Problem with actionsavecart hook updating carrier

[lfelisiak]

i'm dealing with a problem, I need to change the carrier if the cart amount of product with certain tags  is > 500 , so , I'm hooking actionCartSave and checking the amount , but , when I do $cart->id_carrier=(int)$carrier_id; $cart->update(); The ajax stops responding and the request remains in pending... like in a loop, then i need to reload the page to see if a product has been added to the cart, but if i remove the $cart->update() the ajax request is successful but , the carrier does not get updated. How can i solve this?