transform.TransformDirection problem .

function Update () {

          moveMent_Direction=Vector3.zero;
         
         if(char_Controller.isGrounded){    
             anim_Controller.SetBool("jump",false);
             if(Input.GetAxis("Vertical")>0.1){
                currentSpeed=Input.GetAxis("Vertical")*topSpeed;
                anim_Controller.SetFloat("speed",currentSpeed);
                moveMent_Direction= new Vector3(Input.GetAxis("Horizontal"),0,currentSpeed);                   
             }else{               
                currentSpeed=0.0f;
                moveMent_Direction=Vector3(0,0,currentSpeed);
                anim_Controller.SetFloat("speed",currentSpeed);                   
             }
 
            if(Input.GetKey("1")){               
               // moveMent_Direction.y=7;
                  check_Ob();
                  anim_Controller.SetBool("jump",true);
            }
        }

      moveMent_Direction=transform.TransformDirection(moveMent_Direction);
     moveMent_Direction.y-=10*Time.deltaTime;    
     char_Controller.Move(moveMent_Direction*Time.deltaTime);
 

}

Problem is , if i add moveMent_Direction=transform.TransformDirection(moveMent_Direction); , then my character don't move.and stays at that same position . And one thing more , You notice that i am making moveMent_Direction=Vector3.zero; moneMent_Direction to 0 at every update call . Because of this , gravity functionality if affecting . Character goes down very slowly . Please tell me what i am doing wrong .