My “Service 4WD” dash light on in my 2000 GMC Jimmy AWD/4WD Auto. The light would reset after restart and only come back after the Jimmy was in motion. I did not know if a scan tool was needed to diagnose the fault or not, I brought it to a local repair shop and they were able to confirm the symptoms but not the reason. They were going to continue trouble shooting the next day. I decided to research the symptoms on my own and the conditions that were setting the waring light. The wiring diagrams led me to the front wheel drive shaft speed sensor. The front speed sensor only functions in modes other the 2 HI. That would explain why all the functions would work correctly while stationary and while in 2 HI. Once in motion in 2 Hi I would switch to AWD/4 HI/4 LO and the service 4 WD light would set.

Advanced Auto had the speed sensor in stock, I purchased the speed sensor and brought the sensor along with the Factor Service Manual with the pages bookmarked for the front speed sensor (Location, Operation & Troubleshooting) to the shop when they opened. (Advanced opened @ 0700 and the Shop @ 0730) Speed Sensor at Advance was $34.00.

The shop installed the speed sensor and the 4WD system is functioning normally. The ship charged about $50 for Diagnostics $50 for installing the sensor for a little over a $100. All is well, I believe I got off cheap.