When the gear selector lever is moved to the OVERDRIVE (D) range, the manual valve (404) moves and allows line pressure to fill the D4 passage. D4 pressure seats the #6 checkball (372) and the pressure sends fluid through the forward servo feed orifice into the servo applied passage. Servo applied fluid strokes the forward servo assembly (15-22), which applies the forward band (688). D4 fluid also feeds the servo applied passage through the thermo element, which is located on the spacer plate (370). D4 fluid from the manual valve directs fluid to the following valves:

The gear selector lever moves the manual valve, which allows line pressure to enter the D4 passage.

The #6 checkball is located in the valve body (300). The checkball blocks the forward servo applied passage, which forces D4 pressure to the following parts:

When the manual valve (404) is moved from DRIVE to PARK or NEUTRAL, the checkball unseats. This quickly exhausts the servo applied fluid and releases the forward band assembly (688).

The forward servo boost valve opens under high line pressure (high throttle position) This allows D4 fluid to bypass the feed orifice and the thermo element. The D4 fluid then enters the servo applied passage, which fills quickly and applies the forward band assembly. This prevents slippage during abusive shifts from PARK or NEUTRAL or DRIVE.

The forward servo assembly applies and holds the forward band (688) during all forward gear drive ranges.

The forward band assembly wraps around the 1-2 support outer race (687) and holds the 1-2 support outer race during all forward gear drive ranges.

The 1-2 accumulator valve is fed by D4 pressure. The valve regulates fluid pressure from the primary trim to the secondary 1-2 accumulator valve (347) in proportion to changes in modulator pressure.

The secondary 1-2 accumulator valve is fed by fluid pressure from the primary trim. The valve regulates the 1-2 accumulator pressure in proportion to modulator pressure. When primary trim reaches its breakpoint pressure, the secondary 1-2 accumulator valve regulates the pressure.

The 2-3 accumulator valve is fed by D4 pressure. The valve regulates 2-3 accumulator pressure in proportion to modulator pressure and the OFF pressure from the 2-3 shift solenoid valve.

The 3-4 accumulator valve is fed by D4 pressure. The valve regulates 3-4 accumulator pressure in proportion to modulator pressure.

The 2-3 shift valve allows D4 pressure to enter the auxiliary input clutch feed passage.

Spring pressure from the 1-2, 3-4 shift solenoid valve holds the 1-2 shift valve. D4 fluid stops at this valve until a 1-2 shift is made.