NEED HELP Service 4WD light

2003 Trailblazer EXT "Service 4WD" light illuminates only after reaching approximately 15 mph, and stays illuminated at any speed. At restart, the light is off until reaching approximately 15 mph again. All lights on the switch illuminate when selected, and the disconnect actuator and transfer case motor both operate to engage all functions when selected.
I cleaned the disconnect and re-greased it and replaced the seal, and cleaned and replaced the fluid and seal in the differential 13,000 miles ago.
I checked all grounds, checked the power at the TCCM, and changed the transfer case fluid recently, and have driven the SUV about 200 miles since, but I'm still having the same issue with the Service light.
I live in the desert, where it is flat and dry, so I rarely use 4wd, but I do engage it occasionally just to keep the parts lubricated, and the switch contacts clean.
Any ideas what would be triggering the light when the 4wd is operating OK?
Note: The ABS and Brake lights stay on, but will occasionally go off after driving 60+ miles on the highway. They used to go off if I honked the horn 7 times, but that doesn't work anymore. I suspect a bad wheel speed sensor. Also the Airbag light stays on constantly, but these issues have been going on for quite a while. I tested the ignition switch, and have constant power on the red wire with the key off, and power on orange wire only with the key in run.


Well-Known Member
"used to go off if I honked the horn 7 times," seems like it was possessed by demons:goodevil:7 honks will scare them away!

Some of those GM rotary switches don't self clean and the internal contacts oxidize as they age. If the tcm doesn't get a good signal from the switch it sets an error. It should be possible to disassemble the switch and clean the contacts.

A good scantool will give you the codes for the airbag and abs so you have an idea where to troubleshoot. I had a oxidized connector on a wheel speed sensor and a bad connection at the underseat connector for the passenger side airbag.

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