OBSERVER FOR TRANSFER CASE FAULT DETECTION IN A FOUR-WHEEL DRIVE SYSTEM

A system and method are provided for detecting faults in an encoder used for position feedback as part of a transfer case in an electronic-shift 4.times.4 vehicle. Encoder signal failures including open-circuit, short-to-ground, and short-to-power faults and encoder power line failure with open-circuit or short-to-ground fault are detected. The methodology ensures that no fault is set due to transients seen immediately after the power-set up in the system or after turning a relay off at the end of a shift. If the relays are off and an intermittent physical fault changes the encoder code, the fault flag is reset by the methodology when the fault disappears. The physical encoder faults which are transparent from the encoder code in stationary motor are detected by an intelligent recursive index-based algorithm which works when a shift is commanded. The index is increased geometrically when successive invalid codes are received, is increased by a fixed parameter when an out-of-sequence code is received, and is decreased by another fixed value when a proper anticipated code is received. This algorithm is designed and tuned to filter out quick, transient failures, and sets the fault flag when the severity of the failure warrants.