Chevrolet Silverado / GMC Sierra. Service manual — part 1000

possible shift quality. Hydraulic conditions are detected by comparing the speed sensor
information for the current range, to the range of the speed sensor information stored in TCM
memory. The speed sensors are variable reluctance devices that convert mechanical motion to an
AC voltage. Each sensor consists of a wire coil wrapped around a pole piece that is adjacent to a
permanent magnet. These elements are contained in a housing that is mounted adjacent to a
rotating ferrous member.

Shift Selector

Fig. 519: Internal Mode Switch
Courtesy of GENERAL MOTORS CORP.

The vehicle is equipped with a column type shift selector. In addition to the column shifter,
another component associated with the shift selector is the internal mode switch (IMS), which is

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado

mounted internally on the selector shaft. The IMS transmits selector position information to the
TCM. The IMS detects the angular position of the shift selector shaft. This position is
communicated to the TCM so that certain vehicle control functions can be coordinated with the
position of the shift controls. The IMS has redundant circuitry to alert the TCM in the event of a
single wire or switch failure. The neutral signal output of the IMS is typically used as
confirmation that the transmission is in NEUTRAL before the engine starter is engaged. The IMS
is interfaced to the starter circuit.

The operator chooses the transmission range by moving the selector lever to the appropriate gate
position. When properly adjusted, the shifter gates prevent inadvertent shifting between ranges,
and corresponds to the internal transmission detent positions. A positive detent is provided in the
transmission in order to maintain the selector shaft in the selected position.

The TCM shift calibration determines the available forward ranges for each selector position.
Although specific installations vary, typical selector positions for the 1000 Series include the
following:

P - PARK

The parking pawl is engaged. The transmission is in NEUTRAL.

R - REVERSE

REVERSE is selected in order to move the vehicle backward.

N - NEUTRAL

NEUTRAL may be used when starting the engine and for stationary operations. The TCM
disables the starter switch if a range other than NEUTRAL or PARK is selected before
starting the vehicle.

D - DRIVE

DRIVE is the highest forward range used for normal driving. The transmission shifts to first
range for starting, then automatically upshifts through the ranges, as operating conditions
permit, until the highest range is attained.

M - Manual

Manual selection of ranges is provided by moving the selector to the M - MANUAL
position. A driver shift request switch has been added to the column shift lever. The driver

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado

information center (DIC) then shows the 6 available ranges with the current range
bracketed. The bracketed range is the highest attainable range with all gears below
accessible, for example, when 4th range is selected, ranges 1st through 4th are available. By
using the +/- buttons on the gear select lever, the driver can select the range of gears desired
for their current driving conditions.

1 - Manual LOW

Manual LOW provides first gear operation only.

Internal Components

Several components of the 1000 Series electrical control system are located within the
transmission as part of the main control valve body. These components include 3 types of
solenoids for controlling the hydraulic action of the valves. They include the PSM and an internal
wiring harness that links the internal components with the TCM.

Solenoids

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado

Fig. 520: Solenoid Valves
Courtesy of GENERAL MOTORS CORP.

The 1000 Series control valve body contains both normally closed (N/C) and normally open
(N/O) solenoids. A N/C solenoid remains closed until a signal from the TCM energizes the
solenoid. A N/O solenoid remains open until the TCM energizes the solenoid. When a solenoid
valve is in the closed position, the valve blocks the flow. When a solenoid valve is in the open
position, flow is permitted through the valve. Shift solenoid 1 (SS1) (4), shift solenoid 2 (SS2)
(2) and shift solenoid 3 (SS3) (3) are N/C. Both solenoid types have an orifice, electrical
windings, an iron core, and a steel check ball.

Shift solenoids provide the necessary logic to distribute fluid to the correct clutches. The shift
solenoids provide either full control main pressure or exhaust to the head of each of the
corresponding shift valves. Since the valve states, stroked or unstroked, are critical to providing
the correct transmission range, each shift valve has a pressure switch, located in the PSM, which
provides feedback to the computer regarding the position of the valve.

Modulated main pressure solenoid (7) is a N/C solenoid used to modulate the transmission main
pressure. Under specific conditions, such as low throttle setting, low engine torque, low engine
speed, and low transmission output speeds, the TCM commands the solenoid ON. When the
solenoid is applied, fluid is routed to the main pressure regulator valve; this in turn reduces the
main pressure schedule and improves the volume of oil through the overage circuit. By
modulating main pressure, the cooler flow at idle can be increased, allowing improved cooling
and reducing transmission pump noise.

Pressure control solenoid 1 (PCS1) (6), pressure control solenoid 2 (PCS2) (4), and the torque
converter clutch (TCC) (1) are used to control on-coming, off-going, and holding pressure to the
5 clutches and the TCC. These solenoids are referred to as variable bleed solenoids, since the
output hydraulic pressure supplied by these solenoids is proportional to the controlled current
command.

Pressure Switch Manifold

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado