hi guys in the winter i had trouble with my eletric fan control unit , so i purchased another off demon tweeks website a davies craig digital thermatic fan switch ive wired it up and works ok but its been coming in too soon , the overide works fine so today ive set it to 90 , the landy takes alot as its a retro fitted disco 200 ,and the guage goes to 3 quaters and stays there and only up a big long hill it goes up to end of of just past white but comes back quickly ive replaced guage in dash before for a 90 200 one and sender , what temp should i set the fan to come in at i know the thermostat is an 88 one cheers
The fan should only really come on if the radiator cannot cool adequately with the air flow available to it, i.e. driving at any speed over about 10mph your fan should not need to come on.
The way the system should work is when the radiator starts returning significantly un-cooled hot water, i.e. lack of cooling, it then cools the system less, as the system temp creeps up the stat opens wider, if the stat has had to open 100% then the fan should probably really also be on, but this may be system temp up towards 100°C.
So for discussions sake if the stat is passing water in the region of 88°C, and the return temperature is say only 84°C then the cooler return water is not cool enough to make much of a difference, the system temp goes up and the stat would open wider to pass more water in an attempt to cool more at this point the coolant temperature could be about 90-100°C (and indeed even hotter) at this point would want your fan on. So I would set it about 95°C maybe a little higher.
Remember, the engine will be much hotter than 88°C, 88°C is only the opening temp. it will not be wide open at 88°, they do this to create a hysteresis, try bringing a stat to the boil in a pan and watch it. It will really only allow a small volume of water to flow at 88.
The engine itself will happily sit at 100°C if the coolant is made up properly, the engine will come to no harm, a diesel engine is a heat engine and a diesel hot engine is a good thing - to a point obviously! So as long as your stat is wide open by mid 90°'s and your radiator can dissipate heat you will be fine.
Another thing to consider is that all manufacturers now attenuate their temperature gauges and have done for about 30 years, notice a car temp gauge will sit bang on the mid point (assuming cooling system is OK), you can drive like a bat out of hell and the needle just sits there, that is not because the cooling system is so effective, that is because people would call the AA/RAC every time it looked a bit too warm, so they attenuate the gauge so it stays centre within a given range. I had my car plugged into diagnostics once and was looking at engine readings, and when I booted it the water temperature would climb about 10° to around 99° - the dash gauge was still dead on.
I have an electric gauge on my 300tdi and after a long hill climb I can see 4-5° increase, likewise if I am driving light throttle on descents, the temp can drop down as much as 10°C.
Keep the temps under 110°C and you should be fine.
- As I said in my post in your EGT thread - this gauge is useless - fit a PROPER one - my preference is Durite - not cheap - but cheaper than a new head - oh wait 
