Have it checked for trouble codes. It sounds like a bad switch.
2000 GMC Truck GMC K Sierra - 4WD | Sierra, Silverado, Suburban, Tahoe, Yukon (VIN C/K) Service Manual | Driveline/Axle | Preliminary Information | Document ID: 1616894
#PIP3349: 4WD Inoperative DTC U1026 ATC Fuse Open on Light Duty Trucks, Utilities with NVG 236/246 T/case - keywords cold electrical fuse hi4 inoperative lamp light no switch wire - (Mar 1, 2005)
Subject: 4WD Inoperative DTC U1026 ATC Fuse Open on Light Duty Trucks Utilities with NVG 236/246 T/Case
The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in the PI.
Four wheel drive inoperative and the 4WD switch light will not illuminate. DTC U1026 will set as a current code. During cold ambient temperatures, the 20 amp ATC fuse for circuit 1640 will go open when shifted into hi4 or auto mode.
Possible cause is that the current draw of the encoder motor exceeds 20 amps when shifting into hi4 or auto mode during cold ambient temperatures.
Refer to the applicable year and model service manual or eSI transfer case wiring schematic.
Replace the 20 amp ATC / T-case fuse with a 25 amp fuse.
Replace circuit 1640 (orange wire), 0.8 mm2 (18 gauge) wire with an orange wire with a specification of a 1.0 mm2 (16 gauge) wire.
Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.
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Let me know what codes you have.
Wednesday, October 29th, 2008 AT 4:39 PM