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This may have already been done, but I was doing some thinking (dangerous for my age) on electric cooling fans and would want a auto-on-off switch controlled by water temp at the manifold. Anyway I have an idea to make my own ($10) vs paying $30-$50. Use the same relays used for bright light coversions and the switching temp sensor used for idiot lights. A very simple hook up:
1. One lead from the battery to the relay coil.
2. One lead from the battery to the relay switch.
3. from the other side of relay coil to switching temp sensor on manifold. (this would supply the ground for the relay.
4. The other side of the relay switch to the fan power lead.
5. Of course the fan ground wire to ground.

The only thing I would not know for sure would be the temp that the temp sensor would switch on.
 

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The problem is that the idiot light sensor doesn't close until the engine's WAY hot. This would mean the fan wouldn't come on until the engine was over heating.

I've been very happy with my switch (I bought it, though, and so paid alot more than $10) with a probe in the upper radiator hose inlet on the radiator.
 

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Discussion Starter #3
OK what about the cooling fan switch from a donor car?
 
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How timely.

I see in my just out of the mailbox Hot Rod Air catalog that they sell an engine block mount thermostat probe just for this application. They have 185 and 195 degree versions for $35.00. They also have complete wiring kits and fans. Sorry, the catalog does not mention the thread size. Like with everything else, its probably Chebby sized.

For $500.00 you can get a complete radiator/fan/shroud package for '64 1/2 - '66 Mustangs. You still need the T-stat though.
 

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I sapped my thermostat housing to an early model 289 thermostat housing which has the threaded boss in it. I pulled the plug out and threaded in the temp sensor switch sold to me by spal when I bought the fan. works fine. Even with the hot weather here in Sacramento the fan does not run all of the time. Cycles on and off. This avoids having to use the proble in the radiator hose approach. I think it is a cleaner application. Oh, BTW, a neat installation place for the fan relays is under the battery tray. convenient to the fan motor wiring harness and out of sight.
 
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As long as we're all throwing in our 2 cents... I've been through three different forms of temperature sensors so far for my electric fan, all in a one month period.

The first sensor, from Perma Cool, strapped to the radiator hose with a zip tie, and switched the voltage directly, with no need for a relay. It never switched on. Not once, no matter how hot the engine got. Crap.

My next sensor worked OK, I guess -- it was the probe-in-the-fins variety. It had a relay pack, and an integral fuse, and was very well built, unlike the first unit. The only problem with it was that it switched off on the freeway, despite the engine being very hot. Too much air past the sensor was cooling it down, even though the water was still hot. I confirmed it's non-functionality by wiring up an LED inside the car to show me when the fan was off and when it was on. Crap.

Finally, I bit the bullet and bought the Painless Wiring setup. This one uses a threaded probe that screws into the water. I relocated the temperature sensor for the factory temp gauge to a convenient hole in the intake manifold, over on the driver's side. I then screwed the Painless Wiring sensor into the now-empty hole on the passenger side, right behind the thermostat housing / water outlet. Wiring was straightforward, as it always is with Painless products, and the fan turns on and off reliably now. I just wish I had gone to this solution straight away instead of dinking around with the cheaper approaches.

I guess it's true what they say about short cuts. They just cost you more in the long run.
 
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