Chevrolet Silverado / GMC Sierra. Service manual — part 1699

equipment. Anytime aftermarket accessories are installed into the courtesy lamp circuit, it
can cause the inadvertent power timer in the body control module (BCM) to keep resetting.
This would cause the BCM to remain awake and cause a current drain on the battery.

Customer driving habits, such as regular short trips. This does not allow enough time to
properly charge the battery. Refer to Battery Description and Operation.

Verify that the battery and charging system are in proper working order. Refer to Battery
Charging and Charging System Test.

A battery discharging for no apparent reason while the vehicle is parked can be caused by
an intermittent draw, such as a module waking up, or a continuous draw, such as a dome
light or stuck relay. Look and listen for any components that may be operating when they
should turn off, such as a fuel pump.

Some systems and modules such as OnStar®, and regulated voltage control (RVC), if
equipped, are designed to wake-up, perform a task, and go back asleep at regular intervals.
Refer to Body Control System Description and Operation for the system or modules
description and operation.

Remote keyless entry (RKE) will wake up due to an outside input. Refer to Keyless Entry
System Description and Operation for the system description and operation.

The battery run down time will vary depending on cold cranking amperage (CCA) and
reserve capacity (RC). If the CCA and RC are higher, then the battery run down time would
be longer. If the CCA and RC are lower, then the battery run down time would be shorter.
The graph below indicates roughly how many days a 690 CCA battery with at 110 min. RC
(60.5 AH) starting at 80 percent state of charge will last with a constant current draw until it
reaches 50 percent state of charge. Differences in battery rating and temperature will affect
the results.

IMPORTANT: The battery specification listed below is a generic

specification. Refer to Battery Usage when testing the
battery.

Current Drain

Days

25 mA

30.5

50 mA

16.5

75 mA

11

100 mA

8.25

250 mA

3.3

500 mA

1.65

2008 Chevrolet Silverado 1500

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

Load Test

1. Disconnect the battery negative cable from the battery negative terminal. Refer to Battery

Negative Cable Disconnection and Connection (w/Single Battery) or Battery
Negative Cable Disconnection and Connection (w/Auxiliary Battery) or Battery
Negative Cable Disconnection and Connection (w/Dual Batteries).

2. Install the male end of the J 38758 to the battery ground terminal. See Special Tools.

3. Turn the J 38758 knob to the OFF position. See Special Tools.

4. Install the battery negative cable to the female end of the J 38758 . See Special Tools.

5. Turn the J 38758 knob to the ON position. See Special Tools.

6. Road test the vehicle and activate ALL of the accessories, including the radio and air

conditioning. This may take up to 30 minutes.

7. Park the vehicle. Turn the ignition switch to the OFF position and remove the ignition

switch key.

8. Connect a 10A fused jumper wire to the test switch tool terminals.

750 mA

1

1 A

0.8

2 A

0.4

CAUTION: Refer to Battery Disconnect Caution .

NOTE:

Do not turn the parasitic draw test switch to the OFF position with
the engine running. Damage will occur to the vehicle's electrical
system.

NOTE:

The test switch must be in the ON position when removing the fuses
in order to maintain continuity in the electrical system. This avoids
damaging the digital multimeter due to accidental overloading,
such as a door being opened to change a fuse.

IMPORTANT: The switch knob on the J 38758 is marked ON and OFF. See

Special Tools. When the switch knob is in the ON position, the
circuit is closed and electrical current will pass through the
switch. When the switch knob is in the OFF position, the circuit is
open and electrical current will not pass through the switch.

2008 Chevrolet Silverado 1500

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

9. Turn the J 38758 knob to the OFF position. See Special Tools. The current now flows

through the jumper wire.

10. Wait 1 minute. If the fuse blows, install an inductive ammeter and go to step 19.

11. Remove the fused jumper wire.

12. Set a digital multimeter to the 10A scale.

13. Connect the digital multimeter to the test switch tool terminals.

14. Turn the J 38758 knob to the OFF position. See Special Tools. The current flows now

through the digital multimeter.

15. Wait 1 minute. Check and record the current reading.

1. When there is a current reading of 2A or less, turn the J 38758 knob to the ON

position. See Special Tools. The electrical current will now pass through the switch.

2. Then switch the digital multimeter down to the 2A scale for a more accurate reading

when the J 38758 knob is turned OFF. See Special Tools.

16. Turn the J 38758 knob to the OFF position. See Special Tools. Wait 15 minutes for most

vehicles.

17. Check and record the current reading.

18. Note the battery reserve capacity, amp hour rating. Refer to Battery Usage.

1. Divide the reserve capacity by 4, amp hour rating by 2.4.

2. Compare this to the multimeter milliampere reading taken in the previous step. The

parasitic current drain should not exceed this number. Example: If a battery has a
reserve capacity of 100 minutes, (60 A/H) the current drain should not exceed 25 mA.

19. If excessive current drain is not found at this time and there are no other apparent causes,

complete the following:

1. Using the MIN/MAX function of the digital multimeter, monitor the parasitic drain

overnight or during the day. This will determine if something has been activated during
that time frame.

NOTE:

The test switch must be in the ON position when removing
the fuses in order to maintain continuity in the electrical
system. This avoids damaging the digital multimeter due
to accidental overloading, such as a door being opened to
change a fuse.

IMPORTANT: Removing fuses, relays, and connectors to determine the

failure area may wake up modules. You must wait for

2008 Chevrolet Silverado 1500

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

2. When the vehicle has an unacceptable amount of parasitic current drain, remove each

fuse one at a time until the current drain falls to an acceptable level. This will indicate
which circuit is causing the drain. Refer to Power Distribution Schematics to
diagnose exactly which part of the suspect circuit is causing the parasitic drain. In
some cases a non-fused circuit or component, such as a relay, is the cause of excessive
parasitic current drain.

3. Repeat the parasitic current drain test procedure after any repair has been completed

to make sure that the parasitic current drain is at an acceptable level.

4. When the cause of the excessive current drain has been located and repaired, remove

the J 38758 . See Special Tools.

20. Connect the battery negative cable to the battery negative terminal.

BATTERY COMMON CAUSES OF MALFUNCTION

A battery is not designed to last forever. With proper care, however, the battery will provide
years of good service. If the battery tests good but still fails to perform well, the following are
some of the more common causes:

A vehicle accessory was left on overnight.

The driving speeds have been slow with frequent stops with many electrical accessories in
use, particularly air conditioning, headlights, wipers, heated rear window, cellular
telephone, etc.

The electrical load has exceeded the generator output, particularly with the addition of
aftermarket equipment.

Existing conditions in the charging system, including the following possibilities:

A slipping belt

A bad generator

The battery has not been properly maintained, including a loose battery hold down or
missing battery insulator if used.

There are mechanical conditions in the electrical system, such as a short or a pinched wire,
attributing to power failure. Refer to General Electrical Diagnosis .

Electrolyte Freezing

The freezing point of electrolyte depends on its specific gravity. A fully charged battery will not

these modules to go to sleep or use the sleep function
on the scan tool.

2008 Chevrolet Silverado 1500

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