Check Transmission Fluid: Why The Engine Has to Be Running

Check Transmission Fluid: Why The Engine Has to Be Running

Why You Have to Check Transmission Fluid with the Engine Running

Checking automatic transmission fluid (ATF) with the engine running and the transmission in park or neutral is a crucial step to ensure proper fluid levels and maintain the health of the transmission. There are several reasons why carmakers require the engine to be running and the transmission fluid to be at the recommended temperature. This article explains the reasoning behind the requirement to check transmission fluid with the engine running, the park or neutral position shifter, and the fluid at the right temperature.

Why the Engine Must Be Running When Checking The Automatic Transmission Fluid Level

The primary reason to check transmission fluid with the engine running is that the fluid circulates through the transmission only when the engine is in operation. With the engine off, the fluid in the torque converter drains to match the fluid level in the transmission, and the fluid in the upper portions of the transmission drains down into the transmission pan. If you check transmission fluid level with the engine off, you’ll get an inaccurate reading of the fluid level, whether you have a dipstick or are using the fill hole. When the engine is running, the fluid is distributed throughout the transmission, including the torque converter and the valve body, providing a true indication of the actual fluid level in the system.

This image shows a typical transmission and the fluid level in the torque converter

With the engine running, the transmission pump moves the fluid into the torque converter and into the top portion of the transmission. So, the fluid rises to the level required for normal operation, allowing for a more accurate measurement when the dipstick is checked. Measuring the fluid with the engine off would result in a reading that underestimates the actual level needed during operation.

Click here to learn the symptoms of low transmission fluid

Why the Transmission Should Be in Park or Neutral

It is also important to have the transmission in the park position (or sometimes neutral, depending on the vehicle’s design) when you check transmission fluid levels. This ensures the fluid is stabilized at the appropriate level for measurement. In park or neutral, the transmission fluid can flow throughout the system without any pressure build-up from the gear engagement that occurs in drive or reverse. This setup mimics the conditions under which the fluid will function during normal vehicle operation.

In park or neutral, the fluid remains steady within the transmission. If the transmission is in drive or reverse, some pressure may be applied to the fluid, potentially leading to inaccurate readings. Park or neutral is typically the position at which the fluid has been designed to be checked for proper level consistency.

The Effects of Temperature on Transmission Fluid Levels

Automatic transmission fluid expands when heated. Transmission fluid typically expands approximately 6-8% in volume across its normal operating temperature range, which is from cold (around 70°F/21°C) to a typical operating temperature (around 175°F/79°C). A transmission holding 12 quarts cold might expand to about 12.8-13 quarts hot. The expansion coefficient for most transmission fluids is approximately 0.0004 per degree Fahrenheit.

For this reason, many vehicle manufacturers recommend checking ATF when the engine is warm. If the fluid is checked when cold, it may appear lower than it is, leading to overfilling when the vehicle reaches operating temperature.

Checking the transmission fluid while the engine is running and at the correct temperature is also essential to ensure that the fluid is not being measured in an expanded state that would give a false high reading. This ensures that the transmission is filled to the proper level for all temperature conditions.

Why Following the Manufacturer’s Instructions Is Critical

Different vehicle manufacturers may have specific guidelines for checking ATF, and it is important to follow the instructions in the owner’s manual. Some vehicles may have unique requirements for checking fluid levels, such as having the transmission in neutral or requiring the fluid to be checked at a certain temperature range. Some newer vehicles may even have sealed transmissions that do not allow for traditional dipstick checks, requiring specialized procedures to measure or change the fluid.

Failing to follow the manufacturer’s instructions can lead to incorrect fluid levels, which may cause various transmission problems such as overheating, slipping, delayed shifting, or even transmission failure. Thus, referring to the shop manual for the recommended procedure is always best.

Consequences of Incorrect Fluid Levels

If the fluid level is too low, the transmission may not get enough lubrication or cooling, leading to overheating and increased wear on components. Low fluid levels can also cause the transmission to shift erratically or delay engagement. In severe cases, it could result in damage to the transmission that requires costly repairs.

On the other hand, overfilling the transmission can also be problematic. Too much fluid can cause foaming as the gears churn through the excess, which introduces air into the fluid. This can result in inconsistent hydraulic pressure, leading to erratic shifting and poor transmission performance.

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Author

  • Samantha Cole

    Samantha has a background in computer science and has been writing about emerging technologies for more than a decade. Her focus is on innovations in automotive software, connected cars, and AI-powered navigation systems.

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