I have been down this electric fan route and used the sensor that pushes through the radiator fins. I bought the adjustable one. It was a PITA. I could not get the fan to come on at a reasonable temperature both moving and stopped -- one or the other. I was trying to use electric only. My solution was to have the radiator shop solder a bung in the top tank to take a 3/8" pipe fitting. You could also drill the water outlet neck for a 3/8" fitting. That would be cheaper and just as good, I think. Then I went down to the Peterbilt truck dealer and they had a selection of fan switches both normally open and normally closed in various temperatures. I picked a temperature cutoff that was at least 10* above the thermostat temperature. If you pick too low a temperature, the fan won't shut off again while the motor is running or until you go down a hill. So if I remember correctly, the fan comes on at 205* and off at 195* and I can run a 180* t'stat, or something lower, like a 160.
My SPAL pusher fan draws a lot of current so it's powered through a 50A relay wired directly to the battery (with circuit breaker, of course). The ignition switch will not handle that much current. I can tell when the fan comes on because the ammeter swings over to 30A charge and goes back to ~0 when the fan shuts off. Set up this way, it'll come on a couple minutes after you've shut the engine off at a rest area, for example, and stop you from boiling coolant out on the parking lot. There is a thermosiphon action in the cooling system so the coolant will circulate even with the water pump not running.
This is used in conjunction with an 18" six-blade mechanical fan. In hot weather, pulling a hill, loaded, I can watch the temperature gauge climb to about 205 when the electric fan comes on and then stop climbing. Neither mechanical nor electric provides adequate cooling alone. I really would like to use the electric alone for noise reduction but it just isn't adequate on a long pull, say, more than 2 or 3 minutes. There isn't room for a clutching fan in an AD chassis with the 56 and later water pump. At least not in any combination I've found. If anybody knows of one please let me know.
