Sprinting animation

I have this script for sprinting and crouching and I want it to play a sprinting animation when you hold shift.
but it doesn’t play the animation and I clip through the floor when I press shift.

    var walkSpeed : float = 7; //Regular speed
    var crouchSpeed : float = 3; //Speed while crouching
    var sprintSpeed : float = 13; // Speed while sprinting
    private var charMotor : CharacterMotor;
    private var charController : CharacterController;
    private var theTransform : Transform;
    private var charHeight : float; //Initial height
    function Start () 
    	charMotor = GetComponent(CharacterMotor);
    	theTransform = transform;
    	charController = GetComponent(CharacterController);
    	charHeight = charController.height;
    function Update () 
    	var h = charHeight;
    	var speed = walkSpeed;
    	if (charController.isGrounded && Input.GetKey("left shift") || Input.GetKey("right shift"))
    		speed = sprintSpeed;
    	if (Input.GetKey("c"))
    		h = charHeight*0.5;
    		speed = crouchSpeed;
    	charMotor.movement.maxForwardSpeed = speed; // Setting the max speed
    	var lastHeight = charController.height; //Stand up/crouch smoothly
    	charController.height = Mathf.Lerp(charController.height, h, 5*Time.deltaTime);
    	theTransform.position.y += (charController.height-lastHeight)/2; //Fix vertical position

GetKey(“left shift”)

will be calling for a key binding called “left shift” in your custom settings. If that isn’t set, and you don’t want to bother doing it, you can use