What does the engine temp gauge read on your dash when it turns on?
Also, you should check to make sure your engine compartment temp sensor hasn't come loose and is resting on the engine or other hot part. This could be causing the fan to come on unnecessarily. If it is actually coming on due to being that hot, there could be a different problem.
The compartment sensor is #12 in the diagram, it should be in a rubber grommet right on top of the engine between the #4 & #5 intake plenums. You can't miss it, the connector easily seen just by opening your engine bay lid.
"Switch-on conditions for engine compartment fan (this fan pulls air into the engine compartment) The engine compartment fan is switched on when the engine compartment temperature is > 176 degrees F or the coolant temperature is > 216 degrees F.
If the ignition is switched off and the engine compartment temperature is more than 140 degrees F, the DME control module remains in readiness for another 20 minutes. During this time, the engine compartment temperature is retrieved every 10 seconds. If the engine compartment temperature is > 185 degrees F, the engine compartment fan is switched on for 20 seconds. If the engine compartment temperature is still > 185 degrees F after this time, the fan remains on for a further 30 seconds."