Chevrolet Silverado / GMC Sierra. Manual — part 999

DRIVE or REVERSE, the transmission continues to the commanded range until output
speed is low, at which point NEUTRAL is commanded. If the driver attempts to shift the
transmission from NEUTRAL to DRIVE or NEUTRAL to REVERSE, with the transfer
case in NEUTRAL, the PRNDL display blinks immediately.

High Output Speed Direction Change Shifts

REVERSE to DRIVE, DRIVE to REVERSE, and NEUTRAL to REVERSE shifts,
initiated above 300 RPM output speed are inhibited to NEUTRAL.

Low Automatic Transmission Fluid (ATF) Level Low

Low ATF level is caused by insufficient transmission fluid and lack of initial pump prime,
caused by refilling the transmission.

Extremely Low ATF temperature

A shift out of NEUTRAL when the ATF temperature is below -45°C (-49°F) may be
inhibited.

MALFUNCTIONS Which May Cause a Blinking PRNDL

Failure to detect turbine speed pull-down during a shift may occur under the following
conditions:

A clutch is failed and the transmission cannot attain the requested range.

Invalid gear ratio

Pressure control solenoid failure

Turbine or output speed sensor failure

Lack of pressure at startup

Conditions Which May Cause a BLANK PRNDL

The PRNDL display may be blank because of failure of the internal mode switch circuits A, B, C,
or P.

The transmission will command the most appropriate range, based on reverse pressure switch and
the remaining internal mode switch inputs.

ELECTRONIC COMPONENT DESCRIPTION

2008 Chevrolet Silverado 1500

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

Silverado

Transmission Control Module

Fig. 517: Transmission Control Module
Courtesy of GENERAL MOTORS CORP.

A microcomputer controls the transmission by receiving and processing signals from various
switches and sensors. The microcomputer determines shift sequences, shift timing, and clutch
apply and release characteristics. The microcomputer is an independent controller and is referred
to as a transmission control module (TCM). The pressure switch manifold (PSM) and the internal
mode switch (IMS) provide operator input to the TCM. Other data sent to the TCM include
throttle position, engine, turbine, and output speeds, and sump temperature. Any active special
function, such as anti-lock brakes or power take-off, is also an input to the TCM. The TCM
processes this data to determine proper shift points, to monitor the current range, to perform ratio
tests, and to compile diagnostic data. The TCM is programmed to protect the transmission and
other vehicle driveline components by inhibiting actions, such as full-throttle NEUTRAL-to-
range shifts and high speed direction changes. The TCM determines if a system malfunction exists
and stores diagnostic codes related to the malfunction. The codes, accessed by the service

2008 Chevrolet Silverado 1500

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

Silverado

technician, are used in diagnosing persistent or intermittent trouble in the system.

Throttle Position/Torque Management

The TCM receives input on throttle position/torque management from a signal transmitted by the
engine control module (ECM).

The ECM communicates directly to the transmission electronic controls over an SAE J1850 or
J1939 serial communication interface (SCI) data link. The TCM must be calibrated to receive
these signals.

Speed Sensors

2008 Chevrolet Silverado 1500

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

Silverado

Fig. 518: Speed Sensors
Courtesy of GENERAL MOTORS CORP.

There are 3 speed sensors typically required for use with 1000 Series transmissions. They include
the engine speed sensor, the turbine speed sensor, and the output speed sensor. The speed sensors
provide revolutions per minute (RPM) information to the TCM. The speed ratios between the
various sensors allow the TCM to determine the transmission operating range. Speed sensor
information is also used to control the timing of clutch apply pressures, resulting in the best

IMPORTANT: Do not rotate the sensor in the retaining bracket. Changing the

sensor/bracket orientation may cause improper operation.

2008 Chevrolet Silverado 1500

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

Silverado