This invention relates to a monitoring system for a hydraulic system and, in particular, a hydraulically actuated and lubricated vehicle transmission.
It is now possible, due to microprocessor technology, to monitor various vehicle operating parameters according to complex criteria. For example, a clutch monitoring system is described in copending U.S. application, Ser. No. 378,703, filed May 17, 1982, a filter monitoring system is described in copending U.S. application, Ser. No. 378,703, filed May 17, 1982 and an engine oil pressure monitoring system is described in copending U.S. application, Ser. No. 378,704, filed May 17, 1982, all assigned to the assignee of the present invention. However, none of these monitoring systems provide an indication of the condition of the hydraulic fluids in a hydraulic-assist type transmission. Heretofore, typical vehicle function monitoring systems have included sensors, such as pressure-actuated switches, which energize warning devices when undesirable conditions, such as low hydraulic pressures, occur in the function being monitored. The electrical circuits of such monitoring systems have often been coupled to the vehicle ignition switch so that warning signals cannot be generated unless the ignition switch is in a certain condition. However, such systems can produce misleading warning signals when low system pressures are caused merely by low engine speeds and not by any system failure, such as leakage or low fluid quantities. Furthermore, such systems have not been compensated for temperature changes in the fluid being monitored.