There are two motors under the
vehicle.
The front axle actuator moves ONLY during 2HI->A4WD and A4WD->2HI
transitions. The transfer case encoder motor makes a lower pitched
noise in every mode transition: 2HI->A4WD, A4WD->4HI, and
4HI->4LO, and the three transitions upwards.
If EITHER of the motors fails to do what the TCCM commands it to do, the switch light will flash and the Service
4WD light on the dash will come on.
If you're a DIY mechanic, you might look into them
further, or just take it to a mechanic who knows our systems. The
average mechanic is not going to know the theory of operation of the trailblazer/envoy 4x4 system, which is pretty
unique in the industry
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