Automatic Transmission "Shift Quality"

In an automatic transmission, the Shift Valves, Governor, and Throttle Valve, or Vacuum Modulator, all work together to decide what gear the transmission should be in, and when. But, there is more to transmissions than merely "finding a gear" to be in. At light engine loads (light throttle conditions) relatively low hydraulic pressure is needed to apply the bands and clutches (friction components) in order to make the torque required to propel the car forward. As engine load increases (due to increased throttle settings), more hydraulic pressure is required to apply the friction components to make the gear changes. If we used a CONSTANT hydraulic pressure, sufficient to apply the friction components without too much slippage, during high engine load conditions, the transmission would seem to shift progressively harder at lower throttle settings, literally "slamming" into the next gear, rather than shifting smoothly at light throttle. Similarly, if a lower CONSTANT hydraulic pressure was used, that would allow for smooth shifting at light throttle conditions, the transmission would tend to shift "softer" at higher throttle settings, and at high throttle conditions, approaching Wide Open Throttle, may even tend to "slip" severly.

Automatic transmissions monitor the throttle position (or engine load) with a TV valve, and/or a modulator, as discussed in the previous artical. In addition to using this information for shift timing and placement, the valve body has ways of using this "information" to adjust the hydraulic pressure, to compensate for varying engine loads. As mentioned before, the pressure regulator valve has a spring "pushing" on one end of the valve, and hydraulic pressure "pushing" on the other end of the valve (Kind of like "tug-of-war", only insteading of pulling, they "push" against each other.). When the hydraulic pressure is high enough to overcome the spring pressure, the valve moves, and "dumps" the excess pressure.
The Throttle Pressure Valve, or modulator valve, helps the pressure regulator valve, by sending fluid pressure, that varies, depending on throttle position, or engine load. This pressure coming from the Throttle Pressure Valve, or modulator valve, is directed to the pressure regulator valve, and helps the spring "push" on the valve. This, effectively, makes the spring seem "stiffer"; hydraulic pressure must be higher, to overcome the increased pressure, caused by the spring, AND the "throttle pressure" helping the spring, before the valve can "move over" and dump the "excess pressure". (Does this make any sense at all?)
To further complicate matters, the engineers felt that they needed higher pressure in reverse, so a reverse boost valve is also common. In the reverse position, the manual valve (connected to the shift lever inside the car) directs pressure to one side of the reverse boost valve, moving the valve in a direction that physically pushes on the pressure regulator valve, in the same direction that the spring, and TV pressure are pushing the valve. This further boosts main line pressure in reverse. There has been a "raging debate", between me and a certain Physics professor in Germany (I'm in over my head), as to why reverse pressure is boosted so much, but there is no question that reverse boost does exist.
In this way, hydraulic pressure increases with engine load. By the way, this hydraulic pressure is commonly referred to as "Main Line Pressure", and it varies, from transmission to transmission, but tends to be around 50 or 60 PSI at an idle, in neutral, and may go as high as 250 PSI under certain highload conditions. This accounts for some of the shift-quality-improving characteristics of a modern automatic transmission.

There are other ways, commonly used in conjunction with main line pressure "altering", to improve shift quality. One way, is to provide a restriction, in line, to "slow down" the application of the friction components. The fluid must pass through a small hole,or orifice, as it is frequently called, before applying the friction component, similar to "pinching" a garden hose, to slow water flow, thereby filling a bucket with water more slowly.
To further cushion the shift, engineers often incorporate Accumulators. An accumulator can be, in a simple form, a piston with a spring on one side. When a shift is commanded, hydraulic pressure is routed through an orifice and into a chamber, acting on one side of the piston (the side of the piston, opposite the spring). This hydraulic pressure "pushes" on the piston, compressing the spring (on the other side of the piston), and "absorbing" some of the hydraulic pressure that is also acting on the friction component needed to create the desired gear change. This acts to "slow" down the application of the friction component, and smooth the application, and make the gear change less harsh.

To further cushion the application of some clutch packs, and some bands, a spring is used to mechanically cushion the shift. These springs act like a sponge, absorbing some of the "shock" as the clutch (or band) applies.