In order to improve interchangeability, GM designed the output carrier and output shaft to be a simple spline fit ( Figure 6). Many of the planetary components can be removed from the back of the case. The sun gear splines to the intermediate shaft and has a rear thrust bearing that goes next to the output shaft. To say that the output sun gear is small would be an understatement, but small it is. Not only are the park-gear lugs made to the carrier, but the output speed-sensor teeth as well. The planet carrier contains a shaft that splines to the direct/overdrive planet carrier and also has a thrust bearing retained by a metal baffle. The intermediate speed-sensor teeth are cut into the outside of the ring gear. The set components are the output ring gear, planet carrier and sun gear ( Figure 5). The last assembly is the output planet set, which is what the output shaft splines to. The retaining ring is also a spiral design. The planet carrier also has a thrust bearing retained by a metal baffle and the carrier is splined to the output ring gear. The ring gear is splined to the aluminum 1-2-3-4-5/R clutch hub and has a thrust bearing on the front of the assembly. The third assembly is the reaction planet set, which is comprised of the dual sun gear, ring gear and planetary carrier ( Figure 4). The sun gear contains a front and rear bushing and the planet carrier uses two plastic thrust washers. The input sun gear has more than one function as well, since it is splined to the reaction sun gear and is also attached to the 1-2-7-8/R clutch hub. In addition, the aluminum planet carrier is connected to a driving shell that splines to the 1-3-5-6-7 clutch housing and can be driven directly by the input shaft ( Figure 3).
As stated, what drives the input planetary is the previous sun gear/ring gear assembly. The next planetary set down the line is the input planet that is also somewhat unique. The assembly is retained with a spiral snap ring and the sun gear has a press in bushing. The third item has three functions, first as the sun gear, next as the 4-5-6-7-8/R clutch hub and lastly as the ring gear for the input carrier ( Figure 2). On the front side is another thrust bearing retained by a metal baffle. The next item is a planetary that splines to the output planet shaft and has a captured thrust bearing on the backside. There is a thrust bearing between the ring gear and clutch housing. The first item in this set is a ring gear that also doubles as the 2-3-4-6-8 clutch hub. As with other later-model transmissions, gear-set components are not always clear-cut as to the layout. The first planetary gear set in the stack up is the direct/overdrive assembly. Four of the five clutch hubs are also part of the assembly.
From the connecting shell in the front to the removable splined output shaft in the back, the nested planet assembly is certainly compact ( Figure 1). Planetary gear assemblyĪlthough there are four planetary gear sets, GM did a great job in compressing the space needed overall. Part 2 will begin with a discussion of the planetary gears sets. Friction elements are listed accordingly: The 8L90 uses numbers to describe when a friction element is applied like GM six speeds, but the numbering is somewhat more involved with the eight speeds due to more gears. One welcomed departure from ZF was the absence of an internal TCM in the GM units. The 8L90 also has a ZF thumbprint on it, though somewhat less than the previous six speeds. When GM developed the 6L80/6L90 transmission series, they were loosely based upon the ZF 6HP family of transmissions. Currently, GM offers two models of eight-speed transmissions, the light-duty 8L45 and heavy-duty 8L90.
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A year later, the 8L transmission series had spread out to the Corvette and trucks such as the Silverado and Yukon. By 2014 GM released the 8L90 in the Cadillac ATS.
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