Chevrolet Silverado / GMC Sierra. Manual — part 2837

the vehicle is in START or RUN and the Engine control module (ECM) receives the axle switch
signal from the front axle actuator, the ECM sends a CAN signal to the instrument cluster turning
on the 4WD indicator. The 4WD indicator will stay on as long as the axle switch signal input is
present.

Reference Information

Schematic Reference

Transfer Case Control Schematics

Connector End View Reference

Component Connector End Views

Electrical Information Reference

Circuit Testing

Connector Repairs

Testing for Intermittent Conditions and Poor Connections

Wiring Repairs

Circuit/System Verification

1. Place the vehicle in 4WD install a scan tool and using engine data 2 look at the display for

4WD signal.

2. Verify that the scan tool indicates that the vehicle is in 4WD.

If the scan tool indicates that the vehicle is in 2WD go to circuit system testing.

Circuit/System Testing

1. Check the axle switch signal circuit for an open or high resistance.

If everything checks normal then go to the next step.

2. Check the front axle actuator control circuit pin B at the front axle actuator connector or pin

C at the transfer case shift control switch for an open or high resistance.

If everything checks normal then short the axle actuator control circuit to ground this
should cause the actuator to engage. Remove the short to ground and then short the
circuit to ground again, this should cause the actuator to disengage. If the actuator does
not operate properly then refer to front axle will not engage or front axle will not
disengage. If everything checks normal go to ECM and IPC functionality check.

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Transfer Case - MP 1222/1225/1226-NQG - Cab & Chassis Sierra, Cab & Chassis Silverado,

Sierra & Silverado

Instrument Panel Cluster (IPC) and Engine Control Module (ECM) Functionality

If all circuits referenced above check good use the scan tool under engine data 2 to see if the scan
tool still indicates the vehicle is still in 2WD.

If the scan tool indicates the vehicle is in 2WD then replace the engine control module, if
not then replace the IPC.

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Instrument Cluster Replacement (without RPO SLT)

Engine Control Module Replacement for the 4.3L engine

Engine Control Module Replacement for the 4.8L, 5.3L, 6.0L, and 6.2L engine

Engine Control Module Replacement for the 6.6L engine

Control Module References for ECM replacement, setup, and programming

Four-Wheel Drive Does Not Disengage

Four-Wheel Drive Does Not Engage

TRANSFER CASE POPPING NOISE

TRANSFER CASE WHINE OR RUMBLE NOISE

Cause

Correction

DEFINITION: A faulty internal transfer case component that causes a popping noise in the
transfer case.
Review the Symptoms - Transfer Case and perform the necessary inspections.

Chain jump

Replace the chain and sprockets. Refer to Transfer Case
Disassemble and Transfer Case Assemble.

Replace the shaft bearings. Refer to Transfer Case
Disassemble and Transfer Case Assemble.

Inspect for damaged components caused by metal debris.
Refer to Drive Chain, Drive Sprocket, and Driven
Sprocket Cleaning and Inspection and Front Output
Shaft and Input Shaft Cleaning and Inspection.

Cause

Correction

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Transfer Case - MP 1222/1225/1226-NQG - Cab & Chassis Sierra, Cab & Chassis Silverado,

Sierra & Silverado

TRANSFER CASE GROWL OR GRINDING NOISE

DEFINITION: A faulty internal transfer case component that causes a whine or rumbling
noise in the transfer case.
Review the Symptoms - Transfer Case and perform the necessary inspections.

Faulty bearings

1. Inspect all of the bearings and journals. Refer to Rear

Output Shaft Cleaning and Inspection, Front
Output Shaft and Input Shaft Cleaning and
Inspection, and Drive Chain, Drive Sprocket, and
Driven Sprocket Cleaning and Inspection.

2. Replace any faulty bearings and any components with

faulty bearing journals. Refer to Transfer Case
Disassemble and Transfer Case Assemble.

Cause

Correction

DEFINITION: A faulty internal transfer case component that causes a growl or grinding
noise in the transfer case while the vehicle is turning or in a four wheel drive mode.
Review the Symptoms - Transfer Case and perform the necessary inspections.

High/low planetary carrier
assembly

1. Inspect the high/low planetary carrier assembly for the

following conditions:

Chipped, bent, or deformed teeth

Planetary gear needle bearings worn

Planetary gear thrust washers cracked or missing

Seized or binding planetary gears

Refer to High/Low Planetary Carrier Cleaning
and Inspection .

2. Replace the high/low planetary carrier assembly if any of

the above conditions exist. Refer to Transfer Case
Disassemble and Transfer Case Assemble.

1. Inspect the input shaft in the planetary carrier assembly

for excessive thrust end play.

2. Inspect the input shaft teeth for the following conditions:

Damage

Worn or galled thrust surfaces

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Transfer Case - MP 1222/1225/1226-NQG - Cab & Chassis Sierra, Cab & Chassis Silverado,

Sierra & Silverado

TRANSFER CASE CLUNK IN 2HI ONLY

Input shaft

Excessive wear

Chipped teeth

Metal embedded in the root of the teeth

Refer to Front Output Shaft and Input Shaft
Cleaning and Inspection

3. Replace the input shaft if faulty. Refer to Transfer Case

Disassemble and Transfer Case Assemble.

Thrust washers

1. Inspect for missing or damaged thrust washers for the

input shaft.

2. Inspect the thrust washers for galling.

3. Replace the thrust washers if faulty and the mating

component if damaged. Refer to Transfer Case
Disassemble and Transfer Case Assemble.

High/low internal gear

1. Inspect the high/low internal gear teeth for the following

conditions:

Damage

Chipped teeth

Excessive wear

Metal embedded in the root of the teeth

Refer to Transfer Case Front Half Cleaning and
Inspection .

2. Replace the front case half if the high/low internal gear is

faulty. Refer to Transfer Case Disassemble and
Transfer Case Assemble.

Cause

Correction

DEFINITION: During acceleration or deceleration, a clunk is heard or felt in the transfer
case.
Review the Symptoms - Transfer Case and perform the necessary inspections.

1. Inspect the transfer case for the correct fluid level.

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Transfer Case - MP 1222/1225/1226-NQG - Cab & Chassis Sierra, Cab & Chassis Silverado,

Sierra & Silverado