Improving Transaxle Performance at Low Temperature with Reduced-Viscosity Automatic Transmission Fluids

The effects of automatic transmission fluid viscosity on the low-temperature performance of a front-wheel-drive transaxle were determined in a cold room maintained at a temperature of -20°F (-28.9°C), using both a cranking apparatus and a vehicle. Cranking and vehicle tests were conducted to determine the effects of fluid viscosity on the power required to crank a transaxle and on transaxle performance under low-temperature transient operation, respectively. Four automatic transmission fluids were tested, ranging in viscosity from 2 800 to 16 000 cP at -20°F. All test fluids contained the same additive package and were blended using the same types of base oils. Reducing fluid viscosity from 6 000 cP (current factory-fill) to 3 900 cP at -20°F reduced shift times by 10 percent, forward clutch engagement time by 25 percent, apply pressure rise times by 40 percent, and cranking loads by almost 40 percent. Fluids providing such improved low-temperature performance are needed for planned future automatic transmission fluid specifications.