Ajax response text correct but not displaying - ajax

Question

i tried finding this problem using google but no luck so far , so im asking for some help.
im working with a webpage on JSP and im having a little problem with ajax responseText, im testing the page on firefox and using firebug.
the problem its that the operation its completed correctly and the response text show ok on the firebug console, but it doesnt display on the <div> i assigned.
when i put an alert() inside the function handling the onreadystatechange it displays correctly as if the response was just too late, so im a bit lost.
can anyone give me an idea?
my JS handling the submit is
function validate( form ) {
var expresion = /[\b\W\d]+/;
var error=0;
var nombre = document.getElementById("Addname");
var descripcion = document.getElementById("Adddescripcion");
if(nombre.value == "" ){
nombre.focus();
nombre.style.backgroundColor="red";
alert("Los Campos no deben estar vacios");
error = error+1;
}else{
if(!(expresion.test(nombre.value))){
var temp = nombre.value.toUpperCase();
nombre.value = temp;
}else{
nombre.focus();
nombre.style.backgroundColor="red";
alert("La entrada no es valida");
error = error+1;
}
}
if( descripcion.value == "" ){
descripcion.focus();
descripcion.style.backgroundColor="red";
alert("Los Campos no deben estar vacios");
error = error+1;
}
if(error == 0){
var url = "addCarrera";
var params = "name="+ nombre.value +"&descripcion="+descripcion.value;
ajaxPost(url,params,"addres");
document.getElementById("addres").setAttribute("class","visible");
clearing();
}
}
function clearing(){
var nombre = document.getElementById("Addname");
var descripcion = document.getElementById("Adddescripcion");
nombre.value="";
descripcion.value="";
nombre.style.backgroundColor="white";
descripcion.style.backgroundColor="white";
var timer = setTimeout("ocultar()", 1500);
}
an my code handling the Ajax request its
var ajaxCall;
function ajax_request() {
if(window.XMLHttpRequest){
//codigo para IE7+,Firefox,Chrome, Opera,Safari
ajaxCall=new XMLHttpRequest();
}else{
//codigo para IE6 y 5
ajaxCall=new ActiveXObject("Microsoft.XMLHTTP");
}
}
function ajaxPost(url ,params, respuesta){
ajax_request();
ajaxCall.onreadystatechange = statechangeHandle(respuesta);
ajaxCall.open("POST",url,true);
ajaxCall.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
ajaxCall.setRequestHeader("Content-length", params.length);
ajaxCall.setRequestHeader("Connection", "close");
ajaxCall.send(params);
//alert("respuesta? "+respuesta);
}
function statechangeHandle(text){
//alert("text?"+text);
//alert("ready?"+ajaxCall.readyState);
if(ajaxCall.readyState == 4 && ajaxCall.status === 200){
//alert("prueba");
document.getElementById(text).innerHTML=ajaxCall.responseText;
//alert(ajaxCall.responseText);
}
}
the html part
<h3>Agregar Carrera</h3>
<table>
<tr>
<td>Nombre:</td>
<td><input type="text" id="Addname" size="20" /></td>
</tr>
<tr>
<td>Descripcion:</td>
<td><textarea rows="2" cols="20" id="Adddescripcion" > </textarea></td>
</tr>
<tr>
<td><input type="button" value="Agregar" onclick="validate()"></td>
</tr>
</table>
<div class="ocultar" id="addres"></div>
thanks in advance.
by the way ,the ajax at ajaxPost doesnt show a thing but for example at statechangeHandle if active te alert before the asign the responseText the html its modified correctly but if not then the div stays the same, im not quite sure what that means, its like it has to wait i dont know if im just doing something really wrong