Chevrolet Silverado / GMC Sierra. Service manual — part 2251

RADIO/AUDIO SYSTEM DESCRIPTION AND OPERATION

Contents

RPO options

Circuit description

Component description

Theft deterrent feature

Customer tips

RPO Options

The entertainment system on this vehicle is configured with either a base or uplevel audio system.
Both the base and uplevel audio systems contain a radio, antenna, and speakers. The following
shows the Entertainment RPOs that are available for this vehicle:

(UM7) AM/FM Stereo

(U1C) AM/FM Stereo CD

(US8) AM/FM Stereo, CD, MP3, RDS, EQ

(US9) AM/FM Stereo, 6-Disc CD, MP3, RDS, EQ

(UVA) AM/FM Stereo, CD, DVD, MP3, RSE Video

(UQ5) Base Four Speaker, No Amplifier

(UQ3) Uplevel Six Speaker, No Amplifier

(Y91) Premium (Luxury) GMC Denali

(UQA w/o Y91) Non GMLAN Bose Amplifier

(UQA w Y91) GMLAN Bose Amplifier

(UQS) Cadillac Premium Amplifier

(U2K) Digital Radio

Circuit Operation

at the start of the word.

'nine'

Emphasize the 'n' sounds to distinguish from 'five'.

'star'

Emphasize the 'r' at the end of the word. Emphasize the 'ah' sound
in order to distinguish from 'store'.

'pound'

Emphasize the 'p' at the start of the word. Emphasize the 'd' at the
end of the word.

2008 Chevrolet Silverado 1500

2008 ACCESSORIES & EQUIPMENT Cellular, Entertainment, and Navigation - Cab & Chassis Sierra, Cab & Chassis

Silverado, Sierra & Silverado

Radio Power

The main radio power is supplied by a 15A fuse located in the underhood fuse block. The radio
does not require a discrete ignition feed circuit for power moding. The power moding is
accomplished using a structure of Virtual Networks (VN). The Power Mode Master (PMM)
transmits the GMLAN power mode signals.

The radio supports the following signals:

System Power Mode

Infotainment Operation Allowed

The radio also supports the following GMLAN Vehicle Power Modes:

OFF

ACCESSORY

RUN

CRANK REQUEST

Radio Ground

A wire in the main radio connector that is connected to battery negative at all times provides the
main radio ground. Resistance between the ground pin and the vehicle battery negative terminal
must not exceed 0.05 ohm.

Radio Speaker Outputs

At low volume, the plus (+) and minus (-) speaker outputs circuits measure approximately 7-8
volts. If a plus or minus circuit for any speaker output is shorted to ground or voltage, the radio
disables the circuit for component protection and sets a diagnostic trouble code (DTC). The radio
sets the speaker circuit DTC on non-amplified systems only. As the radio volume increases the
voltage on the plus and minus circuits change to create a voltage difference between each other.
The difference in voltage is what drives the voice coil of the speaker producing sound.

Radio Head Unit Dimming

GMLAN signal data received by the radio determines the radio dimming and backlighting levels.
The radio sets the backlight and VF display dimming to the value indicated by the interior
dimming level and interior dimming display level signals in the GMLAN dimming information
frame. The radio provides faceplate and control/graphics backlighting in the OFF (RAP inactive)
when the exterior lighting virtual network (VN) is activated. The radio display is consistent then

2008 Chevrolet Silverado 1500

2008 ACCESSORIES & EQUIPMENT Cellular, Entertainment, and Navigation - Cab & Chassis Sierra, Cab & Chassis

Silverado, Sierra & Silverado

with the surrounding devices that use analog PWM dimming to backlight their control/graphics
when the power mode is OFF and the park lights are ON.

Amplifier Interface UQA without Y91(Non-GMLAN)

The main amplifier power is provided by a 20A fuse located in the body control module (BCM).
A discrete switched 12-volt output is used to control the power - state of the amplifier. To
respond quickly to audio input and control signals, the amplifier is ON in all vehicle power
modes except OFF and CRANK Request. However, when the amplifier is asleep it still must
allow chime functionality. The internal amplifier bridges are fully powered and unmuted when
the amplifier receives the switched 12-volt input.

The radio provides a remote amp mute output circuit to control overall muting of remote non-
GMLAN amplifiers with rear seat audio (RSA) applications. The amplifier receives PWM signals
on the circuit at varying duty cycle percentages for controlling the muted and unmuted functions
of the amplifier. The RSA Enable circuit in the radio enables the rear seat channel muting of the
amplifier. A Diag Sense circuit internal to the radio monitors the remote amp mute circuit for
faults and sets the appropriate DTC when detected.

The function table below shows the four possible amplifier mute functions.

Amplifier Interface UQA (GMLAN)

The main amplifier power is provided by a 30A fuse located in the underhood fuse block. For
vehicles equipped with UQA, the radio commands the amplifier to perform the following
functions through serial data (GMLAN) communication. The radio controls both the overall
muting and the rear seat muting via serial data (GMLAN) messaging.

Volume

Balance

Fade

Bass

Midrange

% Duty Cycle Out

Function

0% (Vlow Vdc)

FOUR_CHANNEL_UNMUTE

50%

REAR_MUTE (input must be 2-channel)

75%

SIX_CHANNEL_UNMUTE

100% (Vhigh Vdc)

ALL_MUTE

2008 Chevrolet Silverado 1500

2008 ACCESSORIES & EQUIPMENT Cellular, Entertainment, and Navigation - Cab & Chassis Sierra, Cab & Chassis

Silverado, Sierra & Silverado

Treble

DSP Modes

Volume Compensation

Chime Output

Amplifier Radio Speaker Inputs

The low-level audio signals (LF, RF, LR, and RR) from the radio are the inputs to the amplifier.
The amplifier boosts these inputs and outputs them to the vehicle speakers. If one speaker plus or
minus low level audio signal circuit is open between the radio and the amplifier, the input to the
amplifier is approximately half. The speakers for that channel then operate at approximately half
the normal volume or in some cases no volume.

The low level audio signals from the radio to the amplifier typically measure in the 4-5 volts
range.

Amplifier Speaker Outputs

At a low volume, the plus (+) and minus (-) speaker outputs circuits measure approximately 2-7V
AC or 7-8V DC at the speaker connector when testing a normal working system. If a plus or
minus for any speaker output is shorted to ground or voltage, the amplifier circuitry will turn
OFF the front outputs or rear outputs for component protection.

Repeat speaker failure can be caused by a damaged amplifier. Test for a damaged quad bridge
output in the amplifier if the wiring between the amplifier and speaker test negative for a short to
voltage. Prolonged exposure to DC voltage can cause a speaker coil to short. An early indication
of a shorted speaker coil is distortion/noise and eventual failure.

Remote Radio Audio Signal Inputs (UE1 only)

Audio output from the OnStar® communications module connects to the remote audio signal
inputs of the radio. When the cellular telephone mute signal goes to 0 volts, the radio over-rides
any other audio signal and uses these inputs as the source for the output to the speakers.

Cellular Telephone Mute (UE1 only)

The OnStar® communications module uses the cellular telephone mute signal circuit to over-ride
the radio for OnStar® communication. When cellular telephone mute is not active, this circuit is
held at 1 volt by the radio. When the cellular telephone mute signal is pulled to ground, the radio
over-rides any other audio signal and uses the remote audio signals as the source for output to the
speakers. If the radio was OFF when this circuit is pulled low, the radio will turn ON.

2008 Chevrolet Silverado 1500

2008 ACCESSORIES & EQUIPMENT Cellular, Entertainment, and Navigation - Cab & Chassis Sierra, Cab & Chassis

Silverado, Sierra & Silverado