I tried you're example but no luck so far.<br>
I've added the strategie;<br>
<pre>kml_layer = new OpenLayers.Layer.Vector("KML", {
        projection: map.displayProjection,
        strategies: [new OpenLayers.Strategy.Fixed()], 
        styleMap: myClusterStyleMap,                            
        protocol: new OpenLayers.Protocol.HTTP({
                url: filename,
                format: new OpenLayers.Format.KML({
                        extractStyles: true,
                        extractAttributes: true,
                        maxDepth: 3                                     
                        })
        })
});</pre>And my style map;<br><br>
<pre>     var myClusterStyleMap = OpenLayers.StyleMap(
        { 
                        'default': new OpenLayers.Style(
                        { 
                                externalGraphic: '${myGrafic}', 
                                label: '${count}'
                        }, 
                        { 
                                context: 
                                { 
                                        count: function(feature) 
                                        { 
                                                if (feature.cluster) 
                                                { // is `.cluster` the array of clustered features 
                                                        return feature.cluster.length; // or feature.attributes.count, is the same 
                                                } else 
                                                { // is not clustered 
                                                        return ''; // no label 
                                                } 
                                        }, 
                                        myGrafic: function(feature) 
                                        {                                                       
                                                if (feature.cluster) 
                                                { 
                                                        return 'OpenLayers-2.12/img/marker.png'; 
                                                } else { 
                                                        return 'OpenLayers-2.12/img/marker-green.png'; 
                                                } 
                                        }
                                } 
                        }  
        }); </pre>

        
        
        
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