to gasket wear, improper transmission or seal installation, or just over time due to age.
You can check for transmission leaks by crawling under the car and looking for dirty, oily residue on or near the transmission. You may notice a buildup of oil and debris around a leaking transmission pan gasket.
Types of Transmission Fluid
Adding the correct fluid is very important for maintaining the proper operation of your transmission. There are a few different types of transmission fluid, and even different viscosities within each type.
Always consult your owner’s manual if you are not exactly sure which fluid to use. Some fluids have friction modifiers in them. This is especially true for vehicles with limited slip differentials that share fluid between the center differential or transfer case and the transmission.
Automatic Transmission Fluid (ATF)
Although there are many types of automatic transmissions, they typically all use automatic transmission fluid (also called ATF). Some CVT transmissions will use an ATF that has additional special additives.
Automatic transmission fluid is typically red in color and serves the purpose of heat control, lubrication, and hydraulic fluid in the valve body. ATF is sometimes also used for power steering systems.
Manual Transmission Fluid (Gear Oil)
Manual transmissions are simpler than automatic transmissions and have different fluid requirements.
Manual transmission fluid, also called gear oil, is a bit like a really thick engine oil. While engine oils may be a 20 or 30 weight at operating temperature, manual gearbox oil tends to be closer to 90 weight.
Thicker oil is needed to support the gears and synchros as you change between gears with the shift lever. This extra thickness also helps it cling to the gears as they spin.
Gear oil is widely considered to be one of the worst smelling vehicle fluids, especially used gear oil. This is due to the high sulfur content and other similar foul-smelling additives.
Checking Your Transmission Fluid
Transmissions with a Dipstick
If you’re worried your transmission fluid is low, park your car on level ground with the vehicle in park and running at operating temperature. Pull the dipstick and note the fluid level. If the transmission fluid level is low, add fluid until you reach the full mark. If the fluid smells burnt, it’s probably time to change it out.
Different vehicles may have slight variances in their transmission fluid checking process. Always check with the owner’s manual to make sure you are getting an accurate reading from the dipstick.
You can only really check the fluid level of a sealed transmission by unscrewing the fill plug and dipping a finger in to feel where the transmission fluid level sits. Fortunately, the fluid level in a sealed transmission will only drop if you have a visible leak.
How to Add Transmission Fluid
With a Dipstick
The process for adding transmission fluid is usually pretty straightforward. If your transmission has a dipstick, you can usually just pour some transmission fluid down the dipstick hole. It helps to grab a small funnel if you choose to try this.
With the car running, place the shifter in park and leave the handbrake on. Place a funnel into the dipstick hole after removing the dipstick. You may need a very long, skinny funnel for this to work depending on where the dipstick is located in the engine bay.
If your transmission requires any additives, add the additives before the transmission fluid.
Slowly pour in half a quart of transmission fluid. Wait a few minutes, then move the vehicle through each gear. This allows transmission fluid to enter each of the little passages inside the valve body.
Recheck the fluid level. If you the level is still low, add another 1/4 quart of fluid. Repeat this process until the level is at the full mark.
Be careful not to overfill the transmission. This could cause excess pressure within the transmission and damage internal components, leading to an expensive repair.
Without a Dipstick
If you don’t have a dipstick, your transmission should have a fill plug. Unscrew the fill plug, bring the bottle up to the fill hole, and squeeze the bottle until transmission fluid starts to leak out. It’s best to have some cardboard or an oil pan ready, as this process can be a bit messy.
You could try using an inexpensive fluid pump that attaches to top of the transmission fluid bottle to make the job much cleaner and easier. There are different types of fluid pumps available to meet your needs, no matter what your use case is.