function lista_modelos( valor ) {
	http.open("GET", "modelos.php?marca=" + valor, true);
	http.onreadystatechange = handleHttpResponse;
	http.send(null);
}

function lista_valores( valor ) {
	http.open("GET", "valores.php?valor=" + valor, true);
	http.onreadystatechange = handleHttpResponse;
	http.send(null);
}

function handleHttpResponse() {
	campo_select = document.forms[0].modelo;
	if (http.readyState == 4) {
		campo_select.options.length = 0;
		results = http.responseText.split(",");
		for( i = 0; i < results.length; i++ ) {
			string = results[i].split( "|" );
			campo_select.options[i] = new Option( string[0], string[1] );
		}
	}
}

function getHTTPObject() {
	var req;
	try {
		if (window.XMLHttpRequest) {
			req = new XMLHttpRequest();
			if (req.readyState == null) {
				req.readyState = 1;
				req.addEventListener("load", function () {
					req.readyState = 4;
					if (typeof req.onReadyStateChange == "function")
						req.onReadyStateChange();
					}, false);
				}
				return req;
			}
			if (window.ActiveXObject) {
				var prefixes = ["MSXML2", "Microsoft"	, "MSXML", "MSXML3"];
				for (var i = 0; i < prefixes.length; i++) {
					try {
						req = new ActiveXObject(prefixes[i] + ".XmlHttp");
						return req;
					} catch (ex) {};
			}
		}
	} catch (ex) {}
	alert("XmlHttp Objects not supported by client browser");
}
var http = getHTTPObject();
// Logo após fazer a verificação, é chamada a função e passada
// o valor à variável global http.