Thermo Unit 688-83591-00-00

For the Yamaha thermo unit (688-83591-00-00), which is typically a temperature sensor or thermoswitch, the two wires generally serve different purposes: Black wire: Yes, it is commonly the ground wire. Light blue wire: This is typically the signal wire that sends the temperature data or activates a warning light. The black wire should be grounded, while the light blue wire connects to the input of the temperature gauge or warning system in the dashboard or control panel. You can always confirm by checking a Yamaha wiring diagram for your specific model, but in most cases, the setup follows this standard.