Being different from the traditional fault diagnosis method based on a gearbox’s vibration signals, a new test method with sensors installed directly on a gear inside a gearbox was proposed here. Thus, gear vibration signals’ amplitude attenuation was reduced effectively due to shortening vibration signals’ transmission path, early faults in gear transmission systems could be detected. In order to verify the effectiveness of the proposed method, a faulty gear transmission system with a 2mm crack on one of its gear tooth roots were simulated dynamically and tested, respectively. The results showed that vibration signals measured with embedded sensors can be used to detect effectively the early fault of this crack while vibration signals measured on this gearbox cannot reveal this crack fault; in fact, after vibration signals passing through a complex transmission path, their amplitudes are attenuated and parts of their frequency components are lost, so the weak fault information is diminished. This study provided an effective method for diagnosis of gearbox early weak faults.