Why transmissions fail after fluid change

Sometimes, when a transmission has been maintained poorly previously, it is good to know and apply an approach that will not cause the transmission to stop functioning either by slipping or not engaging at all.

What is an automatic transmission fluid?

An automatic transmission fluid commonly called ATF which is an abbreviation is a special fluid formulated for torque converter operation, valves operation, transmission clutches and brakes operation, heat transfer and for lubrication of gears.

ATFs also provide hydraulic pressure necessary for the transmission to work.

To differentiate ATFs from engine oils, they are usually coloured red or green.

There are varieties of automatic transmission fluids. Those variety have transmissions they work with hence we don't put any ATF in any transmission without reference to recommendation and specification by vehicle manufacturer.

Why change automatic transmission fluid?

Transmission fluids oxidize and degrade over time due to the effect of temperature, stress and wear particles from clutches, bands and other transmission components.

Due to these, they loose their lubrication properties. Other additives such as friction modifiers for clutches also degrade.

Over time, the transmission oil burns and also get saturated with particles which can cause the filter to get clogged and can block passages in the transmission valve body.

What makes the transmission to malfunction after fluid change?

1. When the oil is not changed at the right time, the oil begins to form varnish that is not soluble. This varnish stick to valve body parts and contributes to wearing the valves. When a fresh fluid is later introduced, the varnish becomes soluble in it and clog the oil filter causing the transmission to be oil starved.

2. A filter that is already clogged due to lack of fluid replacement might allow the old fluid to pass since the fluid has lost some viscosity. When new fluid is introduced, it might not be able to easily pass through the filter causing oil starvation which lowers oil pressure and parts lubrication.

3. Gunks from old fluid that was not replaced on time stick to some areas in a good way by acting as seals. New fluid can dissolve them creating clearances that can make the transmission to malfunction. This is because transmission fluids contains detergents.

4. Transmission flush is not recommended for an old transmission as it washes away gunks that seal parts. Transmission flush is done by using pressure to remove all the transmission fluid using specialty tools. The tool is connected to the transmission through the oil cooler lines.

5. Sometimes, people replace old transmission fluids with fluids having wrong specifications for that transmission. The wrong fluid will have different properties from that of the original fluid. The viscosity modifiers might not be compatible with the clutch materials of the transmission. The wrong oil might not generate same oil pressure that is needed and so on...

6. Condition of the transmission before oil change is another factor. As earlier said, gunks that have settled in parts of the transmission are beneficial. New fluids have detergents that will wash them into the oil, creating transmission problems.

7. Not replacing the filter during the oil change will make the already clogged filter to restrict passage of the new fluid with higher viscosity.8. This is an important cause that is often overlooked. Modern transmissions are adaptive as they adjust shift pressure and timing as transmission parts wear and as transmission fluid ages.

The values of shift pressure and timing are stored in the transmission control unit commonly referred to as TCU or TCM. When new fluid is introduced, the pressure values of the new fluid and that which is stored on the TCU no longer correspond. This makes the transmission not to shift properly. You might experience bangs or transmission slip. To avoid this, after transmission oil has been changed, the TCU should be reset and transmission relearn should be done. Adaptive transmission reset and relearn is a good resource to learn more about this.

When next you intend to change ATF of a transmission that doesn't have history of regular maintenance, do only drain and refill, replace the filter and carry out reset and relearn if the transmission requires the procedure. Some cars do not require following cetain drive pattern. Just drive and after some drive cycles, new shift pressure and timing values are updated. That's the power of machine learning!