Don57
The 200's and 700's have different control systems for the fans. The 240s with mechanical fan and externally mounted electric "pusher fan" on the outside of radiator: This fan is triggered exclusively by a high pressure switch on the ac high side. The's 700's with inside the radiator fans - these are triggered by coolant temperature sensor (on the radiator I believe). At least this is what I have observed in my fleet of 90's cars. I'm sure there were other flavors back in the 80's and maybe in the newer mid nineties 900's etc.
Robb

Make up some jumper wires and apply power directly to the fan plug to see if it runs. Does it spin by hand?

If the fan runs with direct power, try shorting the two wires from the switch at the top of the radiator with the ignition on.

While the fan runs when the AC is on, I think that it should also run if the water gets too hot.

There is a switch somewhere to run the fan with the AC and certainly a fuse - have you read all the listings on the fuse block?

I never heard the fan run on our "90 744 TI until I installed a new one. If I started the car and let it idle, the fan would run after 5 or 6 minutes, even in winter.

BTW on one of our other 740s, I found that a generic fan from Autozone fit almost perfectly inside the fan shroud, I tied it in with repair wire in 8 places around the shroud and had a much better installation than zip ties through the radiator that came with the fan. A generic fan cost about 25% of the price of the Volvo fan, This car had 235K on it and was for sale.

If you use a generic, make sure it blows in the proper direction as it could be mounted in front of or behind a radiator.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 485,000 miles put on 9 bricks