Where did the fluid go?
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!
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
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
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
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.