As the importantelementin the automobile transmission system

the quality detection ofmain reducer is very important.The vibration model for gearmesh of main reducer is established

and the vibration response from fault gearmeshing is discussed.Based on Hilbert transmission demodulation analysis and EMD（Empirical Mode Decomposition）

the IMF（Intrinsic Mode Function）of gear vibration signals is got

and then the gear faults based on analysis of envelope-spectra of the signals are suggested.The experimental application to gear fault diagnosis shows that the tooth surface damage detection for automobile main reducer based on EMD and Hilbert analysis is effective to production quality control.