Herein disclosed is a revolution signal processing system for an electronic controller to ensure safe operation of a vehicle having an automatic transmission. The system includes: an input shaft revolution sensor for outputting an input shaft r.p.m. signal; a vehicle speed sensor for outputting a vehicle speed signal; signal processors for determining the r.p.m.'s of the two sensors in response to the two output signals of the same; a gear position sensor for outputting a gear position signal; a gear position detector for detecting the gear position of the automatic transmission; and a self-testing backup. When the gear position detector detects that the transmission is in a gear position other than the neutral position, and if one of the output signals of the two sensors exhibits an r.p.m. equal to or higher than a predetermined value, whereas the other of the same exhibits an abnormally lower value or an r.p.m. lower than the predetermined value, then the self-testing back up determines that the sensor having the output signal exhibiting the abnormally low value is malfunctioning. Thus, the revolution signal processing system self-tests and backs up the vehicle speed sensor and the input shaft revolution sensor.