sgraber
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| posted on 18/2/04 at 09:03 PM |
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Fan control for stock thermistor
I have removed all references to the stock wiring for my MR2 fans and so I am left with a radiator with a stock Toyota thermistor attached to it.
I thought about using the original MR2 fan control box. But to me it's a black box. Don't know how to wire it into my new harness. I could
buy a fan controller and plug it in, but where is the joy in that?! I like making it up as I go along. 
So I have devised an ingenious (in my own simple mind) plan to activate the fans using 2 relays, the stock thermistor and a variable potentiometer.
 
Control for radiator fan therm
As you can see from my schematic, there is a small relay inline with the stock thermistor located in the radiator, and a potentionmeter. When the
resistance across the thermistor drops to a certain level, enough voltage will flow across the relay to trip the high side. This in turn will energize
the larger relay and start the fans.
Why two relays? We know that the resistance through the thermistor drops as the temperature rises. At some point the amount of electricity passing
through the thermistor will be large enough to energize the relay. By using a small, sensitive relay in series with a potentiometer, I can precisely
determine the temperature at which the little relay will be tripped by increasing the resistance. I do not believe that the thermistor would be able
to trip the single large relay at a high enough resistance.
Any suggestions as to why this idea should not work?
Steve Graber
http://www.grabercars.com/
"Quickness through lightness"
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Graceland
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| posted on 18/2/04 at 09:57 PM |
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looks viable  and should work providing the internal resistance of the potentiometer isn't exceeding the resistance of the large relay which u
are controlling
however, to make matters simpler, B&Q (www.diy.com) sell a thermister that is used to controll hot water boilers - its only about 5 quid and is
adjustable aswell - know a few people using it on twin turbo sierras
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sgraber
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| posted on 18/2/04 at 10:08 PM |
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Can anyone tell me off-hand, what the resistance of the Toyota Thermistor is when the fans normally kick on?
Steve Graber
http://www.grabercars.com/
"Quickness through lightness"
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stephen_gusterson
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| posted on 18/2/04 at 11:38 PM |
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What if the relay starts to chatter?
Another thing to check is the pull - in and release voltages of the relay - it might take longer to release than to come on , voltage wise - or vice
versa.
You can buy transistorised relays - these have a transistor as an input, and can be used more effectively with resistive inputs.
http://rswww.com
look on there for examples
atb
steve
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