Chevrolet Silverado / GMC Sierra. Manual — part 1181

The accumulator fluid forced from the accumulator unseats the #1 checkball and enters the
accumulator fluid circuit. This fluid is routed to the accumulator valve. This is shown by the
arrow directions in the fluid circuit.

Accumulator Valve

Accumulator fluid forced from the 3-4 accumulator is orificed to the end of the accumulator
valve. This fluid pressure, in addition to spring force and torque signal fluid pressure, regulates
the exhaust of excess accumulator fluid pressure through the middle of the valve. This regulation
helps control the 2-4 band apply feel.

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

The solenoid remains open and routes AFL fluid into the 3-2 signal fluid circuit. 3-2 signal fluid
pressure holds the 3-2 control valve against spring force, thereby blocking the 3rd accumulator
and 3-4 clutch fluid circuits.

Torque Converter Clutch Applies

TCC Solenoid Valve

When operating conditions are appropriate, the PCM energizes the normally open TCC solenoid
valve. This closes the solenoid, blocks the converter feed fluid from exhausting, and creates
pressure in the converter feed fluid circuit at the converter clutch apply valve and TCC solenoid
valve.

Converter Clutch Apply Valve

Converter feed fluid pressure moves the valve against spring force and into the apply position. In
this position, release fluid is open to an exhaust port, and regulated apply fluid fills the apply
fluid circuit. Converter feed fluid is routed through the converter clutch apply valve to feed the
cooler fluid circuit.

Torque Converter

Release fluid from behind the pressure plate exhausts through the end of the turbine shaft. Apply
fluid pressure is routed between the converter hub and stator shaft where it enters the torque
converter. This fluid applies the converter clutch against the converter cover and keeps the
converter filled with fluid.

TCC Apply Checkball (#9)

Release fluid, exhausting from the converter, seats the #9 checkball located in the end of the

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - 4L60-E/4L65-E/4L70-E - Cab & Chassis Sierra, Cab & Chassis

Silverado, Sierra & Silverado

turbine shaft, and is orificed around the ball. Orificing the exhausting release fluid controls the
converter clutch apply rate, along with the TCC PWM solenoid valve.

TCC PWM Solenoid Valve

The torque converter clutch pulse width modulation (TCC PWM) solenoid valve controls the
regulated apply valve position. This is done through the use of pulse width modulation (duty
cycle operation). The solenoid duty cycle is controlled by the PCM in relation to vehicle
operating conditions and regulates actuator feed limit (AFL) fluid into the CC signal circuit,
through the #9 orifice, and to the isolator valve. This controls line pressure flow through the
regulated apply valve, into the regulated apply circuit, and provides a smooth engagement of the
TCC.

Fig. 553: Overdrive Range, Fourth Gear - TCC Applied
Courtesy of GENERAL MOTORS CORP.

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

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - 4L60-E/4L65-E/4L70-E - Cab & Chassis Sierra, Cab & Chassis

Silverado, Sierra & Silverado

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

The 3-4 accumulator fluid exhausts from the 3-4 accumulator piston. The orificed accumulator

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - 4L60-E/4L65-E/4L70-E - Cab & Chassis Sierra, Cab & Chassis

Silverado, Sierra & Silverado

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).

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - 4L60-E/4L65-E/4L70-E - Cab & Chassis Sierra, Cab & Chassis

Silverado, Sierra & Silverado