Well from experience 99%. Of the time its not the actuator if it shifts into 4 lo and back to 4 hi. The book really sucks at this diagnosis. And if lights on buttons blink for 2 hi for a bit then stay in 4wd then u have to listen very carefully. When shifting from 4 hi. To 4 low the whine or growling noise from motor is heard. If, when shifting from 4 hi to 2wd this motor noise is heard, then actoator or wiring is the issue. If the time while 2wd light is blinking, ie: its trying to shift, no noise from motor or actuator is heard, then shift module has a burnt output. Now. Before replacing any of the components, if you have a schematic of this system, you would be confused on how any of it elworks even if you are a seasoned professional. I wouldn\'t bother 2 much with it as long as you theory inspect Evey millimeter of all the wires from actuator connector (on transfercase) to where it goes through metal and enters occupant compartment. If all wires look good. Lights in all 3, or 4 if u were lucky in neutral which means error, of the lights work, and if requested position is depressed on switch and motor tries to turn or does and clynks are heard , then actuator position switch is stuck and actuator is needing replaced, as not a serviceable assesembly, and I haven\'t had the ability to tear one apart. But if all lights work and when the selected switch button , example 4 WD, 2 WD or 4 lo, and no motor noise like heard when other shift works, them module is to blame. Now, I am a master mechanic with most experience at a gm dealership, I have fought with this system multiple times, and once I employed this diagnosis it has never failed me again. And the stack of 4wd shift modules in my bottom toolbox dawyer. Most times if won\'t shit at all night be actuator but not likely, 95% of the time when this issue rolled through dealership, it took a module to fix it.
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