Chevrolet Silverado / GMC Sierra. Manual — part 1788

SOC is a function of the acid concentration and the internal resistance of the battery, and is
estimated by reading the battery open circuit voltage when the battery has been at rest for several
hours.

The SOC can be used as a diagnostic tool to tell the customer or the dealer the condition of the
battery. Throughout ignition-on, the algorithm continuously estimates SOC based on adjusted net
amp hours, battery capacity, initial SOC, and temperature.

While running, the battery degree of discharge is primarily determined by a battery current
sensor, which is integrated to obtain net amp hours.

In addition, the EPM function is designed to perform regulated voltage control (RVC) to improve
battery SOC, battery life, and fuel economy. This is accomplished by using knowledge of the
battery SOC and temperature to set the charging voltage to an optimum battery voltage level for
recharging without detriment to battery life.

The Charging System Description and Operation is divided into 3 sections. The first section
describes the charging system components and their integration into the EPM. The second section
describes charging system operation. The third section describes the instrument panel cluster
(IPC) operation of the charge indicator, driver information center (DIC) messages, and voltmeter
operation.

Charging System Components

Generator

The generator is a serviceable component. If there is a diagnosed failure of the generator it must
be replaced as an assembly. The engine drive belt drives the generator. When the rotor is spun it
induces an alternating current (AC) into the stator windings. The AC voltage is then sent through
a series of diodes for rectification. The rectified voltage has been converted into a direct current
(DC) for use by the vehicles electrical system to maintain electrical loads and the battery charge.
The voltage regulator integral to the generator controls the output of the generator. It is not
serviceable. The voltage regulator controls the amount of current provided to the rotor. If the
generator has field control circuit failure, the generator defaults to an output voltage of 13.8
volts.

Body Control Module (BCM)

The body control module (BCM) is a GMLAN device. It communicates with the engine control
module (ECM) and the instrument panel cluster (IPC) for electrical power management (EPM)
operation. The BCM determines the output of the generator and sends the information to the
ECM for control of the generator field control circuit. It monitors the generator field duty cycle

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Electrical - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

signal circuit information sent from the ECM for control of the generator. It monitors a battery
current sensor, the battery positive voltage circuit, and estimated battery temperature to
determine battery state of charge (SOC). The BCM sends idle boost requests to the ECM.

Battery Current Sensor

The battery current sensor is a serviceable component that is connected to the negative battery
cable at the battery. The battery current sensor is a 3-wire hall effect current sensor. The battery
current sensor monitors the battery current. It directly inputs to the BCM. It creates a 5 volt pulse
width modulation (PWM) signal of 128 Hz with a duty cycle of 0-100 percent. Normal duty
cycle is between 5-95 percent. Between 0-5 percent and 95-100 percent are for diagnostic
purposes.

Engine Control Module (ECM)

The ECM directly controls the generator field control circuit input to the generator. The ECM
receives control decisions based on messages from the BCM. It monitors the generators generator
field duty cycle signal circuit and sends the information to the BCM.

Instrument Panel Cluster (IPC)

The IPC provides a means of customer notification in case of a failure and a voltmeter. There are
2 means of notification, a charge indicator and the driver information center (DIC) SERVICE
BATTERY CHARGING SYSTEM message.

Charging System Operation

The purpose of the charging system is to maintain the battery charge and vehicle loads. There are
6 modes of operation and they include:

Battery Sulfation Mode

Charge Mode

Fuel Economy Mode

Headlamp Mode

Start Up Mode

Voltage Reduction Mode

The engine control module (ECM) controls the generator through the generator turn on signal. It
monitors the generator performance though the generator field duty cycle signal circuit. The
signal is a 5 volt pulse width modulation (PWM) signal of 128 Hz with a duty cycle of 0-100
percent. Normal duty cycle is between 5-95 percent. Between 0-5 percent and 95-100 percent are

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Electrical - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

for diagnostic purposes. The following table shows the commanded duty cycle and output voltage
of the generator:

The generator provides a feedback signal of the generator voltage output through the generator
field duty cycle signal circuit to the ECM. This information is sent to the body control module
(BCM). The signal is a 5 volt PWM signal of 128 Hz with a duty cycle of 0-100 percent. Normal
duty cycle is between 5-99 percent. Between 0-5 percent and 100 percent are for diagnostic
purposes.

Battery Sulfation Mode

The BCM will enter this mode when the interpreted generator output voltage is less than 13.2
volts for 45 minutes. When this condition exists the BCM will enter Charge Mode for 2-3
minutes. The BCM will then determine which mode to enter depending on voltage requirements.

Charge Mode

The BCM will enter Charge Mode when ever one of the following conditions are met.

The wipers are ON for more than 3 seconds.

The GMLAN Climate Control Voltage Boost Mode Request is true, as sensed by the HVAC
control head. High speed cooling fan, rear defogger and HVAC high speed blower
operation can cause the BCM to enter the Charge Mode.

The estimated battery temperature is less than 0°C (32°F).

Battery state of charge is less than 80 percent.

Vehicle speed is greater than 145 km/h (90 mph)

Current sensor fault exists

Commanded Duty Cycle

Generator Output Voltage

10%

11 V

20%

11.56 V

30%

12.12 V

40%

12.68 V

50%

13.25 V

60%

13.81 V

70%

14.37 V

80%

14.94 V

90%

15.5 V

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Electrical - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

System voltage was determined to be below 12.56 volts

Tow/Haul mode is enabled

When any one of these conditions is met, the system will set targeted generator output voltage to a
charging voltage between 13.9-15.5 volts, depending on the battery state of charge and estimated
battery temperature.

Fuel Economy Mode

The BCM will enter Fuel Economy Mode when the ambient air temperature is at least 0°C (32°
F) but less than or equal to 80°C (176°F), the calculated battery current is less than 15 amps and
greater than -8 amps, and the battery state of charge (SOC) is greater than or equal to 80 percent.
Its targeted generator output voltage is the open circuit voltage of the battery and can be between
12.5-13.1 volts. The BCM will exit this mode and enter Charge Mode when any of the conditions
described above are present.

Headlamp Mode

The BCM will enter Headlamp Mode when the headlamps are ON. Voltage will be regulated
between 13.9-14.5 volts

Start Up Mode

When the engine is started the BCM sets a targeted generator output voltage of 14.3 volts for 30
seconds.

Voltage Reduction Mode

The BCM will enter Voltage Reduction Mode when the calculated battery temperature is above
0°C (32°F). The calculated battery current is less than 1 amp and greater than -7 amps, and the
generator field duty cycle is less than 99 percent. Its targeted generator output voltage is 13 volts.
The BCM will exit this mode once the criteria are met for Charge Mode.

Auxiliary Battery Charging (TP2)

The auxiliary battery provision (TP2) can be used to supply electrical power to additional
equipment that the customer may choose to add, such as a slide-in camper or trailer, without
discharging the vehicles primary battery. The auxiliary battery relay closes when the engine is
running, in order to allow the generator to charge the auxiliary battery. The relay opens when the
engine is off, so that the accessories will not discharge the vehicles primary battery, which is used
for engine starting. If the vehicle is equipped with an auxiliary battery, the relay will be located
on the driver's side of the vehicle, next to the underhood electrical center. Generally, a fuse

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Electrical - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado