Diagnostic device for a multi-voltage vehicle electrical system, a multi-voltage vehicle electrical system, and a method for operating a multi-voltage vehicle electrical system

A diagnostic device for a multi-voltage electrical system of a motor vehicle having an internal combustion engine, the multi-voltage vehicle electrical system including a first subnetwork acted on by a first voltage and at least one other subnetwork acted on by at least one other voltage, the first voltage being higher than the at least one other voltage, and the first subnetwork and the at least one other subnetwork having a shared first ground connection, the diagnostic device comprising:

• at least one second ground connection;
• and
• processing circuitry that is programmed to perform a diagnostic function, wherein:
• the diagnostic function includes diagnosing a disturbance in at least one of the shared first ground connection and the at least one second ground connection based on (a) an obtained measurement of a current flow via the at least one second ground connection, and (b) at least one operating state of the multi-voltage vehicle electrical system;
• and
• the at least one second ground connection and the shared first ground connection are connected in parallel such that currents flowing via a starter situated in the second subnetwork and a generator situated in the first subnetwork flow via the shared first ground connection and the at least one second ground connection when the internal combustion engine is started under the conditions of the shared first ground connection being undisrupted and the at least one second ground connection being undisrupted.