The V8 Owners Forum -

My electric fan cuts in far too prematurely (with and without the A/C on, in freezing weather and after about 5 miles from cold) and I am having touble finding out what is responsible for triggering it. I have read the fan has a thermal switch (i would guess in the rad) although my local garage assures me the fan is operated by the ECU. The engine has the 'banana' inlet manifold.

Engine temp and air temp readings would appear accurate.

Does anyone know what turns it on?

I'm not familiar with the Disco set up but it is common for the ECU to control the fan albeit via a relay. However, the ECU gets it's information from the Coolant Temperature Sensor and it may be this that is playing up. It's worth checking the specified hot and cold resistances of the sensor. This sensor if different to the one that drives the Temp gauge on the dash.
The sensor also controls the warm up mixture enrichening so if you are experiencing bad performance or flat spots during warm up then this would point to a Coolant Temp Sensor malfunction also.
Cheers Denis

wont have an electric fan unless the standard fit viscous unit has been replaced. If thats the case then there will be an adjuster to select the temp at which it cuts in at

Here is the definitive answer from the Land Rover Workshop Manual:

DESCRIPTION AND OPERATION
4.122 DISCOVERY
Cooling Fan
Cooling Fan Description
The cooling fan is located on brackets forward of the radiator. The fan motor is
operated by a cooling fan relay controlled by the Engine Control Module
(ECM). The main relay supplies a feed to the coil of the cooling fan relay. A
permanent feed is supplied to the coils of the main and cooling fan relays
located in the engine compartment fusebox. On V8 models a coolant
temperature sensor is located in the inlet manifold, on Td5 models the sensor
is located in the outlet housing.
The ECM controls the operation of the main and cooling fan relays. At a preset
temperature the ECM receives an input from a coolant temperature sensor
above the values stored in the ECM. The ECM logic enables the earth path for
the coil of the cooling fan relay. The fan motor then gets a feed from the closed
contacts of the cooling fan relay.
When the engine temperature falls, the sensor gives an input below the values
stored in the ECM. When the input from the coolant temperature sensor low,
the ECM interrupts the earth path to the coil of the cooling fan relay. The
contacts of the cooling fan relay open, this action breaks the feed to the cooling
fan motor, and the motor stops. The cycle will start again when the engine
coolant temperature rises and the sensor sends a high input to the ECM.
The ECM has an engine off function, when the ignition is turned off the ECM
logic goes into a watchdog routine and monitors the coolant temperature for
approximately seven to ten seconds. If the coolant temperature is still high,
over 100 °C (212 °F) V8, 110 °C (230 °F) Td5, the ECM logic can control the
operation of the fan motor.ഊDESCRIPTION AND OPERATION
DISCOVERY 4.123
On V8 engines the ECM will only enable the fan if the inlet air temperature is
over 60 °C (140 °F). The ECM will allow the fan to run for a maximum of ten
minutes, however the ECM continues to monitor the coolant temperature. The
ECM logic will stop the fan if the coolant temperature is below the acceptable
values stored in the ECM. To prevent a flat battery, the fan will be stopped
(regardless of coolant temperature) if the battery voltage falls to 12 V.
Cooling Fan Supply
Circuit supply
A permanent feed from the battery positive terminal is connected by an R wire
to the engine compartment fusebox. A feed from this wire is connected to the
contacts of the main relay and also to fuse 13. The feed passes through fuse
13 and to the inertia switch on a NB wire. The feed from the inertia switch flows
on a WG wire to the engine compartment fusebox and is connected to the coils
of the main relay. in this condition the coil and contacts of the main relay
receive a permanent supply.
The permanent feed from the battery on a R wire to the engine compartment
fusebox is also connected via fusible link 1 to fuse 5. From fuse 5 the
permanent feed is connected to the contacts on the cooling fan relay. In this
condition the contacts of the fan relay receives a permanent feed from the
battery via fuse 5.
Main relay - ignition supply
When the ignition switch is a position II, the ECM switches on the earth path for
coil of the main relay. Current flows through the coil of the main relay and flows
on a UR wire to pin C0635-23 V8/C0658-21 Td5 on the ECM. With the coil of
the main relay energised the main relay contacts close, power is now available
to the coil of the cooling fan relay, via the closed contacts of the main relay.ഊDESCRIPTION AND OPERATION
Cooling Fan Operation
V8 engine
The engine coolant temperature sensor sends an input on a G wire to pin
C0636-22 on the ECM. The engine coolant temperature sensor is earthed on a
B wire to pin C0636-21 on the ECM.
When the coolant temperature is high, the value of input from the coolant
temperature sensor causes the ECM logic to switch on the earth path for the
coil of the cooling relay, on a GW wire to pin C0636-31 on the ECM. With the
coil of the cooling relay earthed the coil is now energised. The cooling relay
contacts close and current flows on a BN wire to the cooling fan motor.
Td5 engine
The engine coolant temperature sensor sends an input on a KG wire to pin
C0158-7 on the ECM. The engine coolant temperature sensor is earthed on a
B wire to pin C0158-18 on the ECM.
When the coolant temperature is high, the value of input from the coolant
temperature sensor causes the ECM logic to switch on the earth path for the
coil of the cooling relay, on a PB wire to pin C0658-4 on the ECM. With the coil
of the cooling relay earthed the coil is now energised. The cooling relay
contacts close and current flows on a BN wire to the cooling fan motor.
All models
The current flows through the cooling fan motor to earth header C0018 on a B
wire. The cooling fan motor will operate as long as the ECM receives a high
signal input from the coolant temperature sensor. When the coolant
temperature is low, the ECM logic interrupts the earth path to the coil of the
cooling relay. The contacts of the cooling fan relay open, the fan motor power
supply is now broken and the motor stops. The cooling fan will cycle on and off
as the coolant temperature rises and falls.

Chris.

Based on the stuff posted by ChrisJC, the first try would be to replace the coolant temp sensor on the engine.
Regards Denis

Now converted to gas and since the installation the fan hasn't cut in again! Keeping quiet and certainly out of earshot of the car but it all seems to be working!