The simple answer is to replace the TCCM (transfer case control module).
Being both cheap and inquisitive, I wanted to investigate further, and I will provide a more in-depth answer.
This answer pertains to various GM vehicles. These include the Chevy Blazer, GMC Jimmy, S10, S15, Sonoma vehicles from 1999, 2000, 2001, 2002, 2003, 2004, & 2005.
Specifically the 15748800 module for the NP8 option 4 button automatic 4X4 NVG-236/246 transfer case.
I doing research online, I find that 15154323 might be an alternate TCCM, and that GM redesigned the module and now sells the replacement for this module with a 12577412 part number for about $300.
This information refers to the 15748800 module.
Symptoms. If the vehicle is started and driven in 2 hi, the vehicle operates normally.
If the driver attempts to switch the transfer case into any other mode, the transfer case makes noise as it shifts, the service 4x4 light comes on and the newly selected mode indicator light starts to flash.
The TCCM is mounted behind the passenger side kick panel cover. It should be removed but left connected. Pin F4 should be monitored with either a lamp or meter. This pin should be high or 12 volts when in 2 wheel drive, and low or ground when in 4 wheel drive.
A quick test of the system is to ground the F4 pin, recycle the ignition, and try shifting the transfer case into and out of 4 wheel drive. ( I used a lamp based test light to be safe. Do not an LED based one.) If the mode indicator light remains steady and the Service 4X4 does not light, proceed with this repair procedure. If this does not resolve the issue, please proceed to troubleshoot the front vacuum system, 4X4 solenoid, 4X4 actuator, front axle, or front axle indicator switch.
If this test appears to resolve the issue, you will need to remove the TCCM module from the vehicle. Bend the 8 tabs away from the back cover. Remove the top cover, unscrew the board from the back cover / plate. turn over the board at a static safe work station, and measure the indicated 10,000 ohm SMD resistor. In this module, the resistor is required to operate near it's limits, and I believe that the conformal coating trapped heat and caused the resistor to fail. You can check other resistors nearby for a relative comparison. If found to be open, the resistor can be replaced with a higher power value 10,000 ohm SMD resistor, or a conventional 10,000 ohm resistor with wired leads.
The TCCM should be checked for corrosion on the connectors and cleaned if nessacary. The module can then be restored to the vehicle, and your 4X4 should work normally.
Please respond if this procedure helps you.
Front view / pin side
f16f1 Connector Part Information1205210716 Way F Micro-Pack 100 Series (ORN) Pin Wire Color Circuit No. Function
E1 Tan 1569 Transfer Case Lock Soleniod Output
E2 - - Not Used
E3 PNK 1561 AWD/4WD Indicator Lamp Output
E4 Lt GRN/BLK 1563 2 HI Indicator Lamp Output
E5 BRN/WHT 1555 Transfer Case Position Switch Encoder Signal
E6 RED/WHT 1556 Transfer Case Position Switch Encoder Signal
E7 YEL/BLK 1558 Transfer Case Position Switch Encoder Signal
E8 DK BLU/WHT 1557 Transfer Case Position Switch Encoder Signal
E9 DK GRN/WHT 1133 Class 2 Serial Data Signal
E10 BRN 241 Fuse Output - Ignition 3 - Type III Fuse
E11 PPL 806 Fuse Output - Crank - Type III Fuse
E12 YEL 400 Vehicle Speed Sensor Signal
E13 PPL 401 Vehicle Speed Sensor Return
E14 BLK/WHT 1695 Four Wheel Drive Front Wheel Lock Telltale
E15 - - Not Used
E16 BLK/WHT 451 Engine Control Module Ground
F1 PPL/WHT 1565 4 LO Indicator Lamp Output
F2 TAN/BLK 1566 4 HI Indicator Lamp Output
F3 BRN 1560 Neutral Indicator Lamp Output
F4 GRA/BLK 1570 Front Axle Actuator Output
F5 LT BLU 2221 Vehicle Speed Sensor Signal
F6 DK GRN 2222 Vehicle Speed Sensor Return
F7 GRA/BLK 1694 Four Wheel Drive Switch Signal - Low
F8 - F11 - - Not Used
F12 BLK/WHT 1554 Transfer Case Encoder Signal Return
F13 LT GRN/BLK 431 Reference Voltage Feed - 8 Volt Reference
F14 LT GRN/BLK 431 Reference Voltage Feed - 8 Volt Reference
F15 - - Not Used
F16 LT BLU 1693 Four Wheel Drive Switch Signal
Front view / pin side
Connector Part Information120528564 Way F Metri-Pack 280 Series (BLK) Pin Wire Color Circuit No. Function
A BLK 1552 Transfer Case Motor Feed - Clockwise
B RED 1553 Transfer Case Motor Feed - Counterclockwise
C BLK 1050 Ground
D ORG 1640 Fuse Output - Battery - Type III Fuse