Without the code that caused the service light to be on it's all guesswork.02 trailblazer LS 4wd.
Selector switch has been acting up. The other day, I needed to use 4wd for some light off roading but it wouldn't switch into 4hi or 4lo. After messing with the switch, all the switch lights went out, but I was still in 2wd.
For the past 2 days, the light(s) has been out on the Selector switch and service 4wd light was on. Today, I was at a stop light, went to accelerate and it decided to switch itself into 4lo!! The switch was on 2wd, I was driving, not in neutral nor was the parking brake set. Limped it home and now I'm confused. The Selector switch light(s) is on 4lo and the service 4wd light went away. Wtf.
When I try and switch to any other drive mode, I can hear the encoder motor or whatever it is, trying to make the switch but it fails. I pulled the #8 fuse, unhooked the battery, and let it sit for 20 minutes. No luck. I am now in the process of replacing the Selector switch but the fact that the lights are working and it tries to shift into whatever mode I select, tells me it's no the issue. Also, I've already replaced the tccm with one from an 06 trailblazer.
Any tips would be much helpful since this is my only road legal vehicle atm.
Things that are helpful to know:
The indicator lights are not a function of the switch position. Those indicator lights are independently wired to the TCCM and it is the TCCM that lights them according to what the range position sensor (encoder) reports to the TCCM.
Removing the ATC fuse for a minute or so resets the TCCM and clears the code and service light but if the fault is still present the TCCM will turn the service light back on and set a code right away.
For certain codes like an encoder fault the TCCM may turn off all indicator lights and disable ALL shifting of the transfer case.
Both the selector switch and the range position sensor work in a similar manner. The TCCM gives the sensor or switch 5 volts and the sensor or switch sends back a lower voltage that corresponds to given switch setting or transfer case shift motor position.
If the range position sensor (encoder) is faulty there is no way for the TCCM to know what mode the transfer case is really in. This 'could' cause the TCCM to rotate the shift motor in manner it should not ever do, like shifting into 4lo while moving and not in neutral.