Ajax only works on localhost

Hello, guys, my ajax only works on localhost, what am I missing? Thank you.

Ajax.js

var url = "http://localhost/sistema/";

function criaAJAX() {
    if (window.XMLHttpRequest)
    {// Codigo para IE7+, Firefox, Chrome, Opera, Safari
        return new XMLHttpRequest();
    } else
    {// Codigo para IE6, IE5
        return new ActiveXObject("Microsoft.XMLHTTP");
    }
}
;

Non-html script ...

<script type="text/javascript">
    $('#search-usuarios').keyup(function () {
        var chars = (this.value);
        $.post(url + 'Ajax/busca-usuarios', {val: chars}, function (busca) {
            $('#content-left').html(busca);
        });
    });
</script>