One of the vehicles is listed as a 79 300TD
From what I can remember from the Forum the fan goes on if the Coolant temp it goes above a certain temp or any time the A/C is on.

Assuming you Coolant is in a normal temp range disconnecting the Wire on the Temp Sensor while the Fan is running should tell you if it is the Sending unit.
If it continues to run after disconnecting the Wire at the Temp Sensor it must be some problem in the A/C circuit.

Another thought is that if the Coolant Temp Fan always has + voltage to it all of the time and is turned on by Grounding it through the Temp Sensor or the A/C circuit it could be the insulation has rubbed through on the Ground Wire and is grounding on the Chassis.

__________________
84 300D, 82 Volvo 244Gl Diesel