Developments in Automatic Transmission Fluid Technology

The single recent event that has most significantly changed the way in which automotive transmission fluids are defined took place in 1990 when General Motors published its Dexron HE specification. This represented the first major modification of General Motors' Automatic Transmission Fluid (ATF) specifications since July 1978. (The original Dexron II specification was first published in 1973.) The driving force behind the timing for the introduction of this specification was the launch, in 1991, of the Buick Park Avenue with a newly redesigned, electronically‐controlled version of the older Hydramatic 440‐T4 Front‐Wheel‐Drive (FWD) automatic transaxle. This unit was designated the 4T‐60E. The majority of designers of computer‐controlled transmissions use electro‐hydraulic solenoid valves to open and close hydraulic circuits to make the transmission shift. For this reason, many of the new transmission designs require ATFs to have superior low‐temperature fluidity properties. Dexron IIE calls for fluids having a −40°C Brookfield viscosity of less than 20,000 cP, compared with the 50,000 cP requirement of the Dexron II. Only fluids approved to the new specification will be officially endorsed by GM from January 1993.