Coolant Fans again?

[cphilip]

Yes... another Cooling fans diagnosis thread. Not sure I have found this exact scenario discussed on search

95 W124 E320 wagon. Newish Waterpump (last fall) and coolant etc. all fresh (even a new recovery tank and new cap a few months back)...

All the sudden last few days (at least today) climbing to near 120 at long stops. It is really hot here in SC so that part is understandable in a way. But it appears perhaps both fans are not coming on. Let me explain later. The wife got home today and reported this. She checked the coolant level and its good. And nice and clear. I rechecked and it is. She said it got to the top white line and then hung there and the light turned and things got better but it always seemed higher than it normally is after that. Never go all the way back down to its normal running temp.

I went out there and cranked it up and I heard a fan come on immediately. Of course the AC is on automatically here as it is a Auto Climate control set up and its calling for it. But the temp gauge is indeed up there a bit more than half way between 100 and 120. But at this time it is not pushing the white line anymore as reported. It had cooled off a half hour by this point though.

I went around front and found the right fan (as you look at the engine or Drivers side) was running but not the second (left/passengers side) fan. On a whim I bumped the fan motor with the butt of my fist and PRESTO.... it came on! I have no idea... it responded to the "hammer" approach. At least this time. This says to me it should have been running right? It was in the temp range I should have expected it to be running. Both of them.

This is new. And recent symptom.

I will say that occasionally the AC does fail to come on after a stop and stopping and restarting does fix it. So we are aware that its likely the belt slips sometimes. We do have a newish belt on there (probably three years old) but probably need some attention there but it does blow cold as ice when it is working. We do not see any sign that the freon is low based upon that.

What say you? Sensor or fan itself? Has anyone seen one start with a persuasive bump?