Chevrolet Silverado / GMC Sierra. Service manual — part 1182

Fig. 554: Overdrive Range, 4-3 Downshift
Courtesy of GENERAL MOTORS CORP.

OVERDRIVE RANGE, 3-2 DOWNSHIFT

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.

2008 Chevrolet Silverado 1500

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

Silverado, Sierra & Silverado

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.

2008 Chevrolet Silverado 1500

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

Silverado, Sierra & Silverado

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).

2008 Chevrolet Silverado 1500

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

Silverado, Sierra & Silverado

Fig. 555: Overdrive Range, 3-2 Downshift
Courtesy of GENERAL MOTORS CORP.

MANUAL THIRD GEAR

A manual 4-3 downshift is available to increase vehicle performance when the use of only three
gear ratios is desired. Manual Third gear range also provides engine braking in Third gear when
the throttle is released. A manual 4-3 downshift is accomplished by moving the selector lever into
the Manual Third (D) position. This moves the manual valve and immediately downshifts the
transmission into Third gear. Refer to Overdrive Range, 4-3 Downshift for a complete
description of a 4-3 downshift. In Manual Third, the transmission is prevented, both hydraulically
and electronically, from shifting into Fourth gear. The following information explains the
additional changes during a manual 4-3 downshift as compared to a forced 4-3 downshift.

Manual Valve

The selector lever moves the manual shaft and manual valve into the Manual Third position (D).
This allows line pressure to enter the D3 fluid circuit.

Transmission Fluid Pressure (TFP) Manual Valve Position Switch Assembly

D3 fluid is routed to the TFP manual valve position switch and opens the normally closed D3

2008 Chevrolet Silverado 1500

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

Silverado, Sierra & Silverado