I know this is an old thread, but I seem to have the same problem. My '98 C280 runs about 90c to 94c and the coolant fan is either full on or full off.

I tried to use your description to find the lowspeed relay/fuse, but mine has several fuses and relays in the described box!

Can someone with a layout tell me which relay and fuse is which?

Thanks again guys!

Kid