Quoted from Jakers:Also, the diagnostics botton on the inside of the coin door does nothing.
Try shorting the tabs on the switch. If that works, the internals of the switch may be corroded and you might need to replace it. Pbresource has them. Cherry e69 switch, if I remember right.
[edit]: http://www.pinwiki.com//wiki/index.php?title=Gottlieb_System_1#Accessing_Bookkeeping_and_Diagnostics
E69-00A; with links to jameco, digikey, and mouser.