OVERDRIVE RANGE, 4-3 DOWNSHIFT

When the transmission is operating in fourth gear, a forced 4-3 downshift occurs if there is a significant increase in throttle position. At minimum throttle, the vehicle speed decreases gradually (coastdown) and the PCM commands a 4-3 downshift. The PCM also initiates a forced

4- 3 downshift when the 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 the 1-2 shift solenoid valve and the following changes occur to the transmission's electrical and hydraulic systems:

1- 2 Shift Solenoid (SS) Valve

De-energized by the 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 upshift position.

2- 4 Band Releases

3- 4 Shift Valve

With the signal A fluid pressure exhausted, the spring force moves the valve into the downshift position. In this position, the valve blocks the 3-4 signal fluid and the 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 the 4th signal fluid pressure exhausted, the 3-4 accumulator fluid pressure moves the 3-4 relay valve into the third gear position. This opens the 3-4 accumulator fluid to an orificed exhaust (#5) past the 3-4 relay valve (shown by red arrows). Because the exhaust is orificed, the 3-4 accumulator fluid pressure momentarily holds the 4-3 sequence valve against spring force before completely exhausting.

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

2- 4 Servo Assembly

The 4th fluid exhausts from the 4th apply piston in the servo assembly. The apply pin spring moves the 4th apply piston and the apply pin in order to release the band from the reverse input drum and shift the transmission into third gear.

3- 4 Accumulator Assembly

fluid pressure and the spring force move the piston into a third gear position.

3- 4 Accumulator Checkball (#1)

As the 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 the 3-4 accumulator fluid exhausts from the accumulator assembly.

Accumulator Valve

Biased by torque signal fluid pressure and spring force, the accumulator valve regulates the D-4 fluid into the accumulator fluid circuit.

2- 3 Shift Solenoid (SS) Valve

This solenoid remains de-energized as in fourth gear and the signal B fluid exhausts through the solenoid.

2- 3 Shift Valve Train

The AFL fluid pressure at the 2-3 shift valve holds the valves in the upshift position. This allows the 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 Solenoid Valve TCC PWM Solenoid Valve

The PCM de-energizes the TCC solenoid valve and operates the duty cycle of the TCC PWM solenoid valve to release the converter clutch for a smooth disengagement, prior to initiating the

4- 3 downshift.

Pressure Control (PC) Solenoid Valve

Remember that the PC solenoid valve continually adjusts the torque signal fluid pressure in relation to the various PCM input signals (mainly the throttle position).

OVERDRIVE RANGE, 4-3 DOWNSHIFT

Courtesy of GENERAL MOTORS CORP.

OVERDRIVE RANGE, 3-2 DOWNSHIFT

2007 TRANSMISSION Automatic Transmission - 4L60-E/4L65-E/4L70-E - H3

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

Energized by the PCM, the normally open solenoid closes and blocks the signal B fluid from exhausting through the solenoid. This creates pressure in the signal B fluid circuit at the solenoid end of the 2-3 shift valve.

2- 3 Shift Valve Train

The signal B fluid pressure from the shift solenoid moves both valves to the downshift position against AFL fluid pressure acting on the 2-3 shift valve. This causes the following changes:

• The AFL fluid is blocked from the D432 fluid circuit and the D432 fluid exhausts past the 2-3 shuttle valve.

• The 2nd fluid is blocked from feeding the 3-4 signal fluid circuit and the 2nd fluid is routed into the servo feed fluid circuit.

• The 3-4 signal fluid is exhausted past the valve. The 3-4 clutch fluid and the 3rd accumulator fluid, which were fed by the 3-4 signal fluid, also exhaust.

3- 4 Clutch Releases and 2-4 Band Applies

3- 4 Clutch Piston

The 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 the 3-4 clutch fluid exhaust and the 3-4 clutch release rate.

2- 4 Servo Assembly

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

3- 2 Downshift Valve and 1-2 Upshift Checkball (#8)

The 3-4 clutch fluid exhausts from the valve and the spring force moves the valve into the second gear position. However, before the spring force overcomes the exhausting 3-4 clutch fluid pressure, the 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 the engine speed RPM to increase sufficiently for a smooth transfer of engine load to the 2-4 band. Therefore, exhaust of the 3rd accumulator fluid must be delayed. However, at lower speeds the band must be applied quickly. In order to provide for the varying requirements for the 2-4 band apply rate, the exhausting 3rd accumulator fluid is routed to both the 3rd accumulator checkball (#2) and the 3-2 control valve.

3rd Accumulator Checkball (#2)

The exhausting 3rd accumulator fluid seats the #2 checkball and is forced through orifice #12. This fluid exhausts through the 3-4 clutch and the 3-4 signal fluid circuits and past the 2-3 shift valve. Orifice #12 slows the exhaust of the 3rd accumulator fluid and delays the 2-4 band apply rate.

3- 2 Control Solenoid Valve and 3-2 Control Valve

These components are used to increase the exhaust rate of 3rd accumulator fluid, as needed, depending on the vehicle speed.

The 3-2 control solenoid valve is a normally closed On/Off solenoid controlled by the PCM. The PCM controls the solenoid state during a 3-2 downshift according to vehicle speed.

Low Speed

• At lower vehicle speeds, the PCM operates the 3-2 control solenoid valve in the Off position.

• In the Off position the solenoid blocks actuator feed limit fluid pressure from the 3-2 control valve.

• With no actuator feed limit fluid pressure, the 3-2 control valve spring force keeps the valve open to allow a faster exhaust of 3rd accumulator fluid through orifice #14 into the 3-4 clutch fluid circuit.

• A faster exhaust of the 3rd accumulator exhaust fluid provides a faster apply of the 2-4 band, as needed at lower vehicle speeds.

High Speed

• At high vehicle speed, the PCM operates the 3-2 control solenoid valve in the On position allowing actuator feed limit fluid to pass through the solenoid. This pushes the 3-2 control valve into the closed position.

• This action permits a slow apply of the 2-4 band by blocking off 3rd accumulator exhaust fluid from entering the 3-4 clutch fluid circuit through orifice #14.

• This allows the engine speed to easily come up to the necessary RPM before the 2-4 band is applied.

3rd Accumulator Exhaust Checkball (#7)

After the downshift is completed, the #7 checkball unseats and allows the residual fluid in the 3rd accumulator fluid circuit to exhaust.

Pressure Control (PC) Solenoid Valve

Remember that the PC solenoid valve continually adjusts torque signal fluid in relation to the various PCM input signals (mainly the throttle position).

OVERDRIVE RANGE, 4-3 DOWNSHIFT

Fig. 494: Overdrive Range, 3-2 Downshift Hydraulic Circuit Diagram Courtesy of GENERAL MOTORS CORP.

2007 TRANSMISSION Automatic Transmission - 4L60-E/4L65-E/4L70-E - H3

Post A Comment