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Hello. Welcome to Just Answer. Please allow me to assist you. The 4WD HI light flashing indicates that the system is not completing the necessary sequence to engage the 4WD. Thsi is most commonly caused by a faulty front axle disconnect unit. The system works as follows:
computer sends signal to front axle disconnect unit
disconnect unit energizes and connects the front axle
The disconnect device sends a signal to the computer indicating that the connection is made
The computer sends a signal to the transfer case shift motor to engage 4WD.
The shift motor moves to engage 4WD
If the light flashes, this indicates that the front axle disconnect unit did not send the "connection completed' signal to the computer.
Experience here: we usually replace the front axle disconnect unit to cure this.
Please confirm you have received my input; thanks