The short of how I installed a Volvo electric cooling fan from a 940 turbo in my 1990 740 GLE:

I acquired a 940 electric cooling fan and the associated wiring from a junkyard. Because the cooling fan is ECU controlled on the 940 I also had to get a temperature sensor from a different vehicle. I ended up getting a temperature sensor from a late 80s Nissan.

To install the fan on the radiator I had to cut the size of the shroud down quite a bit because the 940 turbo has a wider radiator than the 740. I had to re-clock the fan on the shroud so the wires would face the passenger side. The fan was fastened to the radiator using stock mounting fasteners on the top and zip ties on the bottom. It was necessary to use washers on the water pump pulley studs after the mechanical fan was removed.

The 940 fan has two speeds of operation; Slow and fast. Both of these speeds were to be controlled by separate temperature switches. The cooling fan relay is always powered and turns on the fan when an input wire is grounded. My Volvo already had a temperature switch in the radiator which is used to turn on the auxiliary electric fan in front of the radiator. Because this is the temperature at which Volvo feels the engine needs additional cooling, I wired this sensor to the fast speed of the cooling fan. Slow speed was wired to the Nissan coolant temp probe. For simplicity the Nissan probe was threaded into a brass fitting which was then spliced into the lower radiator hose. The relay is directly powered by the positive and negative battery terminals. The auxiliary fan in front of the radiator is no longer hooked up to any sensors; however I have not removed it because I have not determined if it is wired into the air conditioning

This set up has worked well for me. Removing the mechanical fan has freed significant high rpm horsepower. No noticeable increase in gas mileage has been observed. The fan rarely uses high speed and cycles on and off at regular intervals in stop and go traffic. In the future I would like to also wire low speed to a switch in the dash. This is because when I went drag racing it was annoying to have the cooling fan unexpectedly come on while I was staging.