The electric fan only comes on when the a/c is operating. If the a/c is not operating, the electric fan won't come on. The electric fan is controlled by a temp switch at the receiver/dryer. In 1985 the 126 chassis incorporated a 212 switch near the thermostat housing that tied into the electric fan circuit.
1) Check fan clutch per previous posts.
2) Check radiator for cold spots with a digital laser thermometer.
3) Check coolant temp sending unit for accuracy when you test the radiator.
You may have to pay a radiator shop to "shot" your temps (or you should pay them a small fee). Good chance your temp sensing system is reading incorrectly and you are getting bad information.
4) If someone put a manual valve in the heater hose they are a dumb-a**.
You probably have a bad mono-valve. Pull it and check the diaphram for a hole or tear.
5) Pull thermostat, inspect it and test it on your kitchen stove with a cooking thermometer (don't let wifeage see you do this)
You need to test for a proper opening temperature and proper opening space. If its not opening fully, it won't cool properly.