How Crucial Is Transmission Fluid Level

Most enthusiasts can understand why a low-fluid level will kill an automatic transmission. Since automatics operate on the principle of hydraulics, you need both oil and pressure to operate clutches and bands. If the pump begins to suck air instead of oil, line pressure plummets, and the result is often a set of burned clutches that will no longer hold a load.

But a high fluid level can create the same problem. A high fluid level pushes ATF into the spinning internal components, which creates foam instead of pure hydraulic fluid. With air entrained into the oil, hydraulic fluid will tend to compress the air bubbles. This reduces the effective line pressure in the system with the inevitable result, again, being burned clutches and excessively high fluid temperatures.

transmission fluid

We checked the aftermarket dipstick on our Turbo 400 and discovered, not surprisingly, that it was incorrect. The black mark is the “Full” mark but it is almost 1/2-inch below the pan rail. This means it indicates full when the level is not actually at its proper level.

So much like the parable of Goldilocks and the Three Bears, the middle-road fluid level is the correct one. All automatic transmissions operate using the same reference point for the proper fluid level – the oil pan rail. An accurate dipstick will position the “Full” level at this position with the engine running and the fluid at normal operating temperature.

The problem is, with some offshore dipstick tubes and sticks, the indicated “Full” level is not always accurate. Of course, the easiest way to tell if the stick is accurate is to remove the transmission oil pan and with the dipstick in place, compare its indicated level to the pan rail.

Correcting The Level

John Kilgore Racing Transmissions has been building and modifying automatic transmissions for longer than he cares to remember. As a point of reference, his experience traces back to a story in a 1969 Hot Rod magazine, imparting his wisdom on modifying old four-speed Hydros. Through the years, Kilgore has seen a lot of smoked automatics that suffered their fate due to both excessively low and high fluid levels.

transmission fluid

This is Kilgore’s cast-aluminum deep-sump pan. It is 1 1/4-inches deeper than a stock Turbo 400 pan. But the real treat is the small 1/8-inch brass tube extending from the bottom of the pan. This tube is level with the oil-pan rail and will begin to drain fluid when the oil level is at its correct position. Kilgore’s pan comes with a longer pickup tube, O-ring, filter bolt, and pan bolts.

Kilgore’s specialty these days is building what he calls the SuperLite Turbo 400. This is a racing transmission with a manual-shift valve body that uses unique internals that trim the rotating mass roughly in half. For racers, this is worth anywhere from a 0.25- to a 0.50-second reduction in quarter-mile times. This transmission is often used in 8-second quarter-mile cars that often leave with the frontend in the air.

This experience taught him that fluid level is critical to ensure his automatics will survive. To this end, Kilgore developed a very simple way to avoid an erroneous dipstick reading by mildly modifying the deep cast aluminum oil pan’s drain plug with a simple brass tube. The tube’s height inside the pan is equal to the oil pan rail height’s fluid level.

Kilgore’s deeper cast-aluminum pan requires a longer pickup tube to place the filter near the bottom of the pan to ensure it is always below the fluid level. Make sure to remove the old O-ring from the transmission and use the new one supplied with the pan. This also requires a longer filter bolt with a sleeve to place the filter lower.

Accuracy Spilling Over

Admittedly, the procedure for checking fluid level is a bit more cumbersome than using a dipstick, but it is far more accurate. If we are dealing with a new transmission, start by removing the small Allen fitting in the drain plug. Then, with the engine off, fill the pan with ATF until oil runs out of the plug. It’s best to use a drain pan to catch any errant oil.

Here’s the new filter in place. You can see how the longer bolt properly positions the filter lower into the deeper pan.

Now, start the engine and drop it in forward and reverse a few times, and add additional ATF until fluid starts draining out of the tube. With only a small dribble coming out of the tube with the engine running, the fluid level is now accurate.

In case you’ve ever wondered why an automatic’s fluid level should be checked when running, it’s because the pump fills the torque converter with fluid when running. Once the engine is shut down, the fluid in the top half of the converter drains into the pan and will show an over-full condition.

transmission fluid

With the pan in place, remove the small Allen plug and with the engine not running, fill the pan until fluid runs out. Now start the engine and fill until the fluid dribbles out. The trans is now properly full with the engine running.

Filter Low-Down

Kilgore also modifies the standard Turbo 400 filter to reposition it near the bottom of his new cast aluminum pan. In addition, he cuts away part of the lower cover of the filter to allow it to pick up fluid from its bottom instead of the top. The factory placed the filter material inlet at the top of the filter to avoid picking up clutch material from the bottom of the pan. Kilgore assumes that racers will have the transmission apart for service long before this happens. He prefers to be able to pull fluid into the pump from an area that will always have fluid – even under a hard drag strip launch.

Our test machine is a 10-second Chevelle, so rather than use normal Dexron fluid, we opted for 7 quarts of Lucas Oil semi-synthetic ATF. Note, this is an F-type fluid. For drag racing and performance use, F-type fluids do not contain friction-modifier additives, allowing the clutches to apply more firmly.

So Easy, Even We Can Do It

We performed this trans pan swap just to check out how it works. The pan install is a simple remove-and-replace operation that only requires about a half-hour. It’s no surprise that the fill level is more difficult to measure than a simple dipstick, but the accuracy is unquestionable. We installed this pan on our Turbo 400-equipped ’66 Chevelle, and testing shows the deeper pan certainly ensures that the oil pickup for the transmission is always submerged in fluid.

We recently tried our Chevelle at Irwindale Raceway’s 1/8-mile track. On this particular pass, the Chevelle ran a 7.05 e.t. at 98.98 mph. That converts to a 10.85 at 122 mph in the quarter. Not bad for an iron 6.0-liter truck engine with a Kilgore-enhanced Turbo 400.

Of course, you don’t have to go through all this just to accurately set the transmission’s fluid level, but it’s nonetheless imperative that at least you know how to accurately assess your fluid level.

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About the author

Randy Bolig

Randy Bolig has been working on cars and has been involved in the hobby ever since he bought his first car when he was only 14 years old. His passion for performance got him noticed by many locals, and he began helping them modify their vehicles.
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