Where is the temperature sensor on a 1991 940 turbo Volvo?

I'm not 100% sure, but I would expect Volvo to keep them located in the same location and that would be in the Thermostat Housing. The part you would be looking for on your engine (where the thermostat is) would be screwed into it and have wires coming off it. This would be your Temp-sensor and is inexpensive to buy and easy to replace. Go to your local parts store to get.