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300tdi Coolant Fan Switch

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I'm interested to know what coolant fan switches are being used for a 300tdi.

I have fitted a Discovery 300tdi into an early 90 and opted for an electric cooling fan since the engine is mated to the original LT77 and sits further back than it would with a standard vehicle.

There is no shroud behind the radiator.

I have used an Intermotor 50091 which has dual switches (all in Celsius):

Low: on at 88, off at 83

High: on at 92, off at 87

The switch is fitted on the top hose.

I am using the high switch and the problem I have is that once the engine is warm the fan runs constantly.

I have no reason to believe the donor vehicle had problems running hot but that doesn't mean it wasn't. The radiator isn't perfect

Maybe an electric fan should be able to cool the engine sufficiently to close the thermostat but that does seem a little excessive.

From looking at other threads etc I have found the following:

1. The 300tdi is fitted with an 88 degree Celsius thermostat.

From what I have read this is the temperature the thermostat starts to open and is fully open at 94 Celsius.

The would mean that I'm trying to cool the engine to the point the the thermostat closes which seems wrong.

2. The 200tdi is fitted with an 82 degree Celsius thermostat.

Now, assuming it works similar to the 300tdi thermostat it would be fully open at about 88 Celsius.

In my mind this works well with the Intermotor 50091 High switch in that the fan will start running a few degrees above the thermostat fully open temperature and stop when the engine has cooled enough but the thermostat is still pretty much open.

I'm struggling with the Low setting a little as the fan would be on once the engine had reached operating temperature.


Given that, for a 200tdi, the fan comes on 6 degrees above the thermostat fully open and stops 1 degree below fully open surely for a 300tdi I need the fan to come on at 100 Celsius and off at 93 Celsius?

I'm not sure it's good to have the engine run that hot, therefore I'd look at one of the switches where the fan come son at 97 Celsius and off at 92.

Can anyone tell me the switch they are running on a 300tdi or if my thinking is correct?



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Are you using an X-Eng fan switch? If so, needs to be in the bottom hose :)

As above, needs to be on lower hose.

When running at normal opertaing temperature the thermostat is constantly opening & closing so there is an almost constant flow of coolant through the radiator.

The top hose takes coolant from the thermostat into the radiator but at this point it has not been cooled so will therefore be above the thermostat temperature and why your fan is operating. By the time the coolant has flowed through the radiator and out of the bottom it will have been cooled however if it hasn't been brought down to a sufficient temperature (i.e it is a very hot day, you are stationary or the engine is under load) the fan-switch will operate to turn on the fan and increase the cooling effect of the radiator.

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Thanks to everyone for replying.

Ok, so there are a couple of mistakes on my part, the 200tdi thermostat temperature and where I have fitted the sensor.

Just read the X-Eng instructions again and it clearly says bottom hose - Doh! :blush:

I'll move it to the bottom hose and see how it goes.


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With a cooling system that is working correctly the fan will usually hardly ever come on.

I am not for one minute suggesting removing the fan completely (as many do) as no-one knows when a situation will arrise that will cause the temperature to rise (what cost a boiled engine?) but normally it won't operate from one end of the year to the other. My fan (300TDi) is completely automatic (using X-Eng switch) but I also have a manual over-ride to turn it off (for wading etc) or to turn it on. The latter gives me the option to run it from time to time to ensure it is working correctly.

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I moved the coolant switch to the lower hose and took the 90 for a run.

The electric fan didn't run on either the high or low connections so all seems to be well. The air temperature was cooler than when I went for a drive at the weekend but at least I know the radiator is cooling the coolant to less that 88Celsius :).

The only thing I can say in defence of my stupidity in fitting to the top hose is that I used the X-Eng switch as a handy way extending the top hose when I fitted the engine. That was about 10 months ago; I forgot that I hadn't read the instructions and just fitted it to get the engine running when I wired it up the other weekend. Still - at least I know it will work when the coolant temperature rises enough.

Litch - Yes, I have done the same. I have wired the fan to give me 'on-off-auto' options. I have also wired the feed to close the relay to a light on the dash so I know when the fan should be on when I switch it to either on or auto.

Thanks again for everyone's help.


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