I found a solution:

/jqgridwiki/doku.php?id=wiki:toolbar_searching

this is the correct way to add a search toolbar:

$('#jsonmap').jqGrid('filterToolbar',{searchOnEnter:true});

this was the wrong way :

$('#t_jsonmap').height(25).jqGrid('filterGrid','jsonmap',{gridModel:true,gridToolbar:true});