Chevrolet Silverado / GMC Sierra. Service manual — part 1177

The PR fluid pressure moves the valve against the spring force and fills the Lo/reverse fluid
circuit. Lo/reverse fluid is orificed (323) back to the lo overrun valve in order to assist the PR
fluid in moving the valve against the spring force. The spring force provides a time delay for the
PR fluid filling the Lo/reverse fluid circuit. The Lo/reverse fluid is routed to the inner area of the
lo and reverse clutch piston in order to increase the holding capacity of the clutch.

Transmission Fluid Pressure (TFP) Manual Valve Position Switch Assembly

The TFP manual valve position switch consists of five fluid pressure switches: D2 and D3 are
normally closed and D4, Lo and Rev are normally open. All fluid circuits routed to the assembly
are empty and the TFP manual valve position switch signals the PCM that the transmission is in
either Park or Neutral.

Shift Solenoid Valves (1-2 and 2-3)

Both shift solenoid valves, which are normally open, are energized by the PCM and block fluid
from exhausting. This maintains the signal A fluid pressure at the 1-2 shift solenoid valve and
signal B fluid pressure at the 2-3 shift solenoid valve.

Shift Valves (1-2, 2-3 and 3-4)

Signal A fluid pressure holds the 1-2 shift valve in the downshift position and the 3-4 valve in the
upshift (first and fourth gear) position. The signal B fluid pressure from the 2-3 shift solenoid
valve holds the 2-3 shift valve train in the downshift position.

2008 Chevrolet Silverado 1500

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

Silverado, Sierra & Silverado

Fig. 547: Park - Engine Running
Courtesy of GENERAL MOTORS CORP.

REVERSE

When the gear selector lever is moved to the Reverse (R) position (from the Park position), the
following changes occur to the transmissions hydraulic and electrical systems:

Manual Valve

The manual valve moves to the Reverse position and line pressure enters the reverse fluid circuit.
As in Park, line pressure also fills the PR (Park/Reverse) fluid circuit. All other fluid circuits are
blocked by the manual valve.

Lo and Reverse Clutch

As in Park, PR fluid pressure acts on the outer area of the lo and reverse clutch piston to apply
the lo and reverse clutch. Also, Lo/reverse fluid from the lo overrun valve acts on the inner area
of the piston to increase the holding capacity of the clutch (see Note below).

Reverse Input Checkball (#3)

Reverse fluid pressure seats the #3 checkball, flows through orifice #17 and fills the reverse input

2008 Chevrolet Silverado 1500

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

Silverado, Sierra & Silverado

fluid circuit. This orifice helps control the reverse input clutch apply rate when engine speed is at
idle.

Reverse Abuse Valve

Reverse fluid pressure acts on the end of the valve opposite of spring force. At engine speeds
above idle, reverse fluid pressure, which is fed by line pressure, increases and moves the valve
against spring force (as shown). Reverse fluid can then fill the reverse input fluid circuit through
the reverse abuse valve. This bypasses the control of orifice #17 and provides a faster clutch
apply.

Boost Valve

Reverse input fluid pressure moves the boost valve against the pressure regulator valve spring.
The spring acts on the pressure regulator valve to increase the operating range of line pressure in
Reverse. Reverse input fluid also flows through the valve and to the reverse input clutch piston.
Remember that torque signal fluid pressure continually acts on the boost valve to control line
pressure in response to vehicle operating conditions.

Reverse Input Clutch Piston

Reverse input fluid pressure moves the piston to apply the reverse input clutch plates and obtain
Reverse.

Reverse Input Air Bleed Checkball

This ball and capsule is located in the reverse input fluid circuit in the oil pump to provide an air
escape when the fluid pressure increases. It also allows air into the circuit to displace the fluid
when the clutch releases.

Transmission Fluid Pressure (TFP) Manual Valve Position Switch Assembly

Reverse input fluid pressure closes the normally open reverse switch in the TFP manual valve
position switch This signals the PCM that the manual valve is in the Reverse (R) position.

Shift Solenoid Valves (1-2 and 2-3)

Both shift solenoid valves are energized as in the Park range. Signal A and signal B fluids are
blocked from exhausting through the shift solenoid valves to maintain fluid pressure in these
circuits at the end of the shift valves.

Shift Valves (1-2, 2-3 and 3-4)

2008 Chevrolet Silverado 1500

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

Silverado, Sierra & Silverado

Signal A fluid pressure holds the 1-2 shift valve in the downshifted position and the 3-4 shift
valve in the upshifted (First and Fourth gear) position. Signal B fluid pressure from the 2-3 shift
solenoid valve holds the 2-3 shift valve train in the downshifted position.

Pressure Control (PC) Solenoid Valve

The PC solenoid valve continues to regulate AFL fluid into torque signal fluid pressure. The
PCM varies the current at the solenoid to regulate torque signal fluid pressure in response to
throttle position and other PCM input signals. Torque signal fluid pressure is used to control line
pressure at the boost and pressure regulator valves.

Note: The explanation in each gear range is, for the most part, limited.

Fig. 548: Reverse
Courtesy of GENERAL MOTORS CORP.

NEUTRAL - ENGINE RUNNING

When the gear selector lever is moved to the Neutral position (N) from the Reverse position, the
following changes occur to the transmission hydraulic and electrical systems.

Manual Valve

2008 Chevrolet Silverado 1500

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

Silverado, Sierra & Silverado