Clark shift control
How the unit operate
The transmission is controlled by two separate valves connected by hydraulic hoses only.
One valve is the transmission control valve mounted one the transmission, this valve directs oil to the speed and directional clutches and also the torque converter.
The second valve is the shift control valve and is mounted in the cab.
The shift lever is part of the shift control valve. The function of the shift control valve is to hydraulically position a combination of valve spools in the transmission control cover. This permits the valve spools to direct oil pressure from the converter pump to the appropriate clutch in the transmission, engaging the transmission in the range selected by the operator.
Clutch oil pressure tapped from the clutch pressure port in the transmission control cover is supplied to the shift control valve though a supply hose. This oil is directed by control plungers in the shift control valve trough individual hoses connected to the valve spools in the transmission control cover, so that the appropriate valve spool is hydraulically positioned according to the speed range selected by the operator.
A check valve and a small accumulator are incorporate in the shift control housing. The accumulator stores a supply of oil under pressure to assure that shift control valve system has a constant oil supply. The check valve prevents the oil supply stored in the accumulator from flowing backwards toward the clutch pressure port.
NOTE: It must be emphasized at this point that when the shift lever in the 8 speed control is in reverse position, it will only engage four  of the eight  speed ranges in the transmission to control reveres travel speed of machine.
Simplification of the speed ranges engaged according to lever position and quadrant marking for both forward travel speeds and reverse travel speeds can best be explained by referring to the following chart.
Observe torque converter temperature gauge when working machine. If gauge approaches 250 degrees F, shift to a lower operating speed range.
Shifting from one speed range to other can be made at any time during the working cycle. Momentarily let up on accelerator pedal when shifting into a higher speed range, and accelerate slightly when shifting into a lower speed range.
It is not good driver practice to skip speed ranges when shifting, if machine is in motion. If machine is in motion. It is better to make progressive shifts, engaging each speed range before proceeding to the next.