Overdrive Range - 4-3 Downshift

OVERDRIVE RANGE - 4-3 DOWNSHIFT





When the transaxle is operating in overdrive range -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 4-3 downshift if engine load is increased with throttle position remaining the same (for example, driving up a steep hill).


Line Pressure Increases
Pressure Control Solenoid (PC Solenoid): During the downshift, except for a coastdown, the PCM senses the increase in throttle position or engine load and increases the PCS duty cycle. The increase in duty cycle increases output fluid pressure from the PCS, thereby increasing torque signal fluid pressure at the torque signal regulator valve.
Pressure Regulator Valve: Increased torque signal fluid pressure acting on the boost valve increases line pressure at the pressure regulator valve.


Intermediate/4th Band Releases
1-2 Shift Solenoid: The PCM de-energizes the normally open solenoid and 1-2 signal fluid exhausts.
3 - Shift Valve: 1-2 signal fluid pressure exhausts from the 3-4 shift valve and spring force moves the valve into the third gear position. This opens the 4th band fluid circuit to an orificed exhaust to help control the band release.
Intermediate/4th Servo: 4th band fluid exhausts from the servo and spring force moves the servo to the release position, thereby releasing the band.
3 - Accumulator: 4th band fluid exhausts from the accumulator. Spring force and 3-4 accumulator feed fluid pressure move the accumulator piston to the third gear position.
1-2/3-4 Accumulator Valve: The accumulator valve regulates line pressure into the 1-2/3-4 accumulator fluid circuit to fill the 3-4 accumulator. This regulation is basically controlled by torque signal fluid pressure. Increased torque signal fluid pressure regulates accumulator fluid to a higher pressure.
#7 Checkball (3-4 Accumulator): 1-2/3-4 accumulator fluid pressure seats the #7 checkball and forces accumulator fluid through orifice #28.


Torque Converter Clutch
The PCM commands TCC release prior to initiating a 4-3 downshift. When the TCC is in the release position, release fluid pressure is routed to the pressure switch assembly. This fluid pressure signals the PCM that the TCC is in the release position. The TCC is not applied under normal operating conditions in third gear (except for some applications).


3-2 and 2-1 Downshifts
Refer to the manual range explanations for a description of the 3-2 and 2-1 downshifts with respect to the clutches releasing.