Automatic transmissions are the transmissions of choice in today's diesel realm. They've come a long way over the last decade, and are now beefier and more complex than ever before. This is an effort to combat the monster torque numbers factory trucks make. In this section of How It Works, we'll explain the components that make these monsters function, and how they hold up to the massive stresses imposed on them.
Planetary Gearsets
The heart of an automatic transmission is the planetary gearset. Planetary gearsets allow a transmission to have several different gear ratios, with different clutches holding certain components stationary. Planetary gearsets are made up of a sun gear, planet gears and the planet gears' carrier, and the ring gear (also known as the annulus gear). Each of these three components can be the input, output, or can simply be held stationary.
Torque Converter
A torque converter is essentially a fluid coupling device that transmits power from the engine to the transmission. Not only that, a torque converter acts like a clutch in that it allows your truck to stop and the engine to idle with the transmission still in gear. Diesel transmissions feature lockup converters, which effectively couple the transmission directly to the engine, stopping slippage, allowing more power to get to the wheels, and providing better fuel economy. All current diesel-specific transmissions have lockup torque converters. When does lockup occur? For example, on our '97 Power Stroke's E4OD transmission, lockup occurs after the transmission shifts into Third gear, and before it shifts into Overdrive (32 mph). It often feels like a shift in itself.
Valvebody & Shift Kits
Think of an automatic transmission's valvebody as a hydraulically imprinted circuit board. Considered the brain of the transmission, the valvebody is made up of two sides, which look like a maze of hydraulic passages with a separator plate in between. Each passage in the valvebody has a different purpose and directs fluid to the numerous valves that activate the appropriate clutch pack to shift into the correct gear. Aftermarket shift kits can change shift firmness, timing, and efficiency by adding stiffer shuttle valve springs, check balls to block certain passages, and often feature additional holes in the separator plate for increased fluid flow. Shift kits reduce slippage between gear changes and often prolong transmission life.
 Allison 1000 Torque Conve...  Allison 1000 Torque Converter. |  Parts: Planet Gear Carrier,...  Parts: Planet Gear Carrier, Planet Gear, Sun Gear, Ring Gear (Annulus Gear). | 
E40D Valvebody. |