Checking the transmission fluid is fairly simple in standard automatic cars since they come with a normal dipstick, just like the one for engine oil. However, you may be surprised and confused by the hot and cold marks on the dipstick, not to mention erratic readings if you do it incorrectly. So, here is everything you should know.
The reason why you should check transmission fluid when the engine is running is when the engine is running; the torque converter is filled with transmission fluid which changes the fluid level in the pan. If you check the fluid with the engine off, all the fluid that’s supposed to be in the torque converter is stored in the oil pan, and the dipstick will show that the level is too high, thus giving you an inaccurate reading.
Why You Should Check Transmission Fluid When The Engine Is Running
Before we go into why you should check transmission fluid when the engine is running, here is how the system works. The automatic transmission relies on a torque converter to transfer power from the engine to the wheels, unlike a manual transmission that uses two solid shafts and a clutch. And the way a torque converter works is that it uses transmission fluid coupling to turn the wheels.
The fluid coupling means there is no solid connection between the wheels and the engine, and instead, the engine turns a turbine wheel that pushes transmission fluid through an impeller connected to the wheels. The transmission fluid passes through the impeller, returns to the turbine, and the process repeats.
The torque converter uses transmission fluid for this process, plus the transmission fluid is also used to lubricate the transmission gears. That means there always has to be enough transmission fluid in the pan for the gears and to fill up the entire torque converter. Also, keep in mind that the torque converter only spins and is full of transmission fluid when the engine is running.
On the other hand, when the engine is off, the transmission fluid drains from the torque converter and rests in the pan, which is where the dipstick measures the fluid level. And since those are not the relevant running conditions, the dipstick is marked to show high and low levels during operating conditions (when the torque converter is full). That means the dipstick will show an abnormally high level of fluid if the engine is off.
Now, considering that a torque converter holds anywhere between 2 and 5 quarts of transmission fluid while it’s running, depending on its size, the dipstick reading will be drastically inaccurate. So, always leave the engine running when checking the transmission fluid so that the torque converter pulls the excess fluid from the pan and you get an accurate reading.
Why You Should Check Transmission Fluid When The Engine Is Hot
Whether you should be checking transmission fluid hot or cold is equally important as checking it with the engine running. When transmission warms up, it expands and raises the level on the dipstick. That’s something car manufacturers keep in mind and take into account when calibrating the dipstick markings.
If you check the transmission fluid with the engine cold, the dipstick will show a lower level than it actually is. Once you see that, you will add some fluid which under normal conditions will be too much, and that can end up blowing the seals or destroying the torque converter and shift solenoids. And that’s enough reason to never be checking transmission fluid cold.
Also, some cars have a hot and cold mark on the transmission fluid dipstick. The cold markings are there for when you are checking the fluid and when the engine is cold; you will also notice those are lower than the hot marks. However, the cold marks are still set to a certain temperature which you can’t replicate unless the outside temperature is right.
That’s why the engine should be at operating temperature when checking the transmission fluid level, even if your car has a cold mark on the dipstick. And to get the engine to operating temperature, take it for a normal 5-10 mile drive before you return home and check the level.
Other than that, make sure the car is on level ground to avoid the liquid level rising or lowering because of a slope. And lastly, with the engine running, shift through all the gears before checking the fluid level. That allows the transmission fluid to fill up all the shift solenoids and gives you a more accurate reading. To simplify the whole process, here is a step-by-step guide.
How To Check Automatic Transmission Fluid Level Step By Step:
- Take the car for a 5-10 mile drive so the engine is at operating temperature.
- With the engine running, shift through all the gears 3-4 times.
- Whether you check transmission fluid in park or neutral is irrelevant, but if you do it in neutral, put the hand brake on.
- With the engine running, check the transmission fluid level.
- If your car has a hot and cold mark on the dipstick, check the hot fluid level.
Should You Lave the Engine Running When Checking Manual Transmission Fluid
Checking manual transmission fluid is much more complicated than it is in automatic vehicles because manual transmissions don’t have a dipstick. So, here is how to check transmission fluid without a dipstick.
Go under the car and locate the filler plug on the side of the transmission. Then, remove the plug, and if the transmission fluid is coming out, the level is good. Otherwise, start putting fluid into the transmission until it starts spilling from the drain plug. This process is much more complicated, but if you are somewhat mechanically inclined, you can do it fairly easily.
And if you do, the engine should be off because with the engine running, the transmission shafts will throw the fluid out the drain plug making it impossible to measure. Furthermore, the engine should be cold, unlike automatic vehicles. Lastly, make sure the vehicle is on level ground, and shifting through gears is not necessary, nor does it matter which gear the transmission is in before checking the fluid.
Q: Can you check the transmission fluid with the engine off?
Technically, yes, you can check transmission fluid with the engine off, but you will get a drastically inaccurate reading. With the engine off, the dipstick will show that there are anywhere between 2 and 5 quarts of fluid more in the transmission than there is. So, always check the transmission fluid when the engine is hot and running.
Q: How long should I let my car run before checking the transmission fluid?
You should drive your car at least five miles before checking the transmission fluid to ensure the transmission is fully warmed up. Also, avoid warming up the car without driving it because that will only bring the engine up to temperature while the transmission remains cool.
Q: Why add transmission fluid when the engine is running?
First, you should always check transmission fluid with the engine running so that the torque converter gets filled with fluid and removes the excess, so you get an accurate reading. Now, there are no rules saying you can’t add fluid with the engine off, but you want to continually check the level as you are adding. So leaving the engine on until you reach the desired level is the best way to do it.
Q: At what mileage should you not change transmission fluid?
There are no manufacturer recommendations stating when not to change transmission fluid because fresh lubrication is always welcome. However, some manufacturers state that there is no need to change transmission fluid ever, while others recommend flushing it every 100,000 to 150,000 miles.
Q: Does transmission fluid warm up at idle?
Yes, transmission fluid warms up at idle but extremely slowly, and there is no way you can tell when it’s at operating temperature. That’s why you should always drive your car for at least five miles if warming up the transmission fluid is your main goal.
Q: How many miles does a good transmission last?
A good transmission that doesn’t have many factory problems or design flaws with top-notch maintenance and services can last 300,000 miles or more. However, most transmissions will last around 200,000 miles or up to 250,000 miles if it is well taken care of.
In the end, the reason why you always check transmission fluid level with the engine running is pretty simple, the fluid needs to fill the torque converter in order to get an accurate dipstick reading, and that only happens when the engine is running.
Other than that, make sure the transmission is warmed up because the fluid contracts when it’s cold, which will also give inaccurate readings. And lastly, make sure to shift through all the gears a couple of times before measuring so that transmission fluid fills all the solenoids.