Chevrolet Silverado / GMC Sierra. Manual — part 2835

If the value is less than 1,420 ohms, test the VSS high signal and the VSS low signal
circuits for a shorted together condition.

2. Measure the resistance of the VSS high signal circuit to ground. The resistance should be

greater than 10M ohms.

If the value is less than 10M ohms, repair the short to ground.

3. Measure the resistance between the VSS low signal circuit and ground. The resistance

should be greater than 10M ohms.

If the value is less than 10M ohms, repair the short to ground.

4. Reconnect the ECM connector. Disconnect the VSS connector. Ignition ON, test the ISS

high signal circuit for a short to voltage.

If all circuits/connections test normal, test or replace the VSS.

5. If all previous steps for circuit checking and VSS replacement have been made and all

component testing has been performed and the condition persists replace the ECM.

Component Testing

1. Verify the VSS resistance is within the specified range.

2. Using a terminal test kit, connect the DMM between the VSS high signal and the VSS low

signal circuits at the VSS connector. Raise and support the drive wheels. While rotating the
drive wheels by hand ensure the AC voltage reading on the DMM is above 0.4 volts.

If the AC voltage is not above 0.4 volts, replace the VSS.

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Ensure that the VSS is 500 RPM or greater for 3 seconds and does not change more than
500 RPM or greater for 3 seconds.

Vehicle Speed Sensor Replacement

Control Module References for ECM replacement, setup, and programming.

SYMPTOMS - TRANSFER CASE

Visual/Physical Inspection

IMPORTANT: Review the system operation in order to familiarize yourself with

the system functions. Refer to Transfer Case Description and
Operation.

2008 Chevrolet Silverado 1500

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

Sierra & Silverado

Inspect for aftermarket devices which could affect the operation of the manual selectable 4-
wheel drive system. Refer to Checking Aftermarket Accessories .

Inspect the easily accessible or visible system components for obvious damage or conditions
which could cause the symptom.

Ensure all mechanical components are operative before diagnosing the electrical portion of
the system.

In order to determine whether the vehicle is in 2-wheel drive (2WD) or 4-wheel drive
(4WD), lift the vehicle so the wheels can spin freely. With the transfer case control lever in
2HI, turn the rear propeller shaft. If the front propeller shaft does not move with the rear,
the vehicle is in 2HI. If the tires and the front propeller shaft moves, the transfer case is in
4WD.

Ensure the 4WD fuse is not open. If the fuse is open, locate and repair the source of the
overload. Refer to Testing for Short to Ground . Replace the fuse.

Intermittent

Faulty electrical connections or wiring may be the cause of intermittent conditions. Refer to
Testing for Intermittent Conditions and Poor Connections .

Symptom List

Refer to a symptom diagnostic procedure from the following list in order to diagnose the
symptom:

Four-Wheel Drive Does Not Disengage

Four-Wheel Drive Does Not Engage

Four-Wheel Drive Indicator Always On

Four-Wheel Drive Indicator Inoperative

Transfer Case Popping Noise

Transfer Case Whine or Rumble Noise

Transfer Case Growl or Grinding Noise

Transfer Case Clunk in 2HI Only

Transfer Case Clunk in 4HI

Transfer Case Clunk in 4LO

Transfer Case Will Not Shift

Transfer Case Jumps Out of Gear

Transfer Case Leak Diagnosis

2008 Chevrolet Silverado 1500

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

Sierra & Silverado

FOUR-WHEEL DRIVE DOES NOT DISENGAGE

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

The front axle control circuit consists of an electric motor actuator which engages and disengages
the front axle. The front axle actuator consists of the following components within the actuator
assembly:

A permanent magnetic motor

A worm gear controlled plunger

A front axle switch

An electronic control circuit

The front axle actuator consists of the following circuits:

An ignition 3 voltage circuit

A ground circuit

An axle actuator control circuit

An axle switch signal circuit

Whenever a shift to 4WD is selected, the transfer case switch engages the front axle by grounding
the axle actuator control circuit.

Reference Information

Schematic Reference

Transfer Case Control Schematics

Connector End View Reference

Component Connector End Views

Electrical Information Reference

2008 Chevrolet Silverado 1500

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

Sierra & Silverado

Circuit Testing

Connector Repairs

Testing for Intermittent Conditions and Poor Connections

Wiring Repairs

Circuit/System Verification

1. Place the vehicle in 2WD 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 2WD.

If the scan tool indicates that the vehicle is in 4WD go to circuit system testing.

Circuit/System Testing

1. Check the axle switch signal circuit for a short to voltage.

If everything checks normal then go to the next step.

2. Check the front axle actuator control circuit for a short to ground with the front axle

actuator disconnected and the mode selection switch disconnected.

If everything checks normal reconnect the mode selection switch and retest. If the
circuit shows a short to ground then replace the switch. If the circuit indicates no short
to ground then refer to front axle actuator and axle testing.

Front Axle Actuator and Axle Testing

Remove the actuator and attempt shift from 2WD to 4WD, if the actuator moves check the front
axle actuator plunger and fork assembly.

If the actuator does not function properly then replace the front axle actuator.

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Front Drive Axle Actuator Replacement

Transfer Case Gear Indicator Switch Replacement

Front Drive Axle Clutch Fork Inspection

FOUR-WHEEL DRIVE DOES NOT ENGAGE

Diagnostic Instructions

2008 Chevrolet Silverado 1500

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

Sierra & Silverado