How would I Limit the position of the Z axis Transform?

Hello:

  I am creating a 2dUnity Game. My players z position keeps increasing when the game starts and I don't know why .My player falls through the background when his Z transform is above 48, how would I be able to limit the z max position? Thankyou

using UnityEngine;

using System.Collections;

 public class carController : MonoBehaviour {
     Vector3 position;
     public float Speed  = 2f;
     public float maxPos = 5.0f;
 
 
 
     void Start()     {
 
     }
 
     void Update () 
     {
 
         transform.Translate (Input.acceleration.x,0, -Input.acceleration.x);
         var pos = transform.position;
         pos.x = Mathf.Clamp (transform.position.x, -5.0f, 5.0f);
         transform.position = pos;
 
     }
 
 
         
     }