I have an 02 Suburban that my son recently bought and the 4wd worked fine when he bought it but not it will not shift into 4hi or 4lo. When you push the button for 4hi the 4hi led flashes but it never engages. I can hear the encoder motor run but I never hear the front axle actuator do anything. The axle actuator has 4 wires. 1 is ground, 1 has 12v when the ignition is on and another has 5 volts that goes to zero when I push the 4hi or 4lo button and the 4th wire has no voltage on it. Any help will be greatly appreciated.