Output speed sensor diagnostic control for a motor vehicle transmission

In a motor vehicle including an engine drivingly connected to an input shaft of a power transmission controllable to selectively establish one of a plurality of speed ratios between said input shaft and an output shaft thereof, where shifting among such speed ratios is scheduled in accordance with an output speed signal generated by a transmission output shaft speed sensor, a method of operation for diagnosing an apparent loss of the output speed signal, comprising the steps of:

• detecting the occurrence of an apparent failure condition in which the output speed signal indicates that the vehicle is at rest, but the transmission input shaft speed is substantially in excess of an idle speed of the engine;
• and in response to such detection;
• initiating upshifting of the transmission to successively higher speed ratios while monitoring the speed of the transmission input shaft to detect the occurrence of an upshift-related reduction thereof;
• and
• indicating the occurrence of (1) a total transmission speed ratio failure if no shift-related reduction of the transmission input shaft speed is detected by the time the transmission is upshifted to the highest available speed ratio, (2) a partial transmission speed ratio failure in response to the detection of a shift-related reduction of the transmission input shaft speed, if an output speed signal indicative of vehicle movement is observed, and (3) a speed sensor related failure in response to the detection of a shift-related reduction of the transmission input shaft speed, if the output speed signal indicates that the vehicle is at rest and the input shaft speed is sufficiently great to produce vehicle movement.