Can you run either call in the browser?

ajax.php?id=3&val=10 OR index.php/controller/action/id/3/val/12 ?

In other words, can the script that will be returning data to your ajax call be run in the browser, and give you the intended result? I'd debug the PHP script you're calling with ajax first. If you can get this working, then take a look at it when its called via AJAX.