All Automatics run on Fluid Pressure
Pressure is supplied by the pump inside transmission hooked directly to torque converter, so spins at engine RPMs
The Valve body routes this fluid pressure to the Clutches and Bands(brakes)
The pressure holds clutches engaged or disengaged, and brakes(bands) on or off, this is how the different ratios(gears) are selected
There are pressure ports on the outside of transmissions to test pressures in various "gears"
Obviously internal fluid leaks will lower pressure, sometimes overall and sometimes just for one valve, and valve seals can leak
These leaks usually occur in the Valve Body
The 4R44E or 55E were first used in 1995 Rangers, the 5R44E or 55E are THE SAME transmission internally, so any info on either applies to all
Good article to read here:
https://atracom.blob.core.windows.net/gears/2003/2003-05/2003_5_50.pdf
Explains what is happening inside as ratios change, what valves are used and not used
Also explains why answering "why is trans slipping" is not easy or specific, there are multiple reasons/causes and they overlap
Wholesale changing of Valve Body "may" fix it
But not a waste of money because if it doesn't fix it then you need a rebuild and already have the new valve body for that
