The H3 has an Eaton E Locker. When 12V is applied through the locker switch it pops out locking pins into the matching holes in the locker side gear. If you turn on the switch the diff may need to rotate a bit before the pins will line up and insert in the holes. This is why when you try to test with the ruck in the air and not on the ground it may not be locked. When the dash switch lights, power is on for the circuit. So unless your connection at and in the diff is bad, of the wires between the diff and the switch are bad, it is working.

Here is a youtube vid for the Eaton

 

fantastic videos,i could watch all day. too bad the Toyota flik looked like the old black/white westerns with the wheels going in wrong directions. I guess technology still cant fix that. that one confused me and I know how a diff works

 

I forgot to add one thing, when i locked the diff I was prompt by the light and text, that the stabilization system is off.

 

That is good, it is supposed to do that.

 

OK, finally I've go the car. I did check You told me. First I use 4lo lock and made a turn, then I locked rear diff and made same turn again. The feeling was different, felt like lot more drag. In both cases marks on the gravel were similar but it's dark outside so wasn't sure.
Should be any sound of the rear locker engage/disengage?

 

the only time I hear and feel anything is going to low lock. nothing wth the diffs on/off

 

Couldn't you just do a burnout? Not locked rear will make one line from the tire locked you will have two lines from the tires. Should be easy to do a burn out in gravel.

 

H3 Hummers are constant 4x4, and I believe the system is designed to take power away from any wheel that breaks traction, should make doing a burn out quite difficult.

 

when you lock the diff it kills the traction control so theres no braking action. its not needed when in lock mode. still hard to burnout with big tires,small motor!

 

True with the motor and tires just thought since he was I. Gravel it might be a but easier.