jQuery Grid Plugin - jqGrid

Hi, firstly congratulation for the plugin and the community.

I am using Codeigniter 1.7.2 and jqGrid, but I have a problem. I also say that after reading the wiki again and again I have not been able to find a solution for my json data. The respond that I get is OK, using Firebug is the following:

POST:

RESPOND:

[{"idpieza":"1","descripcion":"Primera pieza de mi vida",
"fechaAlta":"2008-12-31 23:59:59","fechaMod":"2009-01-31 23:59:59",
"vista":"1","vertical":"0","width":"500","height":"400","top":"10",
"left":"20","color":"blue","background":"#efefef","border":"4px solid #A9A9A9",
"bloque_background":"#fff","usuario":"0"},{"idpieza":"2","descripcion":"Segundona",
"fechaAlta":"2009-02-27 23:59:59","fechaMod":"2009-11-06 23:59:59","vista":"1",
"vertical":"1","width":"600","height":"600","top":"500","left":"300","color":"red",
"background":"white","border":"1px solid green","bloque_background":"white","usuario":"0"},........

CODE:

     jQuery("#lista").jqGrid({
            ajaxGridOptions : {type:"POST"},
            serializeGridData : function(postdata) {
                postdata.page = 1;
                return postdata;
            },
            url:'piezas/cristina',
            datatype: "json",
            colNames:['ID','Descripcion', 'fechaAlta', 'fechaMod','vista','Vertical','width',
                'height','top','left','color','background','border','bloque_background'],
            colModel:[
                {name:'idpieza',index:'idpieza', width:55},
                {name:'descripcion',index:'descripcion', width:90},
                {name:'fechaAlta',index:'fechaAlta', width:100},
                {name:'fechaMod',index:'fechaMod', width:80, align:"right"},
                {name:'vista',index:'vista', width:80, align:"right"},
                {name:'vertical',index:'vertical', width:80,align:"right"},
                {name:'width',index:'width', width:150, sortable:false},
                {name:'height',index:'height', width:150, sortable:false},
                {name:'top',index:'top', width:150, sortable:false},
                {name:'left',index:'left', width:150, sortable:false},
                {name:'color',index:'color', width:150, sortable:false},
                {name:'background',index:'background', width:150, sortable:false},
                {name:'border',index:'border', width:150, sortable:false},
                {name:'bloque_background',index:'bloque_background', width:150, sortable:false}
            ],
            rowNum:20,
            width:900,
            rowList:[10,20,30],
            pager: '#paginacion',
            sortname: 'idpieza',
            viewrecords: true,
            sortorder: "desc",
            caption:"Piezas registradas en el sistema por el usuario"
        });
        jQuery("#pjqgajax").jqGrid('navGrid','#pjqgajax',{edit:false,add:false,del:false});

>I know that maybe this json is not available for jqGrid, but i am trying all possibilities. I tried different versions of the jsonReader but not successfully.

So if something is wrong, can anybody help me?

Thanks indeed.