Good info, Grigg!
Since I quit working for a living and started teaching school, I've forgotten a lot of the details I once knew about working on big rigs. The 4X4 transmission setups I used to work on for a sand & gravel hauler in central California had air shifters on the Brownie, and they worked pretty good unless a driver lost count and forgot which gear he was in. Those things had 4-speed main boxes with a regular stick shift, and a 4-position air splitter on the shift knob for shifting the Brownie. I believe the air cylinder tensioned a spring, and when the driver eased off the power, the Brownie would shift up or down, similar to a 2-speed rear axle.
There was only about 200 RPM between gears, so it was essential for a driver to know which gear he was using at all times. The Cummins and Detroit engines we ran in the late 1970's only had an operating range from 1750 to 2150 RPM, so a 200-RPM spread between gears allowed a driver to stay in either the high speed max horsepower range, or the low-speed max torque band by picking out the right gear. A few of the really big rigs or the ones used off road had 5X4 setups. Lots of fun with those!
Jerry
