If your temperature gauge suddenly stops working there is a good chance that the sensor on the engine block has failed. The sensor is located on the left side of the block under the fuel injection lines and behind the glow plug. Need help with it’s replacement? Check the links below!
Check out the article here: http://www.pelicanparts.com/techarticles/Mercedes-W123/11-WATER-Coolant_Temperature_Sensor_Replacement/11-WATER-Coolant_Temperature_Sensor_Replacement.htm
Purchase W123 CTS here: http://www.pelicanparts.com/catalog/SuperCat/W123/MBZ_W123_WATMIS_pg2.htm#item1