Governor Valve
The governor valve (fig. 4-39) senses engine speed (transmission output shaft speed) to help control gear shifting. The vacuum modulator and governor work together to determine shift points. The governor gear is meshed with a gear on the transmission output shaft. Whenever the vehicle and output shaft are moving, the centrifugal weights rotate. When the output shaft and weights are spinning slowly, the weights are held in by the governor springs, causing low-pressure output and the transmission remains in low gear. As the engine speeds increases, the weights are thrown out further and governor pressure increases, moving the shift valve and causing the transmission to shift into higher gear.
Figure 4-39.- Operation of the governor.
Figure 4-40.- Shift valve.
Shift Valves
The shift valves (fig. 4-40) are simple balance-type spool valves that select between low and high gear when the manual valve is in drive. Using control pressure (oil pressure from the regulator, governor, vacuum modulator, and manual valves), they operate the bands, servos, and gearsets. Oil pressure from the other transmission valves acts on each end of the shift valve. In this way, the shift valve is sensitive to engine load (vacuum modulator valve oil pressure), engine speed (governor valve oil pressure), and gearshift position (manual valve oil pressure). These valves move according to the forces and keep the trans-mission shifted into the correct gear ratio for the driving conditions.
Kickdown Valve
The kickdown valve causes the transmission to shift into a lower gear during fast acceleration. A rod or cable links the carburetor or fuel injection throttle body to a lever on the transmission. When the operator depresses the throttle, the lever moves the kickdown valve. This action causes hydraulic pressure to over-ride normal shift control pressure and the transmission downshifts.
