bentlywarren, if that is your real name
, I would be appy to explain.
When the ECU decides to turn on the fans, it does not do so directly, it can't handle the current draw of the fans. So what the ECU does is put a HI (12V) or a LOW (ground) on a wire running to a relay. This will either turn on the relay or turn off the relay. The relay is basically a switch that can handle higher current that is activated by a lower current. Note that voltage is not a factor here, simply current, I.E. the ECU can supply 12 volts, but not 5 Amps, however, a relay can handle as many amps as it is rated too, and it is independant to the current capacity of the activation signal (ECU in this case).
Hope I didnt loose you there.
To turn on the fan, you simplay have to active the relay, just like the ECU would. Now here's the catch: you can use the same type of signal the ECU uses, but you dont want your signal to go to the ECU, as you don't know what the circuitry looks like inside, and you may burn something up by putting an external signal on the line to the ECU (also keep in mind that it is irrelevant wehre you hook up your wire, it will go back to the ECU).
This is easily avoided by the use of a diode. This is a component that only allows current to flow in one direction, effectively preventing any voltage flow as well.
So you isolate the ECU output to the fan relay from your wire to activate the relay by placing the Diode in a certain manner (insert a year of school here) to allow the ECU signal AND your signal to BOTH activate the fan relay.
Now, logic will tell you that knowing there are two fan speeds, low and high, you dont want to have both relays acivate at the same time. The ECU won't, so why should you?
So what we will do in install a two way switch, and one position activates the low fan and the other position activates the High fan, and the middle is off.
Now this all sound fine and dandy, but what happens when say the ECU decides to turn on the high fan but you have the low fan on with your manual switch? (BTW, I dont care what ANYONE says, you will NOT get 24 volts
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So we must add one more component, and that is a transistor. A transistor is basicaly a switch. In this case, the switch would sense the ECU putting a signal on the line to turn on the high fan, and turn off your signal for low. This setup would require a few more components to allow for what I call the "ECU knows best" circuit that allows you to overide a low fan setting, but not a high fan setting, IE, if the ECU says high, then it goes high, and your switch is ignored. If you are high and the ECU says low, then we ignore the ECU, unles you turn off the switch, then the fan would do whatever the ECU says.
Simple, huh?
I could build something like this, and it is in fact my next project for the proto board. My KEM addon is going in the car tommorow, so I will let everyone know how that works out
I am basically making the lights flash longer, and adding a auto-locking feature when you put the car in gear (auto only, sorry SVT guys
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Iraq-Land of the lost!
