Looking for 4x4 help on my 02 s 10 zr2
 

i have an 02 s10 with 4x4 the problem i am have is there is a clicking noise coming from the front diff. all the time...... the 4x4 acutator has vaccuum all the time! locking the front diff is this how it works r dose some thing tell the acutator to have vacuum w/i push the 4x4 button? i can feel the clicking in the actuator. it made a load pop one day then this all started can anyone help?

does the 4x4 engage and disengage?