4WD how to turn it on on 2006 Mercury Mariner

I am trying to use the 4WD in my 2006 Mercury Mariner and am not doing it correctly. Please give instructions on how to use 4WD

Asked by for the 2006 Mercury Mariner
The 4wd is automatic. The truck is normally front wheel drive. The rear diff will kick in when the front wheels start slipping. It is also gradual, as the more the front wheels slip, the more torque will be sent to the rear wheels. They should have put an override switch on it for full time AWD.
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put in fire order an set timming