Abstract: An improved diagnostic control method for a motor vehicle powertrain including an electronically controlled multi-range transmission and a multi-range transfer case, wherein a default determination of transfer case range enables reliable evaluation of transmission ratio, for powertrain configurations with or without a transfer case range indicator switch. An overall speed ratio is computed according to a ratio of the transmission input speed and the transfer case output speed, and the computed ratio is monitored to identify a period of stable operation at the conclusion of transmission shifting and in steady state operation during which the rate of change of the computed ratio is stable over a predefined interval. If the computed ratio is also deemed to be stable, it is compared with an expected overall ratio based on an assumed speed range (low or high) of the transfer case.