1-2 SHIFT VALVE
The 1-2 shift valve assembly , or mechanism, consists of: the 1-2 shift valve, governor plug, and a spring on the
end of the valve. After the manual valve has been placed into a forward gear range, line pressure is directed to the
1-2 shift valve. As the throttle is depressed, throttle pressure is applied to the right side of the 1-2 shift valve assem-
bly. With throttle pressure applied to the right side of the valve, there is now both spring pressure and throttle pres-
sure acting on the valve, holding it against the governor plug. As the vehicle begins to move and build speed,
governor pressure is created and is applied to the left of the valve at the governor plug.
When governor pressure builds to a point where it can overcome the combined force of the spring and throttle
pressure on the other side of the valve, the valve will begin to move over to the right. As the valve moves to the
right, the middle land of the valve will close off the circuit supplying the throttle pressure to the right side of the
valve. When the throttle pressure is closed off, the valve will move even farther to the right, allowing line pressure
to enter another circuit and energize the front servo, applying the front band.
The governor plug serves a dual purpose:
•
It allows the shift valves to move either left or right, allowing both upshifts and downshifts.
•
When in a manual selection position, it will be hydraulically “blocked” into position so no upshift can occur.
The physical blocking of the upshift while in the manual “1” position is accomplished by the directing of line pressure
between both lands of the governor plug. The line pressure reacts against the larger land of the plug, pushing the
1-2 Shift Valve - Before Shift
1-2 Shift Valve - After Shift
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AUTOMATIC TRANSMISSION - 48RE - SERVICE INFORMATION
DR/DH