The standard fan will turn faster than the visco clutch fan, as it is not slipping at any speed and the visco clutch is a drag clutch. It will never turn the fan at engine speed, it's under load moving air.

Do not discard the electric fan, find another one. It will only come on at 100C, it's there to prevent serious overheating and to help with head pressure with the AC on. Replacement from the boneyard won't be too bad. Make sure the switch, relay, and fuses are good, of course.

Do not operate without a thermostat, you will seriously overheat the engine when it gets really hot. MB has always used and internal bypass system, and a considerable amount of the water flow will simply circulate around the block and heater core without the flap on the back of the thermostat closing off the bypass when the thermostat opens.

Most likely you need to do a citric acid flush (5 lbs of citric acid dissolved in enough water to fill cooling system, drive 10 min or so with thermostat open or with the special "jamed open" one installed), then flush with water and new coolant installed. If you still have high running temps AND you know the gauge is accurate, I would think you need a new radiator. Assuming here that you have an aluminum one in a W123 chassis. Brass and copper replacements are available, I'd get one.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!