25.23.
Automatic Transmission Fluid
Automatic transmission fluid performs four different jobs. In the converter it transmits engine power from the impeller to the turbine. It hydraulically operates the holding devices used to control planetary gear drives. The fluid lubricates the transmission bearings and carries heat from the hot transmission parts to the cooler, usually located in the radiator tank. Automatic transmission fluids are compounded to fulfill a variety of functions. Matching the fluid to the transmission is essential for good clutch plate life, band durability, elastomer seal life and for smooth operation. The fluid is made from 75% paraffinic-based and 25% napthenic-based oil to control seal swelling. Complex automatic transmission fluids are formulated for low temperature fluidity, oxidation resistance, anti-foaming, corrosion resistance, effect on seals, and effect on friction. Red dye is added to transmission fluids to differentiate it from engine oil, which has a neutral colour. The correct fluid must be used in each transmission to minimize a squawk or hump as the discs and plates reach the same speed.
Fig. 25.58. Accumulator action.
The automatic transmission fluid level should be checked at regular intervals. Low fluid level is one of the first causes of transmission problem. The fluid expands from normal temperature to the normal operating temperature of 391 K. Therefore, the fluid level is checked with the engine running and the transmission in neutral position so that fluid is thoroughly warm.
Transmission fluid deteriorates with use. Deterioration results from the shearing action on the fluid as it moves through the pump, converter, and fluid passages. The main cause of fluid deterioration is the oxidation of the oil base and the breakdown and neutralization of the additives that constitute about 10% of the fluid volume. The higher the operating temperature, the more rapidly the oil breaks down. With 40,000 km to 48,000 km running of the vehicle, the fluid reaches a stage where it does not function properly and hence requires replacement. Fluid contamination is another reason for changing the fluid. Materials worn from the bands and clutch discs are carried by the oil. Very small particles circulate with the fluid as large particles are removed at the filter. Transmission should not be filled with excess fluid, which causes foaming and seal damage. Hence excess fluid should be removed if overfilled.
Transmission should be checked for no external oil leaks and if found should be repaired. The external leakage causes the loss of fluid, so that after a certain stage the holding devices do not properly apply causing transmission slip and consequently increase in rate of wear.
