Weird problem-- 2000 Blazer started throwing Service 4WD light intermittently. Couldn't pull scanner codes so I started punching buttons. Got stuck in 4LO, not good.
Replaced TCCM and 4-button selector switch, got to 2HI and left it alone. I live in Texas and I have only used 4wd once when I got stuck in mud.
Now the Service light still comes on, sometimes causing the switch lights to go out,. If or when the Service light goes out the switch lights work normally. Other times the Service light comes on and the switch lights work normally.
I checked the connector on the solenoid, looked fine, no corrosion.
Anybody have any ideas?

 

check that you are getting vacuum through the switch on the firewall , check that the terminal for the TCCM doesn't have corrosion on the wire ends......A better scanner is required to read the TCCM codes.