off/on fan relay switch

[fatfenders]

Hello,
I have just now joined this forum in an attempt to get a little advice on a project I want to complete. I have a good understanding of 12 volt automobile electrics (you can view my latest project **broken link removed**...) and limited knowledge about electronics.
So here is my current "shadetree" project. I would like to add secondary transmission cooler, one with a fan. Of course, I can buy one for around $250 but thats no fun. I would like to use the internal transmission temp. sending unit to control a (power transistor??) that would switch on above say, above 176 degrees. The shop manual for my transmission indicates that the apx. resistance value at that temp. is 332 ohms (140 degrees; 667 ohms... 194 degrees; 241 ohms).
Any advice on a simple method to control a fan relay appreciated.

 

[crutschow]

You need to know the load that the temperature sending unit is driving (I assume the unit output presently goes to some other location). The resistance of the sensor is only useful if you know the load it's seeing. This will tell you the voltage across the sensor at various temperatures. You could then use a comparator to sense the voltage at the desired temperature to control a transistor to power the fan relay.

 

[Willbe]

Your power supply needs to damp out the load dump transient if it is to have a reasonably long service lifetime.

 

[fatfenders]

Crutschow,
That sensor is not currently hooked up to anything. Originally it went to a pin on the PCM but I faked that input with a 1/4 watt resistor as I installed a seperate sensor that came with an Autometer transmission fluid temp. gauge. But armed with your reference to a "Comparator", I will attempt to reseach what one of those are, thanks.

Willbe,
Re: "...and limited knowledge about electronics." I have no idea what you just said.

 

[Willbe]

I have no idea what you just said.

Auto power is very "dirty"; sometimes it goes a few hundred volts negative and this needs to be taken care of if you make a circuit that runs off the auto with the engine running.
There is also some 3Φ AC voltage from the alternator riding on the DC battery voltage, but this is small.

Are you related to the Fendermen?
https://www.youtube.com/watch?v=JdhAkmrTsJY

 

[Boncuk]

Hi fatfenders,

just one question concerning the temperature probe: Is it connected to chassis by means of a threaded package with one wire connection?

If so, I've just made a neat board for you. Its size is 2.1X2.225" including a safe power supply with sufficient filtering, a refernce voltage source and the relay to connect a fan up to 10A.

Hans

 

[fatfenders]

Boncuk,
Thanks for that but after a lot of reseach I just tonight found what I need and at the price (less than 1/2 of everything else I hac found). Its a fan switch (on@180 off@170) for just 24 bucks from Makco Distributing. Perfect. Case closed.