These three components are important to the earlier pre-computerized transmissions. They provide the hydo-mechanical inputs that tell the transmission when to shift.
The Governor is driven by the output shaft and regulates hydraulic pressure based on vehicle speed. It accomplishes this using centrifugal force to spin a pair of hinged weights against return springs. As the weights pull further out against the springs, more oil pressure is allowed past the governor to act on the shift valves that are in the valve body which then signal the appropriate shift timing and down-shifts.
Simply put, the vehicles speed is not the only thing that controls when a transmission should shift, the load that the engine is under is also important. The more load you place on the engine, the longer the transmission will hold a gear before shifting to the next one.
There are two types of devices that serve the purpose of monitoring the engine load: the Throttle Velocity Cable(TV)and the Vacuum Modulator. A transmission will use one or the other but generally not both of these devices. Each works in a different way to monitor engine load.
The TV Cable simply monitors the position of the gas pedal through a cable that runs from the gas pedal to the throttle valve in the valve body.
The Vacuum Modulator watches the engine vacuum by a rubber vacuum hose and metal line which is connected to the engine. Engine vacuum reacts very quickly to engine load with high vacuum produced when the engine is under light load and decreasing to zero vacuum when the engine is under a heavy load. The modulator is attached to the outside of the transmission case and is pushed internally by a shaft which is located in the transmission case and works with the throttle valve in the valve body. When an engine is under a light load or no load, high vacuum acts on the modulator which moves the throttle valve in one direction to allow the transmission to shift early and soft. As the engine load increases, vacuum is lowered which moves the valve in the other direction causing the transmission to shift later and more firmly.
Each manufacturer uses their own variations and different combinations that work for each different transmission depending on it’s design. More useful real world information is in the GotTransmissions.com Blog. Specifically to help understand how a transmission works and perhaps learning to do few home repairs to save some money.
