Ajax in subsite

I have the Ajax content working in a sub-site set up as sub-directory. When I change to domain mapping the Ajax stops working.

The following code is in my sub-site child theme header.php:

jQuery.ajax({
     url:"<?php bloginfo('url')?>/wp-admin/admin-ajax.php",
     type:'POST',
     data:'action=my_special_action&main_postid=' + mainPostID,
     dataType: 'html',
     beforeSend: function() {
         jQuery('#test').html('<img src="<?php echo get_stylesheet_directory_uri(); ?>/images/ajax-loading.gif" alt="pagina is aan het laden..." />');
             if(!mainPostID) {
                 $('#test').html('<p>Oops! There is no data...</p>');
                 return false;
             }
     },
     success:function(result) {
     // alert(result);
     jQuery("#test").html(result);
     }

 });

It probably has something to do with the call to admin-ajax.php.
So I tried using the full domain of the main website, that did not work.
I also used the domain of the sub-website. That did not work.

Hope someone can point me into the right direction.