Auto Parts: 4x4, manual hubs, auto 4x4

Question
I have a 2001 F250 with a 7.3 liter diesel that has auto 4x4 and it won't go into 4x4 when I turn the switch on. I have also tried the Manuel hubs and it still won't work. What else can I do to fix this.

Answer
    There are a lot of possibilities here, but one that is very common.  Firstly, if you have manual hubs, you must always engage them before turning on the 4WD.  If you operate the 4WD with the hubs disengaged, you can screw up the transfer case.  There is a motor, mounted on the transfer case, that engages the 4WD.  They fail all the time, and most big parts stores carry them or can get them in a day or so.  This is the most likely cause of failure, I used to sell a lot of these.  But, before you buy a new one, make sure that the fuse or the relay that operates that circuit isn't bad.  Just check at the motor with a voltmeter while someone turns the switch on and off in the cab (with the ignition on and the engine not running).  If you don't get any voltage, then it's a switch or the relay, if you do see 12V, then the motor needs to get replaced.