how to change an engine class in 1.8

Hello i have to change ElggPAM with myne i had it in my plugin start.php page, elgg_register_class("ElggPAM",$CONFIG->pluginspath ."myplugin/classes/ElggPAM.php");
But in the file sessions.php the line with $pam => new(ElggPAM) returns an error sayiong that ElggPAM doesn't exist.
Have i forgotten something in my styart.php?
Thanks for your help