Controlling auto cooling fans

I’d like to build a project using an Arduino to control a pair of auto cooling fans. The fans are two speed and activate at the same time. So what I need to do is read coolant temperature and at a certain temp switch on the 60amp relay that turns on the low speed circuit. At a temperature of about 20 degrees above that, I want to switch off the low speed relay and switch on the high speed relay, then switch back from high to low when the temp drops. And of course, shut off at the initial start up temp.

After I get the above functionality figured out I’d like to experiment with running the fans for a period of time after the engine is shut down, but that’s not a priority. And of course I need to protect the Arduino from the vehicle’s electrical system and all it’s vagaries. Adding things like LED’s to indicate fan operation, or even an LCD to indicate temperature might be fun too.

Sound feasible?

I’ve got computer programming experience but no Arduino experience. I’m willing to start small with a project kit and learn on small experiments. There’s nothing urgent, the vehicle (a Jeep CJ-7) is running now on the relays and simple on/off temp sensor and just the low speed circuit).

Any advice is welcome, including on which Arduino or kit to start with and any simple starter projects that would help me learn and build up to the main project.


Hey 3rdpig, yeah that is defiantly doable. I use for all of my parts. They have a variety of temperature sensors you can use ( You will also probably need at least one relay to step you up from 5v to whatever the fans you have need. Hooking up LEDs would also be no problem, they could be triggered at the same time the Arduino triggers the 60 amp relay you mentioned. Overall this is a pretty simple and reasonable project to start out with in my opinion.

Thanks for the info and the link. The fans are 12v and on high they pull a steady 28-30 amps and they spike upwards of 40 from a dead stop when high is switched on. I'm using 60amp Durakool relays, the contacts in a typical 30/40 amp Bosch auto relay burns out after a week or two. The relay control voltage is also 12 volts. Not sure what amperage the control circuit pulls, but I doubt it's much since I'm running it through 18 gauge wire.

I'm thinking of setting this up in small scale on my workbench using a small 12 volt fan and a similar set of relays before I move it out to the vehicle.

Is the Duemilanove a good starting point for this project? And maybe a sensor shield?