How to Add Transmission Fluid

mechanic checking fluid level with dipstick

Unless you drive an electric vehicle, your vehicle has some type of transmission fluid. Usually, when people mention “transmission fluid,” they’re referring to automatic transmissions, but it does one good to note that all transmissions use transmission fluid of one type or another. What that transmission fluid or gear oil does depends on the type of transmission, and we’ll get to that in a moment.

Like all engine fluids, transmission fluids have a limited lifespan, which means they must be periodically replaced. Some transmissions include a filter, to remove metal flakes and carbon, as well as magnets, to catch steel particles from internal wear. Depending on the vehicle, transmission fluid replacement may be recommended every 30,000, 60,000, or 100,000 miles – some have no recommended interval. If there is a transmission leak, caused by worn seals or an impact, then adding transmission fluid will keep the transmission running until the leak can be repaired.

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Types of Transmission Fluid

mechanic repairing automatic transmission

There are generally two types of transmission fluid, formulated for either manual or automatic transmissions, and they are not interchangeable. The reason for this is because manual and automatic transmissions use transmission fluid in different ways. Manual transmissions use transmission fluid mainly for lubrication and heat moderation, while automatic transmissions use transmission fluid for these, and as hydraulic fluid, for pressure-operated valves, clutches, and brakes.

Within each group of transmission fluids, manual or automatic, there are several types and additives, depending on transmission type, gear type, and automaker. The most basic manual transmission fluid is simply a heavy gear oil, something like 75W-90 or GL-5, but some manual transmissions require additive friction modifiers for the smooth operation of gear synchronizers. Differentials use similar gear oil, but likely different additives for limited-slip clutches and the like. Automatic transmission fluid types vary widely, such as Mercon V, T-IV, and Dexron 4, depending on YMM (year, make, model) of the vehicle in question.

Whatever vehicle in question, it is critical to only use the appropriate transmission fluid for that application. In a pinch, substituting 100-weight gear oil won’t hurt a manual transmission requiring 75W-90, though you might experience slower shifting and decreased fuel economy. On the other hand, adding Mercon V to an automatic transmission requiring T-IV could be disastrous – it might run for a while, but it would eventually destroy any incompatible seals or clutch materials, costing thousands in transmission rebuilding costs. Always refer to a YMM-specific repair manual or owner’s manual for transmission fluid specifications.

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How to Check Transmission Fluid Level

mechanic using scan tool to access vehicle systems

Generally, there are three ways to check transmission fluid level and condition, but you should always check the repair manual for specifics.

  • The most common method is to use the dipstick, exactly the same as the engine dipstick, which you can usually find under the hood, though it may be harder to find in some vehicles. Honda, Ford, and GM automatic transmissions short dipsticks that weren’t obvious on opening the hood. Checking automatic transmission fluid level usually requires parking on a level surface and leaving the transmission in Park or Neutral. Pull out the dipstick, clean it with a rag, then reinsert it completely for a couple of seconds. Pull out the dipstick and check the fluid level. You can wipe it on a white paper or paper towel to check the fluid for color and suspended particles, both good indicators of transmission health.
  • Most manual transmissions don’t have a dipstick but are checked at the fill port. On a level surface or with the vehicle lifted and level, remove the transmission fill plug, the higher of the two, and use your finger to check that the fluid is less than 5 mm or 1/2" from the bottom of the hole. Wipe your finger on a white paper or paper towel to check the color and condition of the fluid.
  • Some newer automatic transmissions have done away with the dipstick altogether and aren’t easy to check. These “maintenance free” transmissions have drain and fill ports, like typical manual transmissions, as well as a third fluid level check port, usually in the transmission pan. The problem with checking this type of transmission is the procedure required to do so, and getting it just right requires a sensitive scan tool. The Toyota / Lexus procedure, for example, requires running the vehicle level on a lift, then monitoring transmission fluid temperature, via the scan tool or flashing light on the instrument cluster, until it reaches a certain temperature. When the transmission temperature reaches 95 F, you pull the “check” plug and let it drain, but if the temperature goes over 113 F, this will cause too much fluid to drain off.
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How to Add Transmission Fluid

mechanic using a transmission fluid pump to add fluid to vehicle

When adding transmission fluid, such as after draining out old fluid or to correct fluid level for a leak, there are three main ways to go about it.

  • On transmissions equipped with dipsticks, a transmission funnel will be needed, usually less than $5 at your local auto parts store. Remove the dipstick and insert the funnel, then add fluid as necessary. Wait a few minutes before checking transmission fluid level again, because it takes time for all the fluid to make it down into the transmission.
  • On transmissions equipped with fill plugs, both manual and automatic transmissions, a fluid pump will be needed, usually less than $10 at your local auto parts store. On manual transmissions, add any required additives first, then add transmission fluid until it just starts to come back out of the fill port.
  • Adding transmission fluid to some modern automatic transmissions, such as today’s Toyota / Lexus, can be more difficult. While carefully watching the transmission temperature, add fluid while it is still below 95 F and open the check port. Allow fluid to run out the check port before it reaches 113 F. Some fluid may be wasted, but this will result in a proper fluid level for long-term performance and reliability.

As with all things automotive, these procedures are only general guidelines. You’ll need to check your YMM-specific repair manual or owner’s manual for specifics. The details can vary, requiring different fluids, additives, and procedures, but most DIYers should be able to handle adding transmission fluid to most vehicles. Still, if there is any doubt, play it safe and protect your investment by going to the professionals at your local trusted auto repair shop.