I recently experienced a problem with the 4 x 4 of my 2007 Ford F150. The circuit is pretty simple. Basically the 4 x 4 is activated by a dashboard switch. When switched into 4 HI, the light should illuminate and shift into 4 wheel drive. In my case the light would not illuminate and when switched quickly into 4 Hi, it flashed. When the switch is activated it activates a solenoid located on the firewall just behind the battery on the passenger side of the vehicle. This solenoid is connected to tubing and has one connector. After replacing this solenoid, my 4 wheel drive worked.