I have a 95 C280 and I am trying to figure out the aux fan operation. My ECT sensor, mounted on the t-stat housing, has 4 wires, and does not turn on the fan when it is unplugged.... should it? If the variable resistance decreases, pulling the plug is open for no fan. If I unplug the switch on the expansion bottle, the fans come on and stay on until the temp gauge shows about 87 degrees. If I let the engine idle with the air on max, the fans will come on for about 5-10 seconds and then go off - obviously not enough time to help. Given this scenario, this summer I will be in big trouble when the temps are high and the wife is driving around with the air running high. It seems like the cooling temperature circuit is not working, and the air cond pressure/temp circuit is not working very well.

I also think the viscous clutch is not working...seems to be at the same speed all the time, hot or cold. How do I check this fan?