Needing a little help.

I have a 05, 2500 duramax, my first diesel and my first 4x4 so when the 4x4 stopped working it had me wondering.

It is a 3 push button unit, when I switch from 2 > 4H the light blinks then stays on 4H but does not engage. When I go to 4 L from either 2 or 4H the light just keeps blinking, there is no indicator light that comes on in the instrument panel.

I have cleaned up the grounds under the driver’s side floor and have also taken out the actuator motor, checked for power and ground and found them to be good.

Pushed the 4H button while it was hanging down and the motor energizes and plunges out and in, testing 4L it does nothing.

I have tried to engage the 4H at low speeds and you can hear a shrill type sound, at no point did I notice any kind of noise or bang, it just stopped working.

I am fairly capable of pulling a wrench but just don’t know where to go next.

Anybody able to help ?

Thanks

 

I have heard of this before and my truck also has 4x4 problems, I bet you it is the encoder motor on the transfer case, when you try 4x4 low, does it not go into the 'granny' gears? Like 2wd granny gears as the 4x4 doesnt work? When the truck is on jack stands and in 4x4 does the front drive shaft spin when the truck is in drive?