Ask Away! with Jeff Smith: Should I Check My Automatic Transmission Fluid Level with the Engine Running?

I’m new to the car scene and one thing I have always been told is to check the automatic transmission fluid with the engine running, but to check the engine level with the engine turned off. I can see why you would want to check the oil level with the engine off but I don’t understand why an automatic transmission is different?

D.C.

First of all, welcome to the sport of high performance cars. There are plenty of things to learn about cars in general and if you are motivated, SummitRacing.com, OnAllCylinders.com, and other similar sites are a great place to learn. Don’t let the old guys make fun of you or give you grief for not knowing. There was a time when they were newbies too.

How to Check Engine Oil

Yes, you do want to check the engine oil on an engine with it not running so that you have an accurate estimate of the oil in the sump of the engine. If the engine is running, oil will be pumped into the rest of the engine and what is left in the oil pan will be splashed around the crankshaft and will coat the dipstick so the level will not be accurate.

Once the engine is off, it’s best to let it sit for a minute or so to allow the oil to drain into the pan before you check it. Pull the dipstick, wipe it clean with a rag, insert it back into the tube, and remove it again. This will offer an accurate reading of the actual oil level. Sometimes the level will be different on the two sides of the dipstick. This is not unusual. If this occurs, flip the dipstick over and check it again. Likely the results will become more stable. Sometimes oil travels up into the dipstick tube that can cause erroneous readings.

Dangers of Overfilling Engine Oil

It’s important to not overfill the engine oil level. We have an old school autocross racer friend who prefers to overfill the small block Chevy engine because he thinks that will better protect his engine in the sharp cornering situations where the oil can push over to one side of the oil pan. The problem with this approach is that the additional oil level in the pan brings it closer to the crankshaft which will act like a blender and churn up the oil and turn it into foam.

The definition of foam is entrapped air bubbles in the liquid oil. This is not good for several reasons. First, the air is compressible so it tends to reduce the running oil pressure and it also reduces the oil’s ability to lubricate. Both of these situations are bad. Even though you may not abuse your engine like our autocrosser friend, it’s still not a good idea to overfill the oil level.

Keeping it near the full mark is the best approach.

How to Check Automatic Transmission Fluid Level

Checking fluid level in an automatic transmission is completely different. Automatics use a torque converter which is a large, round fluid coupling bolted to the flexplate on the engine and is driven at engine speed. In order for the torque converter to work properly, it must be full of automatic transmission fluid (ATF). This is filled by the automatic transmission pump that is engine-driven. The top half of the torque converter is above the input shaft centerline and the input shaft is several inches above the automatic’s oil pan rail level. This requires the fluid level to be checked only with the engine running.

All automatics work off of hydraulic pressure created by the transmission pump. In order to ensure there is enough fluid in the transmission to fill the torque converter and operate all the hydraulic circuits. The only way to fill the converter full of fluid is with the engine running. So this means checking the fluid level with the engine running.

In all automatic transmissions that I am are aware of, the proper oil level will be even with the oil pan mounting rail. So logic dictates that the only way to check the automatic’s oil level is when the engine is running and has filled the torque converter and energized all the circuits.

Because the transmission oil level is sufficiently below the spinning drums in the transmission, the oil level should appear stable on the dipstick.

Don’t Overfill Your ATF, Either

Again, it’s important not to overfill the transmission for the same reasons as not overfilling the engine oil level. Overfilling an automatic transmission will cause foaming which is even more problematic for any automatic transmission because all the circuits operate on hydraulic pressure. Air in the oil will reduce the transmission line pressure and can cause inaccurate servo operation and perhaps clutch application slippage that could quickly damage these components.

So again, it’s best to keep the fluid level as close to full as possible.