For this SAE Recommended Practice, the transmission is defined as the complete automatic transmission or transaxle assembly between the engine and the driveshaft(s) used to effect a ratio change in transmitting power.
This test procedure deals with the aspect of conducting complete transmission and transaxle assembly testing. However, by its very nature a transmission should be viewed as a compilation of three major component systems: pump, torque converter, and gearbox (all ratio change elements).
From a design perspective, it is important that the losses associated with each of these components be determined by conducting separate tests of each component under controlled test conditions that simulate the in-transmission operating conditions. Torque converter testing is described in SAE J643. If done with strict attention to detail it is possible to subtract off the pump and torque converter losses from the transmission assembly losses in order to obtain gearbox losses only, eliminating the need to conduct a separate gearbox test.