I'm confident that this topic has come up before, but I'm apparently not using the correct search terms to find it.

I put dual electric fans and an electric water pump in my '87 V12 a few years ago. The system works fine, but the controller I use (a Davis Craig unit) will run the fans at times when I don't think it's necessary, i.e., when the car is moving at "X" or so miles per hour. What I would like to do is have the fans shut off at speed "X" since at some point they are useless, i.e. airflow due to vehicle motion is pushing enough air through the radiator to do the job.

The electrical part of shutting the fans off I can do, but I don't know at what speed they become useless or, "inefficient". I envision a relay(s) that would cut power to the fans when speed "X" is exceeded, and then restore fan power when vehicle motion drops below speed "X".

So...my question is "at what speed are running electric fans a waste of energy"? I realize I can probably figure that out by lots and lots of experimenting, but I'm sure the group and kicked this around and come up with an approximate speed.

Thanks,

John
1987 XJ-S V12

 

Speed is not the only factor. Ambient temperature and engine load come into play, too. I'd use the temperature sensor to manage your electric fans. That's what I do. However, with A/C on everything is different, again. Your A/C may not get enough airflow when the fans are off. Something you may want to test.

 

 

This would be a great Arduno project. You would input a speed signal from the VSS and input a temperature signal from the coolant temp sensor. Apply some logic to those signals control a relay for fan 1 and fan 2. Something like this... If speed is > 35 MPH and Coolant temp is < 200 than fan 1 off.
If speed is < 35 MPH and Coolant temp is < 200 than fan 1 on.
If speed is < or > 35 MPH and Coolant temp is < 205 than fan 1 and 2 on.

the logic possibilities are endless....

 

I'm doing something similar with a propeller chip. I have a GPS module mounted just above the rear view mirror (inside the windscreen of course), and have it wired so that whenever vehicle speed is above 3 MPH it flips on the high beams to serve as driving lights. It's a simple matter to use the same electronics to cut off/turn on the fans at speed "X", but I just don't know what "X" should be. I can't imagine that the fans contribute that much at 35 MPH or above, so I'll probably set it to 35, and just monitor the temperatures and see how it performs.

Thanks,

John
1987 XJ-S V12

 

 

A temp sensor takes out the guesswork. The temp is not guaranteed to be down at X speed because of differences in humidity and ambient temp. It is way easier just to say the system is satisfied at X temp, which will vary slightly but be at the desired speed you are looking for.

 

I just started working on this problem myself. I found a great little gadget on eBay (search for W1209 Digital Thermostat). I think I will secure the sensor on radiator tank (driver's side, where the outlet is), making sure it has good thermal contact. The idea is if the coolant temperature is still too high after passing the radiator then it's time to turn on the fans. Need experiment a little to find out what shall the triggering temperature be.

 

There should already be a thermal switch on the waterpump, you could use that with a relay to control the fans

 

If you really want to control the two things, aircon and coolant temps are separate, and really need separate controls. While my own view is that this is not needed, if you are able to control them using the sort of sensors you described, and you want to do it, then I would have two separate systems..
Something like for aircon: speed less than 40 mph, aircon compressor working, aircon pressure higher than x = electric fan (1) on.
For coolant: temp at inlet of engine (ie in the spout below the water pump) higher than (say) 87 degrees C = fan (2) on. (Assuming you have two electric fans fitted, which you will need). I would then go to: if either of the two measurements is more than (eg) 5% above the first fan trigger point, activate both fans.
Now a word of caution: the only mods I have done that have failed have been electronic things. So I am a firm believer in using something less complex but more robust. I now use Grant Francis' system which is: Smaller electric fan (14 inch) on whenever aircon compressor is on. Large fan (17 inch) on whenever water inlet temp above 87, and I use an "on at 87; off at 85" temperature switch in the OEM 22mm temp switch fitting hole in the water pump inlet. This is just to prevent the large fan from cycling too much in traffic, where a surge of cold or hot water can have the thing switching on and off far too frequently as you surge forward then stop in traffic.