Investigation of Failure Modes in the DEXRON® III-H GM Cycling and Oxidation Tests

To develop an automatic transmission fluid (ATF) that meets DEXRON® III-H specifications, the ATF must pass two critical tests, the GM oxidation test (GMOT) and the GM cycling test (GMCT), in addition to many other performance tests. The specification on the GMOT is that delta TAN (difference in total acid number compared with the fresh oil) at the end of the test does not exceed 3.25 while the specifications on GMCT are that delta TAN cannot exceed 2.0 and the 1-2 shift time must stay between 0.30 and 0.75 seconds throughout the test. For this work, we analyze oil oxidation and changes in oils' surface tension, drum and band surface degradation and deposit formation. We have found that with respect to the delta TAN limits of the DEXRON® III-H specification, the GMCT is more severe than the GMOT. The effect of base oil chemistry on oxidation in these tests has been quantified. Oil oxidation is not responsible for the GMCT 1-2 shift time increase. There are two failure modes identified to be responsible for the GMCT 1-2 shift time increase. One is an increase in air dispersed in the oils as they age and the other is thermal degradation of the band friction material. The surface tension of oils decreases as they are aged in the GMCT and the decrease in surface tension is correlated to an increase in 1-2 shift time. As surface tension decreases more air can be dispersed in an oil. As band friction materials are aged the decomposition temperature of the band material (as measured by pressure differential scanning calorimeter (PDSC)) decreases. The drop in PDSC decomposition temperature is associated with the degradation of the band friction material due to oligomerization caused by thermal effects rather than deposit or glazing of the friction material.