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Have you ever finished a transmission service and had the customer bring the vehicle back for a low fluid concern? If it was an external leak, the source was probably not difficult to find. Clean the area with solvent and run the vehicle, following the leak to its highest point until we find the source.

What if the leak is internal? You have likely wondered where it disappeared. The fluid doesn't just evaporate; it must go somewhere. For the seasoned techs reading this I'm sure you have seen faulty seals between the transmission and transfer case in a 4WD application. The fluid can drain into the transfer case until it is completely full. One easy way to check for this is to pull the fill plug on the transfer case. If fluid gushes out, you must remove the transfer case and replace these seals. As much as 4 quarts of fluid can be transferred from the transmission to the transfer case. If your transmission is 4 qt. low, you have big problems!

 Case study

After installing a remanufactured 5R110W transmission in a 2007 Ford Super Duty with a 6.0L diesel, the transmission was filled until the fluid indicated full on the stick. After a short amount of idling and switching between the reverse and forward ranges to circulate the fluid, the level was rechecked. Another 1.5 qt. were added until it read full on the stick again. After a quick clean-up, a road test was performed. Towards the end of the road test, a small amount of slipping was observed. The fluid level was low 2 qt. after being rechecked. After a quick top-off of the fluid, it was retested, and the transmission worked great.

After driving the vehicle that night, and letting it sit, the fluid was low again, and the slip was back. Once again, fluid was added to the transmission and the slip was eliminated. After several more days of continuous monitoring and refilling, it finally stabilized and did not slip anymore. All told roughly 5 or 6 qt. of transmission fluid were added to the initial 6 qt. used at installation. In addition, fluid was left in it from dynamometer testing.

Let's not forget the possibility mentioned above of faulty seals causing transfer case overfilling. The transfer case level was spot on, so that was not where the fluid ended up.

Let me throw you a curveball. A high-capacity radiator and oversized transmission cooler were fitted, which may have exaggerated the condition.

What we have surmised is the cooler circuit had air pockets. As the thermal bypass opened when the fluid reached operating temperature it would purge the air, and fluid would fill the coolers, leaving the pan low. The moral of this story is that sometimes you need to drive the vehicle longer than a 5-mile road test before delivering it to the customer. You may need to run it through a couple of heat cycles before it is ready to deliver. If this vehicle had been delivered to a customer, I'm sure it would have come back on a tow truck. You know the customer would not be happy. 

Low fluid level conditions may result in drivability complaints like:

· Loss of engagement while cornering or coming to a stop due to G-forces moving the fluid to the sides of the pan and not reaching the fluid pick-up.    

· Delayed engagement into gear due to air build-up in clutch drums, fluid worm tracks, servos, and other channels that should be full of fluid.

· No engagement due to fluid not reaching the pump.

· High torque converter stall speed or (slipping). A torque converter is a fluid coupler that uses ATF to transmit torque from the engine and spin the input shaft of the transmission. It is very difficult to do that job without an abundant supply of fluid in the pan.

Okay, we covered some drivability issues, but here are some harsh realities of low fluid levels.

Think about what a transmission pump does. It supplies pressurized fluid to the transmission, continuously pumping gallons per minute. Everything is happy in this situation until you introduce air into the equation. In the pump, there is compression, heat, and fuel (ATF). The only missing ingredient in combustion is air! Air is 20,000 times more compressible than fluid, compressing air molecules makes heat, which leads to hot spots in the pump cavity, enhancing the possibility of pressure spike due to combustion.

For those who have never used ATF as fuel, I can personally attest to its ability to power a diesel engine. Many years of working in a transmission shop gave me access to gallons of used ATF. All I did was filter it and pour it into my truck's fuel tank. I'm not saying you should do this with your new common rail diesel, that would ruin your fuel system. However, this works well in older diesel engines.

The reason I discuss this is that you can have the same (diesel) combustion effect inside your transmission pump due to unwanted air. However, the pump was not designed to handle this stress and will ultimately fail, leaving the transmission needing major repairs.

Photo courtesy of ATRA


The theory of dieseling in the pump was introduced to me years ago by Steve Garrett. Steve is a brilliant man whom I have been fortunate to learn from over the years. I remember the day he mentioned this scenario to me, and a light bulb came on. This explains why a transmission pump may fail 500-1000 miles after installation. The main thing to take away from this is how bad air can be in the hydraulic circuit. It is also how imperative it is to get the fluid level correct before delivering the vehicle to the customer.



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