Overdrive Range: 3-2 Downshift

Overdrive Range - 3-2 Downshift:

Description:

OVERDRIVE RANGE: 3-2 DOWNSHIFT

Similar to a forced 4-3 downshift, a forced 3-2 downshift can occur due to minimum throttle (coastdown conditions), heavy throttle or increased engine load. To achieve a forced 3-2 downshift, the PCM energizes the 2-3 shift solenoid and the following changes occur:

2-3 Shift Solenoid: Energized by the PCM, the normally open solenoid closes and blocks actuator feed limit (AFL) fluid from exhausting through the solenoid. This creates pressure in the AFL fluid circuit at the solenoid end of the 2-3 shift valve.

2-3 Shift Valve Train: AFL fluid pressure from the shift solenoid moves both valves to the downshifted position against AFL fluid pressure acting on the 2-3 shuttle valve. This causes the following changes:
- AFL fluid is blocked from the D432 fluid circuit and D432 fluid exhausts past the 2-3 shift valve.
- 2nd fluid is blocked from feeding the 3-4 signal fluid circuit and 2nd fluid is routed into the servo feed fluid circuit.
- 3-4 signal fluid is exhausted past the valve. 3-4 clutch fluid and 3rd accumulator fluid, which were fed by 3-4 signal fluid, also exhaust.

3-4 CLUTCH RELEASES and 2-4 BAND APPLIES

3-4 Clutch Piston: 3-4 clutch fluid exhausts from the piston and the 3-4 clutch plates are released.

3-4 Clutch Exhaust Checkball (#4) Exhausting 3-4 clutch fluid seats the #4 checkball and is forced through orifice #13. This orifice controls 3-4 clutch fluid exhaust and the 3-4 clutch release rate.

2-4 Servo Assembly: 3rd accumulator fluid exhausts from the servo assembly. 2nd clutch fluid pressure moves the 2nd apply piston against servo return spring force to move the apply pin and apply the 2-4 band.

3-2 Downshift Valve and 1-2 Upshift Checkball (#8): 3-4 clutch fluid exhausts from the valve and spring force moves the valve into the Second gear position. However, before spring force overcomes exhausting 3-4 clutch fluid pressure, 2nd fluid feeds the 2nd clutch fluid circuit through the valve. This bypasses the control of orifice #16 at the #8 checkball and provides a faster 2-4 band apply. Remember that the #8 checkball and orifice #16 are used to help control the 2-4 band apply during a 1-2 upshift.

DOWNSHIFT TIMING and CONTROL

At higher vehicle speeds the 2-4 band apply must be delayed to allow engine speed RPM to increase sufficiently for a smooth transfer of engine load to the 2-4 band. Therefore, exhaust of 3rd accumulator fluid must be delayed. However, at lower speeds the band must be applied quickly. To provide for the varying requirements for the 2-4 band apply rate, exhausting 3rd accumulator fluid is routed to both the 3rd accumulator checkball (#2) and the 3-2 control valve.

3rd Accumulator Checkball (#2): Exhausting 3rd accumulator fluid seats the #2 ball and is forced through orifice #12. This fluid exhausts through the 3-4 clutch and 3-4 signal fluid circuits and past the 2-3 shuttle valve. Orifice #12 slows the exhaust of 3rd accumulator fluid and delays the 2-4 band apply rate.

Note: The #12 orifice is not used on some models. For these models, all of the exhausting 3rd accumulator fluid is routed to the 3-2 control valve when the #2 checkball seats.

3-2 Control Solenoid and 3-2 Control Valve: These components are used to increase the exhaust rate of 3rd accumulator fluid; as needed depending on vehicle speed. The normally closed 3-2 control solenoid is Pulse Width Modulated (PWM) and controlled by the PCM through a duty cycle operation. The PCM decreases the solenoid's duty cycle during a 3-2 downshift from approximately 90% to as low as 10%. Once the shift is complete the solenoid returns to a 90% duty cycle. The duty cycle value during the shift controls the 2-4 band apply rate.
- At lower vehicle speed the PCM operates the 3-2 control solenoid at a lower duty cycle during the shift.
- At a lower duty cycle the solenoid regulates 3-2 signal fluid to a lower pressure.
- With lower 3-2 signal fluid pressure, 3-2 control valve spring force opens the valve farther to regulate a faster exhaust of 3rd accumulator fluid into the 3-4 clutch fluid circuit (as shown).
- A faster exhaust of 3rd accumulator exhaust provides a faster apply of the 2-4 band, as needed at lower speeds.

3rd Accumulator Exhaust Checkball (#7): After the downshift is completed, the #7 checkball unseats and allows residual fluid in the 3rd accumulator fluid circuit to exhaust.

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