Some fans are AC only fans (fact on my old Disco 300 ES with AC), which is why they come on when you flick the switch. However look for a sensor near the thermostat housing (I think) as its the engine temp that needs to direct additonal cooling and an air pocket could affect one on the Rad. Must admit this off the top of a tired head and I seem to remember seeing such a sensor location before. I'll go look at my Haynes later.