Chevrolet Silverado / GMC Sierra. Service manual — part 1700

freeze until the ambient temperature gets below -54°C (-65°F). However, a battery with a low
state of charge may freeze at temperatures as high as -7°C (+20°F). Since freezing may ruin a
battery, the battery should be protected against freezing by keeping it properly charged. As long
as the green eye is visible in the hydrometer, the freezing point of the battery will be somewhere
below -32°C (-25°F).

Battery Protection During Vehicle Storage

Certain devices on the vehicle maintain a small continuous current drain on the battery. A battery
that is not used for an extended period of time will discharge. Eventually permanent damage will
result. Discharged batteries will also freeze in cold weather. Refer to Battery Inspection/Test.

In order to maintain a battery state of charge while storing the vehicle for more than 30 days,
disconnect the battery ground to protect the battery from discharge by parasitic current drains.

When the battery cannot be disconnected:

1. Maintain a high state of charge.

2. Establish a regular schedule for recharging the battery every 20-45 days.

A battery that has remained in a discharged state for a long period of time is difficult to recharge
or may be permanently damaged.

CHARGING SYSTEM TEST

Diagnostic Instructions

Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

Review Strategy Based Diagnosis for an overview of the diagnostic approach.

Diagnostic Procedure Instructions provides an overview of each diagnostic category.

Circuit/System Description

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

2008 Chevrolet Silverado 1500

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

generator has field control circuit failure, the generator defaults to an output voltage of 13.8
volts.

Auxiliary Battery (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
should not be used in the STUD 1 Fuse 70 position of the underhood fuse block, if the vehicle is
equipped with an auxiliary battery. A plastic plug may be installed in this position instead of a
fuse. If a fuse is installed in this position, the accessories will discharge the primary battery in
addition to the auxiliary battery.

Reference Information

Schematic Reference:

Starting and Charging Schematics

Auxiliary Battery Schematics

Connector End View Reference:

Component Connector End Views

Description and Operation

Charging System Description and Operation

Battery Description and Operation

Electrical Information Reference

Circuit Testing

Connector Repairs

Testing for Intermittent Conditions and Poor Connections

Wiring Repairs

Circuit/System Verification

2008 Chevrolet Silverado 1500

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

1. Ignition ON. Verify that no generator or battery current sensor DTCs are set that would

cause a charging system concern.

If DTCs are set, go to Diagnostic Trouble Code (DTC) List - Vehicle .

2. Ignition OFF, measure the voltage across the battery terminals. The voltage should read

12.0 volts or greater at room temperature.

If not within the specified value, charge and test the battery before testing the charging
system. Refer to Battery Inspection/Test.

3. Go to Circuit/System Testing.

Circuit/System Testing

Generator

1. Verify that the battery cable terminals are clean and tight.

2. Connect a carbon pile tester to the battery.

3. Start the engine and increase the engine speed to 2,500 RPM. Place the vehicle in tow/haul

mode. Observe the voltage reading on the tester. The voltage should read between 12.6 -
15.0 volts.

If not within specified range, replace the generator.

4. Adjust the carbon pile tester to the specified load test output value, refer to Generator

Usage.

If less than the specified value, replace the generator.

5. If equipped with an auxiliary generator, repeat this test, with the carbon pile tester's

inductive pickup clamped around either generator harness.

Auxiliary Battery Charging/Isolation (TP2)

Perform this test if there is a concern with the charging of an auxiliary battery.

Do not perform this test if the vehicle is not equipped with an auxiliary battery (TP2).

1. Disconnect the auxiliary battery. Refer to Auxiliary Battery Replacement (6.2L, and

6.6L).

2. Verify that there is not a fuse installed in the STUD 1 socket of the underhood fuse block.

There may be a plastic plug installed in this position, instead of a fuse.

If there is a fuse installed, remove the fuse.

3. Verify that the trailer harness, or other accessory, is connected to the underhood fuse block

stud 1 terminal X7-1, if desired. Verify that the connection is clean and tight.

2008 Chevrolet Silverado 1500

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

4. Ignition OFF. Verify that there is 0 volts present at the underhood fuse block stud 1 terminal

X7-1.

If there is voltage present at stud 1, test the auxiliary battery relay and the run relay for
a stuck closed condition. If OK, test the auxiliary battery positive cable for a short to
voltage.

5. Start the engine. Verify that there is greater than 12 volts present at the underhood fuse

block stud 1 terminal X7-1.

If less than the specified value, test the following components for an open/high
resistance:

SEO B1 fuse

125 A MEGA fuse

Fusible link 2

Fusible link 3

Auxiliary battery relay

Run relay

6. Reconnect the auxiliary battery.

Component Testing

Auxiliary Battery Relay and Run Relay (TP2)

1. Ignition OFF, disconnect the relay.

2. Test for 60-180 ohms between terminals 85 and 86.

If not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

30 and 86

30 and 87

30 and 85

85 and 87

If not the specified value, replace the relay.

4. Install a 20 amp fused jumper wire between relay terminal 85 and 12 volts. Install a jumper

wire between relay terminal 86 and ground. Test for less than 2.0 ohms between terminals
30 and 87.

If greater than specified range, replace the relay.

Repair Procedures

2008 Chevrolet Silverado 1500

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