The solenoid on a KZ900 has a black wire hanging off which is a ground that connects to the ground wire on the regulator. Some Suzuki GS models use the same solenoid which bolts up directly but doesn't have the black ground wire. The circuit grounds through the starter bolts I think.
Don't want to create confusion but the trigger wire is black as well. Not sure about the solenoid on a GPz900 but I suspect that it is very similar to the solenoid on the KZ900.
