Overfilling a transmission causes the fluid to become aerated and foamy. It results in leaks, buzzing and whining sounds, and erratic shifting. Fluid will seep past the dipstick or bypassing axle seals.
Transmissions are sealed and do not consume transmission fluid. When the fluid is low, look for a leaking transmission pan gasket, axle, or pump seal.
Contaminated Transmission Fluid
Pieces of black particles in the fluid indicate slipping friction materials. Worn and damaged metal parts result in silvery metal pieces in the fluid. If the fluid on the stick is pink and milky, coolant has seeped into the transmission cooler and mixed with the transmission fluid.