Abstract: A microcomputer-based electronic control system for a full time four wheel drive torque transfer case monitors the relative slip between the front and rear output shafts of the transfer case, and generates a signal to engage an electromagnetic clutch for a predetermined time period in the event a first predetermined slip threshold is exceeded. In the event the driver has actuated the vehicle service brakes, the electromagnetic clutch is only engaged if the relative slip is less than a second predetermined slip threshold greater than the first predetermined slip threshold.

Abstract: A microcomputer-based electronic control system for a full time four wheel drive torque transfer case monitors the relative slip between the front and rear output shafts of the transfer case, and generates a signal to engage an electromagnetic clutch for a predetermined time period in the event a predetermined slip threshold is exceeded. Below a predetermined vehicle road speed, the electronic control utilizes a pulse counting technique to determine whether or not an excessive slip condition occurs. Above the predetermined speed, the control compares the front and rear output shafts speeds to determine whether there is excessive slippage. During the first portion of the clutch engagement period, the clutch is fully engaged while, during the last portion, a partial clutch engagement signal is generated to cause the clutch to gradually move from a fully engaged condition to a fully disengaged condition. The length of time the clutch is engaged can be a function of the vehicle road speed.