Ajax function not returning any results

Hello everyone!

Im trying to create an ajax call when I click a link, the link is the post thumbnail, and the result should show the post content.

When I use admin-ajax.php it works fine, but the problem then is that the URL doesnt become accessable.

This is my ajaxcall

$.ajax({
			//url: admin_url +  '/wp-admin/admin-ajax.php',
			url: href + '?action=ajax',
			dataType: 'json',
			data: {
				//action: 'ajax',
				post_id: post_id
			},

			success: function(data) {
				console.log("Succes");
				row.before('<div id="case" class="twelve columns omega alpha"></div>');
				$("#case").slideUp(1);
				$("#case").html(data).slideDown(speed);

				$("#header, #featured, .row").animate({'opacity': halfOpacity}, speed);
				return false;
			},
			error: function() {
				console.log("Error");
				window.location = href;
			}
		});

And this is the php in the functions.php

function ajax() {

		global $wpdb;
		global $post;
		$id = $_GET['post_id'];

		$querystr = "
		    SELECT $wpdb->posts.*
		    FROM $wpdb->posts
		    WHERE $wpdb->posts.ID = $id
		";

		$post = $wpdb->get_results($querystr);

		$response = $post[0]->post_content;
		$response = json_encode($response);

		header( "Content-Type: application/json" );
		echo $response;
		exit;
	}