2005 Sierra 5.3....I can hear it when i hit the buttons for 4x4 but tried it out and back wheels are only active....4 low runs in low gear but still no action from front wheels...Could this be the actuator? What could be my other issues? Thanks

 

I can tell you clearly how the circuit for a 1988 works. I'm going to assume that you have a electrically actuated transfer; rotate the selector on the dash and you go in to 4L, 4H, or 2wd. What should be happening is that when you select 4L or H the signal 1st goes to transfer to select the correct L or H range. As that transfer gear range is selected another signal is sent to the IFS 4WD actuator causing it to engage the IFS for 4WD operation. As you would expect, the IFS actuator (some call it a thermal switch) only engages the IFS axles for 4WD. The IFS actuator may have one or two connectors with between 2 and 4 wires. On my 88 I have a manual transfer lever. Once I move that it indicates 4L or 4H only after the IFS actuator successfully engages.

Grab your voltage tester (multimeter) and check for voltage at the transfer when the 4WD selector is in the various positions. You can also test voltage at the IFS. If there is someone you trust near that will help you, lay under the truck and have them select 4L then 4H. You should at least feel the IFS actuator attempt to work. You can even remove the actuator and test it there.

Too bad we don't have a section or "sticky's" where electrical schematics are stored.