| Automotive Column: Control Systems |
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| Written by Christopher Stryker | |
 Top left: One variation of an output shaft mounted governor. Within the housing are two weights which move outward as they spin faster, thus causing allowing more fluid to flow past the internal valve. This week we will handle the control systems. One thing worth noting; the scope of this article is still dealing primarily with older, all hydraulic units that are found in most vehicles manufactured before the early 90’s. Modern transmissions do away with many of the hydraulic controls and have gone to electrical control systems which vary significantly. Directly behind the torque converter, you will find an oil pump. This pump circulates oil from the pan and pressurizes it so that it will feed through the torque converter, as well operating all the control systems and lubricating the internal parts. Regarding control systems, the “brain” of any automatic transmission is the valve body. The valve body is usually a cast chunk of aluminum w/ a tremendous amount of passages that resemble a maze when viewed from above. Using a series of valves and checkballs, this assembly takes your gear selector movement and actuates a series of clutches and bands which determine which gear sets will be applied. Other than the gear selector, there are several other elements which “input” into the valve body to help select which gear the transmission should be in and how much pressure is needed to the clutches. First and foremost is the governor. This device is usually mounted to the tailshaft (most GM and Chrysler designs) or in the tail housing (most Chevy designs). It is essentially two weights which spin faster and faster as the output speed of the transmission increases. As these weights move, they actuate a valve which allows either more or less fluid to flow, depending on output shaft speed. It is this valve that essentially controls shift points. Movement of the governor weights is usually controlled by a spring, and by altering the stiffness of this spring and the mass of the weights, a variety of shift points can be obtained. Fortunately, most transmissions come outfitted very close to optimally from the factory, and little variation in this area is required. The other inputs to the valve body are the throttle position and kickdown (aka passing gear) controls. The throttle valve is designed to raise the operating pressure of the transmission as the engine applies more load. This ensures that when you floor it, it will be your tires slipping on the pavement as opposed to the clutches in your transmission slipping against each other. As line pressure increases, it also moves the shift points further up in the rpm band, which allows for more optimal acceleration. The kickdown function, as the name implies, “kicks it down” a gear when you step on the throttle. (Assuming that dropping down a gear would not over-rev the engine. Fortunately the valve body compensates for this). Transmissions are generally calibrated to operate in as high a gear as feasibly possible in order to lower engine rpms, thus saving gas and prolonging engine life. This is not always optimal for acceleration, however, and shifting down a gear allows you to accelerate as quickly as possible when passing or otherwise needing to accelerate quickly. The physical operation of these two circuits is accomplished differently on different transmissions. On some, such as the Chrysler Torqflight, they are both operated by the same mechanical linkage which is attached to the throttle lever on the carburetor or throttle body. On others, such as the Ford C4 and C6 and many GM designs, a mechanical linkage from the carb controls the kickdown, while the throttle valve is actually controlled by a vacuum pod. Since vacuum strength is proportional to engine RPM, this works rather well. Note: On transmissions which use a mechanical linkage to control the throttle valve, driving with them disconnected will cause the transmission to operate at lower pressure and cause a great deal of slippage, quickly toasting the transmission. On vacuum operated models, they will generally operate at higher pressure if the vacuum line is disconnected, which reduces internal damage. Fortunately, all of these systems are very visible after the transmission is disassembled, and relatively easy to visually determine how they work (compared to the sealed unit of the torque converter). Regarding modifications, chances are if you’re putting it back in a street car, the most radical you will want to get is installing a “shift kit.” Installing a shift kit usually involves drilling new holes in the valve body and adding or replacing some of the contained therein. Most of the name brand kits (such as B&M or Trans-go) have very detailed instructions which make the task a breeze. For full race vehicles, another option is an aftermarket valve body. These valve bodies usually feature calibrations optimized for drag racing as well as full manual operation. This means that you are actually the one determining when to shift gears, and essentially makes your transmission a hydraulic operated manual. The advantage to this is that you are in complete control of your shift points and you still get the cleaner shifting and off-the-line torque multiplication of an automatic unit. Generally, full manual valve bodies which do not have a low-band apply are not recommended for street use, as there is the very real possibility of throwing chunks through the side of your transmission case if they are not operated properly. That about sums it up for control system. Next on the list (and last, in terms of general operating principles), is the actual parts which transmit the power: the gears sets, and the clutches and bands which operate them. After that, it should finally be time to tackle a full transmission rebuild. |
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