How can I retrieve a response page from an ajax object ?

Hello, volks...

I haven't suceed to open my "test page" yet. I'm using Joomla 1.5...Here goes the details:
In the body of my article there's a form which should connect my mySql DB. My template
is configured to receive contents inside the central <div>.
The submit of my form calls openPage1("TESTE1.php")
Here's the function :

<script type="text/javascript">

function openPage1(value) {
var url = value;
var ajax = new XMLHttpRequest();
var conv = document.getElementById("slcConv");
var financ = document.getElementById("slcFinanc");
var frmContr = document.getElementById("slcFrmContr");
var prazo = document.getElementById("slcPrazo");

ajax.open("POST","url?$slcConv="+conv+"$slcFinanc="+financ+
"$slcFrmContr="+frmContr+ "$slcPrazo="+prazo);


ajax.onreadystatechange = function() {

document.getElementById('content').innerHTML = ajax.responseText;
alert ("ALL RIGHT");
}
ajax.send(null);
}

Piece of code from the file "TESTE1.php", which receives the parameters from the ajax object:

<?php
$conv = $_POST["slcConv"];
$financ = $_POST["slcFinanc"];
$frmContr = $_POST["slcFrmContr"];
$prazo = $_POST["slcPrazo"];
echo $financ;
echo $frmContr;
echo $prazo;
?>

He presents the "ALL RIGHT" message, but returns the home content inside the central <div>, not the content from the "TESTE1.php" page.
What's wrong ? Where can I define the "content" <div> ?
Another thing: If I use the conditional if(ajax.readyState == 4 && ajax.status == 200) he only returns the home content, without the alert.

Thanks