had this "problem" when i got my w123. the previous owner couldn't be bothered to replace the temperature-actuated switch which kicks the fan in at 90 degrees (i think) and just bypassed it by "jumpering" one of the relays so as to make it run all the time.
bad idea. it lead to a couple of hours of tracing why the engine sometimes wouldn't kill itself after i pulled out the key.
replacing the switch solved the problem.