Samsung RS21DCSV Compressor Fan?

I have Samsung RS21DCSV. Noticed that the wall between the freezer and the fridge was very warm. Checked at the back of fridge and found that the compressor was very warm. The fan next to the compressor was not working. Switched fridge off and back on again, result the fan turned half a turn only. Have placed a small desk fan next to compressor. This cools down the compressor and the temp in both compartment is ok and constant. Compressor switches on and off as required. I leave the fan running all the time. Have changed the fan next to the compressor still the same problem, only turns a half turn. Question: is there a sensor in the compressor that tells the fan to turn on or is the fan on all the time? any help would gratefully received.