Chevrolet Silverado / GMC Sierra. Service manual — part 1316

DESCRIPTION AND OPERATION

CRUISE CONTROL DESCRIPTION AND OPERATION

Cruise control is a speed control system that maintains a desired vehicle speed under normal
driving conditions at speeds above 40 km/h (25 mph). Steep grades may cause variations in the
selected vehicle speeds.

The following are the main components of the Cruise Control System:

The accelerator pedal

The stop lamp switch

The body control module (BCM)

The cruise control switches

The engine control module (ECM)

The throttle actuator control (TAC) motor

The vehicle speed sensor (VSS)

Cruise Control Engaged

The Cruise Control System will engage and adjust vehicle speeds, based on the activation of the
following cruise control switches, which are located on the steering wheel:

The on/off switch

The + RES switch

The - SET switch

The body control module (BCM) monitors the signal circuit of the cruise control switches. The
BCM relays the cruise control switch status to the engine control module (ECM) via the GMLAN
serial data circuit. The ECM uses the status of the cruise control switch to determine when to
capture and maintain the vehicle speed. The ECM monitors the vehicle speed signal circuit in
order to determine the desired vehicle speed. The ECM uses the throttle actuator control (TAC)
motor in order to maintain the vehicle speed. For further information on the TAC System, refer to
Throttle Actuator Control (TAC) System Description for the 4.3L engine or Throttle
Actuator Control (TAC) System Description for the 4.8L, 5.3L, 6.0L, 6.2L or 7.0L engines.

Voltage is supplied to the cruise control switch via the steering wheel control switch reference
voltage circuit supplied by the BCM. The cruise control function switches are arranged in a
resistive ladder design, with each cruise control function switch having a different resistance

2008 Chevrolet Silverado 1500

2008 ENGINE Cruise Control - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

value. The BCM detects a specific voltage value that is associated with the cruise control function
switch being activated. When the normally open cruise control on/off switch is turned ON, the
switch closes and the BCM supplies a ground to the cruise control switch ON indicator circuit as
it becomes illuminated. The BCM sends a GMLAN serial data message to the ECM indicating
that the on/off switch is active. Similarly, when the normally open + RES switch or the normally
open - SET switch are pressed, the switch closes and the BCM detects the predetermined voltage
signal on the cruise control set/coast and resume/accel switch signal circuit. The BCM sends a
GMLAN serial data message to the ECM indicating that the + RES switch or the - SET switch is
active. To engage the Cruise Control System, ensure that the vehicle speed is above 40.2 km/h
(25 mph), turn the cruise On/Off switch ON and momentarily press the - SET switch. The ECM
will engage the Cruise Control System and record the vehicle speed. The ECM sends a GMLAN
serial data message to the driver information center (DIC) in order to illuminate the CRUISE
SET TO ### message. The ECM sends a GMLAN serial data message to the instrument panel
cluster (IPC) in order to illuminate the Cruise Engaged indicator in the IPC. The - SET switch
and the + RES switch will remain inactive when the BCM has not received the predetermined
voltage signal from the on/off switch. Pressing the accelerator pedal, while the Cruise Control
System is engaged, will allow the driver to override the Cruise Control System in order to
accelerate the vehicle beyond the current set vehicle speed. When the accelerator pedal is
released, the vehicle will decelerate and resume the current set vehicle speed. The driver can also
override the current set vehicle speed via the - SET switch and the + RES switch. When the
Cruise Control System is engaged, pressing and holding the - SET switch will allow the vehicle
to decelerate from the current set vehicle speed without deactivating the Cruise Control System.
When the - SET switch is released, the ECM will record the vehicle speed and maintain the
vehicle speed as the new set vehicle speed. When the Cruise Control System is engaged,
momentarily pressing the - SET switch will allow the vehicle to decelerate at 1.6 km/h (1 mph)
increments for each time that the - SET is momentarily pressed, with a minimum vehicle speed of
37 km/h (23 mph). Pressing and holding the + RES switch, when the Cruise Control System is
engaged, will allow the vehicle to accelerate to a greater vehicle speed than the current set
vehicle speed. When the + RES switch is released, the ECM will record the vehicle speed and
maintain the vehicle speed as the new set vehicle speed. When the Cruise Control System is
engaged, momentarily pressing the + RES switch will allow the vehicle to accelerate at 1.6 km/h
(1 mph) increments for each time that the + RES switch is momentarily pressed, with the
maximum acceleration total of 16 km/h (10 mph) over the current set vehicle speed. Momentarily
activating the + RES switch will recall the previous vehicle speed, after the cruise control system
is disengaged by pressing the brake pedal or by activating the CANCEL switch.

Cruise Control Disengaged

The engine control module (ECM) disengages the cruise control operation based on the signals
from the following switches:

2008 Chevrolet Silverado 1500

2008 ENGINE Cruise Control - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

The stop lamp switch

The cruise control On/Off switch

The cruise control cancel switch

The body control module (BCM) determines when the cruise control cancel switch is activated.
When the normally open cancel switch is closed, the BCM detects the predetermined voltage
signal on the cruise control function switch circuit. The BCM sends a GMLAN serial data
message to the ECM in order to disengage the cruise control system.

The stop lamp switch is mounted on the brake pedal bracket. When the brake pedal is applied,
the normally open stop lamp switch closes. The ECM monitors the stop lamp signal circuit
through a discrete input and a GMLAN serial data message signal from the BCM indicating the
brake status. When both signals indicate the brake pedal is applied, the ECM will disengage the
cruise control system.

The Cruise Control System will disengage when the cruise control on/off switch is switched OFF.
The vehicle speed stored in the memory of the engine control module will be erased when the
cruise control On/Off switch is turned OFF, or the ignition switch is turned OFF.

The Cruise Control System will disengage when the ECM detects that a driver override function
has been active for approximately 60 seconds. When the Cruise Control System has been
disengaged, the ECM sends a GMLAN serial message to the instrument panel cluster (IPC) in
order to turn OFF the Cruise Engaged indicator, and to the driver information center (DIC) in
order to turn OFF the CRUISE SET TO ### message.

Each time the Cruise Control System is disengaged, the ECM will keep track of the reason for
system disengagement. The last 8 disengagement reasons will be recorded within the ECM
memory, and can be viewed with a scan tool.

Cruise Control Inhibited

The engine control module (ECM) inhibits the cruise control operation when any of the following
conditions exist:

The ECM has not detected a brake pedal activation from the body control module (BCM).

A Cruise Control System DTC has been set.

The vehicle speed is less than 40.2 km/h (25 mph).

The vehicle speed is too high.

The vehicle is in PARK, REVERSE, NEUTRAL, or 1st gear.

2008 Chevrolet Silverado 1500

2008 ENGINE Cruise Control - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

The engine RPM is too low.

The engine RPM is too high.

The system voltage is not between 9-16 volts.

The Antilock Brake System (ABS)/Traction Control System (TCS) is active for more than 2
seconds.

2008 Chevrolet Silverado 1500

2008 ENGINE Cruise Control - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado