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The fan in my car dosen't turn on automatically anymore when the car gets too hot, but it will turn on when i switch on the A/C. So what i had in mind was to place a temperature sensor in the coolant tube just where the thermostat is to measure the temperature and turn on the cars fan with an AtTiny85 and a relay. My question here is: can i leave the cars built in cables to the fan and add some new ones from the AtTiny and the relay, cause i still want the fan to kick in when i turn on the A/C. I'm not quite sure if it will damage the fan cause at some time the car might be hot and therefore the AtTiny will add power to it and the A/C turned on at the same time, that will result in the fan getting two voltages? But it may not damage anything since all the power and ground comes from the same place?

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  • \$\begingroup\$ You should replace the thermostat \$\endgroup\$
    – crasic
    Commented Nov 6, 2015 at 23:30
  • \$\begingroup\$ Why not just wire the fan to a switch and turn on the switch when the temperature gauge on the dashboard looks like getting a little high? \$\endgroup\$
    – Andy aka
    Commented Nov 6, 2015 at 23:33
  • \$\begingroup\$ It would make more sense to replace the original faulty temperature sensor. If you do it your way, however, you should put the sensor on the radiator, since you only want the fan to come on if the radiator can't cope with the cooling on its own. \$\endgroup\$
    – HandyHowie
    Commented Nov 6, 2015 at 23:39
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    \$\begingroup\$ In modern cars, most things are controlled via a computer or ECU. "manually" controlling the fan may confuse the computer, so it would be best to find and repair the actual fault, rather than making a haywire repair that bypasses the fault. \$\endgroup\$ Commented Nov 6, 2015 at 23:50
  • \$\begingroup\$ Why use a switch and turn it on manually when it can be easily made so it will do it automatically? The temperature sensor is not faulty, actually it's brand new! In my car it's the ecu deciding when to turn on the fan and I found out why the fan doesn't turn on.. I accidentally hit the relay box and suddenly it came on so I believe it's a bad connection in the wires somewhere around the relays. \$\endgroup\$ Commented Nov 7, 2015 at 14:19

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If you use a simple multimeter you can test the continuity (short-circuit) between older conection and the new one. Make sure that battery of car is unppluged... the batteries of cars have a lot of capacity and can cause serious damages!

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All of the power is coming from the same source: the car battery (plus the alternator). All you're doing is putting a new switch in parallel with the old switch. Nothing wrong will happen, so long a you wire it correctly.

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