I know that when installing starter solenoids in Fords and Chevys it's an easy thing to overtighten the solenoid to starter wire as the studs are so large it seems as they need to be torqued but the size of them is actually for the purpose of electrical continuity. When the nuts holding the wire on these solenoids are torqued excessively, their internals bend to the point of contact and therefore negates the ability to switch off when the relay is open. The fix is to buy another relay and when tightening the connections, do so carefully.