Potentially damaging operation of a viscous converter clutch (VCC) is detected in relation to the slippage thereacross. The slippage is compared with a reference slip value indicative of excessive VCC slippage to develop a net overslip indicator. The value of the net overslip indicator changes in the direction of a first limit value so long as the detected slip is at least as great as the reference slip, and in the direction of a second limit value so long as the detected slip is less than the reference slip. When the net overslip indicator reaches the first limit value, the friction clutch is disengaged to release the VCC. According to a first embodiment, the transmission fluid temperature at the time of such deactivation is measured and stored, and the VCC is reactivated when transmission fluid temperature falls below the stored temperature, assuming other criteria for activation are also met. According to a second embodiment, the VCC is reactivated a predetermined time after its deactivation and measurement of the transmission fluid temperature is not required.