Thanks,
I'm planning to try the thermometer, but my thought is this: There is slight evidence that the rad has been replaced, along with the repaint of the support on top. There is no evidence of body work anywhere else on the car, nor any record of insurance claim. I think that someone slapped the rad backwards in a minor way, but bent the bimetal strip on the fan clutch in the process of the rad striking it, effectively accomplishing the same thaing as a longer pin, i.e. lower engagement temperature. The thermometer will tell me. Even if it is a bad sensor, the temperature shouldn't wander. It is winter, so the cooling effect of the clutch fan is quite dramatic. I have never seen it even engage on my c280, no matter what the weather and load, and I know it works fine, that's why this one surprises me.