When checking transmission fluid should the car be running

There are two main things about tranny fluid:

1. Transmission fluid is checked when it's hot (up to running temperature). As the fluid gets up to temperature it expands. If the fluid is cold, it won't give the proper level reading, thus overfilling will ensue.

2. One of the main fluid compartments of the transmission is the torque converter. When full, the fluid level of the torque converter is much higher than is the level of the pan (almost entirely above the pan in most cases). Due to this, when the engine is stopped, a significant amount of the fluid will drain down into the pan, showing the wrong amount of fluid on the dipstick. With the engine running, not only is the transmission paths filled with fluid, but the torque converter is as well. Then a check of the fluid will give you a proper level.

Checking engine oil is a lot different. It needs to be checked when it's in the oil pan. In order for it to be in the oil pan, it needs to drain after it's been up in the engine. This takes a little bit of time. Engine oil doesn't expand as much as transmission fluid, so can be checked hot or cold. While engine oil is important to how an engine operates, the level isn't as imperative as transmission fluid. Transmissions are very sensitive to line pressure. As you get more transmission fluid into the transmission, the pressures rise. If left unchecked, the fluid will find an escape hatch somewhere and that somewhere is usually a seal. Blowing out the front main seal (or where the torque converter rides) requires a transmission pull to fix. This isn't something you want to do every day.

Engines don't have the pressure effect which transmissions do. The pressure is regulated at the pump and mainly needs to have oil flow in order to operate efficiently. Most engine manufacturers are good down to even 5psi of oil pressure. There is also a lot of space in an engine for oil to hide. As oil gets pumped around, it sticks to walls and is all over the place. It needs to drain back into the sump, thus the 5 minutes of drain time. Not everything will drain down, but the vast majority of it will. This isn't how every manufacturer likes to have it read, but leaves things in a pretty good state as far as the level goes.

I don't read this as a discrepancy in either case. I look at it as how they are designed/engineered to be checked.

Most vehicle owners know that in order to keep their ride roadworthy, they need to stay up-to-date on routine maintenance, such as oil changes and tire replacement. But you should also keep tabs on your automatic transmission—you know, that thing with the gears that you shift into drive, reverse, and park multiple times a day. Keeping your transmission humming smoothly can pay dividends, as it's one of the most expensive components on your car to repair or replace.

Thankfully, checking on your transmission's health is neither as difficult, as time consuming, nor as costly as you may think. Here's how.

Checking the Fluid

Just as your engine uses oil to lubricate and cool its internals, automatic transmissions use specially designed transmission fluid for the same purpose. Conventional automatic transmissions, dual-clutch automatics, and continuously variable automatic transmissions each use a specific type of transmission fluid. If you're unsure which fluid is used in your transmission, consult your owner's manual; typically, transmission-fluid requirements can be found in the specifications section.

You don't need to be a mechanic to gain insight into the condition of your vehicle's transmission—a simple visual check will do. You'll need to look at the level and condition of your transmission fluid.

Locate the Dipstick

First, locate the transmission dipstick, which can be found under the hood, in the engine compartment. Make sure you are locating the transmission dipstick and not the engine-oil dipstick; the transmission dipstick is usually further back in the engine bay, toward the firewall (the bulkhead at the front of the cabin). The transmission dipstick is typically marked with a specific color or a transmission symbol.

Note: if you can't find the dipstick, don't be alarmed. Many modern vehicles use a sealed-for-life transmission that never requires checking or fluid replacement—so they don't have a dipstick. (Refer to your owner's manual for your model's specific service schedule and to double-check whether it has a transmission dipstick.)

If your vehicle has a sealed transmission, you can slam the hood shut and drive. But if your vehicle does have a transmission dipstick, here's what to do next:

Check the Level

With the engine warmed up, leave the car idling in park on a level surface. Pull out the dipstick, wipe it clean, replace it slowly, and then pull it back out. Check the fluid level—how high the fluid comes up on the dipstick—against the "full" and "low" or "fill" marks on the dipstick.

The color of transmission fluid can tell you a lot about the health of your car's transmission.

Fluid Condition

Now lay the dipstick on a white surface, such as a paper towel, to analyze the color of the fluid. The condition of your transmission fluid—and to some extent, the transmission itself—is indicated by the color of the fluid. If your fluid is healthy, it should have a reddish-pink color; if it's getting to the point of needing replacement, it will be brownish red. If the fluid is dark brown or black, then it's quite possible that you will be replacing more than just your fluid. Dark fluid with a burnt smell is bad news; in the worst case, you might find fine metal shavings in the fluid as well. Both of those symptoms point to possible damage of your transmission's internal components. This is usually a result of failing to follow the recommended service interval for replacing the transmission fluid, but it's not impossible that a transmission could have a premature mechanical issue, just like any other component on the vehicle.

Low Fluid?

If your fluid is low, it doesn't necessarily mean you're headed for disaster, but it does most likely mean there is a leak somewhere in the system. Filling up the transmission and then checking it daily to see how quickly the level goes down can be a good way to assess the severity of a potential leak. Also try to visually inspect your transmission by looking under the car for any fluid oozing out of it. Does the car leave spots of reddish fluid on the ground after it's parked? If the fluid is black, it's engine oil. If it's water, it's likely condensation from the air-conditioning system.

If you do notice some transmission-fluid loss or observe that your transmission is using an abnormal amount of fluid, contact a mechanic as soon as you can. We'd recommend seeing a reputable car dealer who sells and services your vehicle as a first stop. Its service department will have the most experience with your make and model and might have seen this issue before. If you have the luxury of letting the car sit until it can be inspected, do so.

Contrary to what some internet mechanics may tell you, a transmission-fluid replacement will not destroy an older vehicle's aging transmission. Typically, when a transmission suddenly has issues after fluid replacement, it's because there was already an internal problem, such as a worn clutch pack. If your transmission is healthy, then a fresh change of fluid will only help its longevity.

FYI, if your transmission fluid is low and needs topping off, this is usually done through the same tube that the dipstick fits in. Adding fluid (which is available at auto-parts stores) will require a funnel with a narrow—and most likely long—spout.

Just like any machine, a transmission needs proper maintenance in order to operate as the manufacturer intended. As they say, take care of your transmission, and it will take care of you.

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