Overdrive Range: 4-3 Downshift

Overdrive Range - 4-3 Downshift:

Description:

OVERDRIVE RANGE: 4-3 DOWNSHIFT

When the transmission is operating in Fourth gear, a forced 4-3 downshift will occur if there is a significant increase in throttle position. At minimum throttle, vehicle speed will decrease gradually (coastdown) and the PCM will command a 4-3 downshift. The PCM will also initiate a forced 4-3 downshift when throttle position remains constant but engine load is increased, such as driving up a steep incline. To achieve a 4-3 downshift, the PCM de-energizes shift solenoid "A" and the following changes occur to the transmissions electrical and hydraulic systems:

Shift Solenoid "A": De-energized by PCM, the normally open solenoid opens and signal "A" fluid exhausts through the solenoid.

1-2 Shift Valve: As in Fourth gear, D432 fluid pressure and spring force hold the valve in the upshifted position.

2-4 BAND RELEASES

3-4 Shift Valve: With signal "A" fluid pressure exhausted, spring force moves the valve into the downshifted position. In this position, the valve blocks 3-4 signal fluid and 4th signal fluid exhausts past the valve.

3-4 Relay Valve and 4-3 Sequence Valve: These valves control the timing of the 2-4 band release. With 4th signal fluid pressure exhausted, 3-4 accumulator fluid pressure moves the 3-4 relay valve into the Third gear position. This opens 3-4 accumulator fluid to an orificed exhaust (#5) past the 3-4 relay valve (shown by red arrows). Because the exhaust is orificed, 3-4 accumulator fluid pressure momentarily holds the 4-3 sequence valve against spring force before completely exhausting.

When exhausting 3-4 accumulator fluid pressure decreases sufficiently, spring force will move the 4-3 sequence valve into the Third gear position as shown. This opens both the 3-4 accumulator and 4th fluid circuits to a quick exhaust past the 4-3 sequence valve. In this position the valve blocks 2nd fluid from entering the servo feed fluid circuit.

2-4 Servo Assembly: 4th fluid exhausts from the 4th apply piston in the servo assembly. The apply pin spring moves the 4th apply piston and apply pin to release the band from the reverse input drum and shift the transmission into Third gear.

3-4 Accumulator Assembly: 3-4 accumulator fluid exhausts from the 3-4 accumulator piston. Orificed accumulator fluid pressure and spring force move the piston to a Third gear position.

3-4 Accumulator Checkball (#1): As accumulator fluid fills the 3-4 accumulator it seats the #1 checkball and is forced through orifice #18. This orifice controls the rate at which accumulator fluid pressure fills the 3-4 accumulator and 3-4 accumulator fluid exhausts from the accumulator assembly.

Accumulator Valve: Biased by torque signal fluid pressure and spring force, the accumulator valve regulates drive fluid into the accumulator fluid circuit.

Shift Solenoid "B": This solenoid remains de-energized as in Fourth gear and AFL fluid exhausts through the solenoid.

2-3 Shift Valve Train: AFL fluid pressure at the 2-3 shuttle valve holds the valves in the upshifted position. This allows servo feed fluid to exhaust through the valve, into the 3-4 accumulator fluid circuit and past the 4-3 sequence valve.

Torque Converter Clutch: The PCM releases the converter clutch prior to initiating the 4-3 downshift. However, if vehicle speed is above approximately 121 km/h (75 mph), the PCM will command TCC apply in Third gear.

Pressure Control Solenoid: Remember that the force motor continually adjusts torque signal fluid pressure in relation to the various PCM input signals (mainly throttle position).