what requirements must be met in order for the transfer case motor to engage the 4x4
The only thing that can even possibly cause a problem is engine temp not within specs, but the most common failure is the transfer case motor, lightly tap on the motor with a small hammer while someone pushes the 4x4 button, if the motor runs then you have found your problem, replace the shift motor and that should fix it. If the motor won't then you will have to run a systems circuit test on the 4x4 module assembly.
remove shift relay block under hood, look for corroded wires to UNDERSIDE of relay sockets. common issue, under reported. in mean time, take cover off two relays, use your finger to close relay contacts, shift should work fine. what happens is open condition closes when downward pressure from volt/ohm meter is used to check continuity......then reopens when pressure is removed, drives you batty.
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