function ajaxobj(){
  try {
    salida = new XMLHttpRequest();
  } 
  catch (trymicrosoft) {
    try {
      salida = new ActiveXObject("Msxml2.XMLHTTP");
    } 
    catch (othermicrosoft) {
      try {
        salida = new ActiveXObject("Microsoft.XMLHTTP");
      } 
      catch (failed) {
        salida = false;
      }
    }
  }
    
  return salida;
}


function cargaModelos(id_moto, combo){
  var cadena = new String();
    
  var ajax = ajaxobj();
  ajax.open("GET", "cbomodelos.php?id_moto=" + id_moto, true);
    
  ajax.onreadystatechange = function(){
    if (ajax.readyState == 4) {
      cadena = ajax.responseText;
            
      var elemento = document.getElementById(combo);
      elemento.style.display = 'inline';
      elemento.length = 0;
      elemento.options[elemento.length] = new Option('', 0);
          
      modelos = eval('(' + cadena + ')');
        
      for (var clave in modelos) {
        elemento.options[elemento.length] = new Option(modelos[clave], clave);
      }
    }
  }
    
  ajax.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
  ajax.send(null);
}