There have been several threads concerning the fan clutch going on and off while people are driving at highway speeds. My rig (FJ60) certainly does it. There was a previous thread where I asked this question and did not get a response. It would seem to me that there would be no benefit for a radiator fan to come on when you are driving down a highway as you already are pushing a 50-70 mile per hour wind through your radiator. Thus, I am concerned that fan clutches that are doing this on/off thing are defective. Perhaps, there is a thermocouple in the fan clutch that is over-reacting? How does a new fan clutch behave?