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Long ago my 2003 GMC Envoy wound up stuck in 4LO randomly. The selector was set to 2HI but you could tell it was 4LO. The Transfer case motor was trying to switch ranges but was audibly struggling as if it was locked up. Long story short, I pulled the motor off and manually adjusted the splines into the correct range. Been in 2HI ever since. While doing this I replaced the Transfer Case motor and TCCM under the dash from a junked out 04 trailblazer.

The issue: On startup all lights for the 4WD selector switch flash but after that they're good as dead. Just a service 4WD light on the Instrument Cluster. They don't respond to any input. No noises or anything. I've disconnected the battery and left the ignition on for a few minutes to try and reset the computer. No good. Pulled the ATC fuse (fuse #8) a few times. Also, no results.

I'm not sure what else to try, winter is coming up and having 4wd would be optimal.
 

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2004 gmc envoy_slt
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Are you saying that the selector light situation started right after you installed the used TCCM? If so, you may have picked a bad TCCM. There could also be a wiring issue that is not allowing the TCCM to wake up correctly. I think this problem may be too difficult to diagnose remotely.
 

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2002 chevy trailblazer_lt
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but after that they're good as dead.

If this means they all go out then I would expect there is a permanent fault in the system. With such a fault the TCCM will disable the system and not allow any shifts. I would read the TCCM trouble code(s) with an OBD adapter and the Android App "Car Gauge Pro".
 
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