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Automatic transmission fluids can oxidize with use, causing marginal transmission performance and eventual transmission malfunction. Periodic fluid changes are presently recommended to alleviate this problem. Fluid oxidation is promoted in current transmissions because they breathe air freely through a vent tube. To reduce fluid oxidation, and thereby improve fluid and transmission durability, a one-way check valve, called the Transmission Air Breathing Suppressor (TABS), was designed to restrict the intake of air into the transmission and to replace the conventional vent tube. The effectiveness of the TABS valve in reducing fluid oxidation was determined in high temperature transmission cycling tests and in taxicab tests. Fluid oxidation results with the TABS valve-equipped transmissions were compared to those with normally-vented transmissions. By reducing the amount of oxygen in the transmission gas, the TABS valve nearly eliminated fluid oxidation.

Clutch plate surface temperatures have been measured during clutch engagement using a clutch machine that simulates clutch application in an automatic transmission. Clutch surface temperatures were found to depend upon such parameters as clutch plate sliding speed, number of clutch plates in the clutch, rate of clutch engagement, lubricant type, and lubricant sump temperature. With this same machine, friction between clutch plates was found to be affected by lubricant type, lubricant sump temperature, and number of clutch plates used. It was also found that a correlation existed between friction and automatic transmission shift performance.

As part of General Motors effort to improve fuel economy, the effects of engine and power train lubricant viscosities were investigated in passenger car tests using either high- or low- viscosity lubricants in the engine, automatic transmission, and rear axle. Fuel economy was determined in both constant speed and various driving cycle tests with the car fully warmed-up. In addition, fuel economy was determined in cold-start driving cycle tests. Using low-viscosity lubricants instead of high-viscosity lubricants improved warmed-up fuel economy by as much as 5%, depending upon the differences in lubricant viscosity and type of driving. Cold-start fuel economy with low-viscosity lubricants was 5% greater than that with high-viscosity lubricants. With such improvements, it is concluded that significant customer fuel economy gains can be obtained by using the lowest viscosity engine and power train lubricants recommended for service.

Controlled-slip differentials (CSD) improve car operation under wheel slipping conditions. The performance of CSD's is dependent upon two criteria associated with clutch friction: “chatter” and “effectiveness.” “Chatter” is an undesirable noise which may occur during differential action. “Effectiveness” is a measure of the ability of the CSD clutches to transfer torque, during wheel slippage, to the wheel with the greater traction. The objective of this investigation was to definitely establish the cause of chatter, measure CSD effectiveness, and relate friction characteristics of lubricants to CSD operation. In tests with an instrumented car, it was found that both chatter and effectiveness are strongly influenced by the lubricant. Chatter occurred with lubricants that produced an increase in clutch friction with decreasing sliding speed. Chatter did not occur with lubricants containing friction modifiers which produced a decrease in clutch friction with decreasing sliding speed.

In order to determine transmission fluid requirements, fluid performance in automatic transmissions has been investigated using engine-transmission-dynamometer test apparatus. It was found that the rate of fluid oxidation was essentially the same using two-speed and three-speed transmissions. Further, the rate of fluid oxidation was very similar in part throttle and full throttle cycling tests. In addition, the effect of fluid frictional deterioration on shift performance was similar between the two transmission types. It is concluded that fluid performance in the two-speed transmission can be evaluated using the three-speed transmission, or vice versa. The significance of the full-scale laboratory test results was confirmed by establishing a correlation with car test results.

Engine oil and automatic transmission fluid viscosities are major factors in assuring good starting and running performance in cold weather. To determine the contributions of the engine and transmission to the cranking and running effort, instantaneous torque and power, obtained with an instrumented engine-transmission apparatus, were determined for five engine oils ranging in viscosity from 4 to 184 poise (SAE 5W to SAE 20W) and for four transmission fluids ranging in viscosity from 3200 to 83,000 cp at -20 F. Specific engine and transmission cranking variables - engine friction, compression and expansion, engine rotational inertia, and transmission friction and rotational inertia - were analyzed in detail. The engine required most of the cranking effort, which increased with increasing engine oil viscosity. Increasing engine oil viscosity increased engine friction torque but decreased engine friction power because of decreased cranking speed.

The General Motors Dexron-II automatic transmission fluid specification, issued in August 1973, defines physical, chemical, and performance requirements of a new class of fluids developed to meet increasingly severe service in passenger car and commercial automatic transmissions. Four new tests for determining fluid performance and durability have been developed for the specification. Results from these tests with Dexron-II prototype fluids are compared to those with Dexron fluids. It was found that the prototype fluids are much more oxidation-resistant than typical fluids in the Turbo Hydra-matic oxidation test; a 60% improvement in fluid durability has been realized in the Turbo Hydra-matic transmission cycling test; and Dexron-II prototype fluid friction and wear characteristics are about equivalent to those for Dexron fluids in the high energy, friction characteristics and durability test, and the wear test.

Clutch plate surface temperatures were measured with estimated accuracy of ± 10 F in a controlled-coupling automatic transmission. Power input to the transmission was from a 394 cu in. displacement engine, and power output was absorbed by a flywheel and dynamometer. Surface temperature as a function of time was obtained by incorporating a thermocouple into the surface of one of the steel clutch plates in the transmission. Operating parameters studied with respect to their effect on clutch surface temperature were dynamometer load, engine throttle setting, and transmission sump temperature. Dynamometer load had little effect on maximum clutch surface temperatures during clutch engagement for all transmission sump temperatures investigated. On the other hand, maximum clutch surface temperatures were found to increase with increasing throttle opening. Results agree with calculations of energy dissipated by the clutch during clutch engagement.

Performance-based test techniques were developed to determine high- and low-temperature automatic transmission fluid viscosity requirements. High-temperature fluid viscosity requirements were determined using engine-transmission-dynamometer tests. Low-temperature fluid viscosity requirements were established with a motor-driven 3-speed transmission apparatus. It is concluded that the current used-fluid Dexron viscosity limit of 5.5 cs minimum at 210 F provides a good safety factor for preventing excessive internal leakage at high temperatures. Since some Dexron fluids have -40 F viscosities approaching the 55,000 cp specification limit, lowering it will be considered in future specifications.

In certain types of city bus service some automatic transmission fluids can fail in less than 10,000 miles. In order to provide satisfactory transmission performance for longer mileage, improved fluids are required. An investigation was undertaken to obtain improved fluids. Fifteen different fluid formulations were evaluated in 30 city buses operated in normal service for more than 2,000,000 miles. It was determined that fluids fail because of frictional deterioration and oxidation. Based on these evaluations, only two fluids were found to be satisfactory for more than 40,000 miles; one additional fluid was satisfactory for more than 30,000 miles. The remaining 12 fluids failed in less than 20,000 miles.