Gearbox Characteristics

The ungoverned torque output of the Vortec 8100 8.1L gas engine and the introduction of the Duramax Diesel 6.6L engine necessitated the introduction of the ZF S6-650 (ML6) in the 2001 model year. Supplied by ZF as the S6-650, the ML6 is a high-torque capacity 6-speed manual that puts GM ahead of its competitors in the midsize, heavy-duty truck segment in terms of smooth shifting, high quality and overall vehicle performance.
The use of six forward speeds allows for an additional gear between first and direct drive fifth gear. First gear is a creeper gear, for starting out heavily laden vehicles. Second gear provides enough torque for launching an unladen or lightly loaded vehicle. The ZF S6-650 has dual cone synchronizers on second and third gears for easier downshifts from fourth to third and from third to second.
The shifter has received upgrades to limit vibration and coarse feel, and internal parts are coated for lower friction. There are ongoing improvements to continue to improve shift feel, as well.…