Pressure and Harsh Shifting
Mainline pressure is controlled by the pressure regulator valve. Mainline pressure is the source pressure for the torque converter, valve body, and the clutches and bands in an automatic transmission. As pressure builds from the transmission pump, the regulator regulates and exhausts this excessive pressure to the sump, preventing any damage to friction materials and internal components.
Always check the condition of the fluid while checking its level. Use a piece of paper towel or a lint free cloth. Automatic transmission fluid should be red. Only in a few transmissions is the dye left out. If the fluid has been overheated, it will appear dark brown or black. Contaminated fluid can cause valves like the pressure regulating valve to stick in their bores. Pieces of black particles in the fluid indicate slipping band and clutch friction materials.