Diagnostic system for rotational speed sensors in drive train of four wheels drive vehicle having central differential device

Claim 1

In a four wheels drive vehicle comprising an engine, a transmission, a front wheel drive shaft, a rear wheel drive shaft, a central differential device having an input rotational member adapted to be driven by said engine through said transmission and two output rotational members adapted to drive said front wheel drive shaft and said rear wheel drive shaft, respectively, under distribution of rotational power supplied to said input rotational member between said two output rotational members, a first rotational speed sensor for detecting rotational speed of one of said two output rotational members of said central differential device, and a second rotational speed sensor for detecting rotational speed of either said input rotational member or the other of said two output rotational members of said central differential device, a diagnostic system for detecting failure of at least said first rotational speed sensor, comprising: a first means for comparing an output signal of said first rotational speed sensor with an output signal of said second rotational speed sensor to detect a first condition that the rotational speed detected by said first rotational speed sensor is lower than the rotational speed detected by said second rotational speed sensor, and a second means operable when said first condition was detected to detect if the rotational speed detected by said first rotational speed sensor is below a threshold value therefor while the rotational speed detected by said second rotational speed sensor is above a threshold value therefor over a first predetermined period so as thereby to determine a failure of said first rotational speed sensor.