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Electronic Radiator Fan Switch

Cheaper than most other ways of doing it and far more versatile

by Julian Edgar

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At a glance...

  • Uses the Simple Voltage Switch kit
  • No need to fit a new temp sensor
  • Full control over switch-on and switch-off points
  • Much cheaper than commercial radiator fan thermostats
  • Uses the ECU or dashboard temp sensor
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There are a couple of reasons why you might need an electric radiator fan temp trigger. You could have needed a cooling system upgrade and have sourced a bare fan from a wrecker, installed it – and then wondered how you’re going to have it automatically turn on and off. Or the temp switch in your current car’s cooling system may have failed – and you’re not too rapt with the cost of a genuine replacement. (And plus it’d be nice to have a bit of control over when the fan actually comes on, too.) Or maybe you’ve replaced the radiator with an upsized design which turns out can’t take the original rad’s temp switch.

Of course, there are lots of solutions – commercially available adjustable radiator temperature switches (eg see Thermo Fan Install) or even the DIY adjustable temp switches we covered in DIY Adjustable Temp Switches. But the approach in this story has significant advantages over those other techniques.

Firstly, it’s easy to alter the temp at which the fans will cut in. In fact, you just turn a ‘pot’ (potentiometer) on the circuit board. Secondly, you can adjust that point very finely indeed – in some systems, to the individual degree Celsius. Thirdly, the temperature at which the fan switches off again (ie the hysteresis – the difference between the switch-on and switch-off temps) is also adjustable. And finally – and this is a killer – you don’t need to install a new temperature sensor. Instead you can take the temp signal straight off the engine management ECU.

Neat, huh?

How it’s Done

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The approach uses the Simple Voltage Switch kit developed by Silicon Chip electronics magazine. The kit – and the book in which it is featured – are available from the AutoSpeed Shop and Jaycar stores.

So how does the system work? Well, the ECU sends out a regulated voltage to its coolant temperature sensor, which is a device that changes in resistance with temperature. If the resistance is low, the ECU sensor voltage is pulled lower. If the resistance is high, the ECU sensor voltage stays higher. Since most sensors have a resistance that gets lower as the temperature gets higher, the voltage sensed by the ECU gets lower with increasing temp.

Say all that through twice really fast!

But hey, you don’t need to worry about any of that anyway. All that you really need to know is that one of the wires connecting to the ECU’s temp sensor has a voltage on it that gets lower as the coolant temp gets higher. The Simple Voltage Switch (SVS) continually monitors the voltage on this wire, tripping when it falls to a level that you’ve pre-set. In turn that switches on the radiator fan. When the coolant temp again falls (and so the voltage rises), the SVS switches the fan off. A red LED on the SVS shows whether it’s in its tripped state or off.

Fitting and Set-Up

In this case the guinea pig car was a 1988 Maxima V6 Turbo with twin factory fitted electric fans. A change in radiator meant that the original temp switches could no longer be used.

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The first step in the installation is to back-probe the connections to the coolant temp sensor, either at the ECU or near the sensor itself. (On the Maxima we did it near the sensor because it was quicker and easier to find the right wire without having a dedicated workshop manual). As mentioned, you’re looking for a voltage (normally between 0-5V) that decreases as the car warms up. So you can easily see this change, start off the measuring process with the engine cold. Here the voltage shown on the meter is 2.369V – it was falling rapidly as the car warmed up.

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Once you’ve found the right signal wire, solder a new wire to it and then run it back into the cabin.

The next step is to build the kit and test that it works. As mentioned, in this application the kit needs to be configured to trip on a falling voltage. This requires that during the build process a diode is orientated in a specific direction and a moveable link placed correctly. Follow the kit instructions carefully – in fact unless you are an experienced electronics kit builder, we suggest that you buy the built and tested version (although note a link will need to be moved and a diode orientation swapped, as the pre-built version is configured to switch as the signal voltage rises).

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The next step is to install the module in the car. Connect 12V and earth wires correctly, and connect to the SVS signal input the wire that you’ve run to the coolant temp sensor. (Note that the SVS won’t cause any additional load on this output – the rest of the EFI system will keep working happily.) At this stage you don’t need to connect up the fan, although as shown here you can put in a pilot light if you can’t see the SVS’s indicator LED.

Turn the hysteresis pot fully anticlockwise, then start the car and turn the voltage adjustment pot until the SVS’s LED comes on. If you had the radiator fan connected, it would be on now too. Let the car warm up and then adjust the pot until the SVS just turns off. That is, you’ve set the SVS so if the car gets any warmer, the LED will come on.

With the hysteresis pot set to its minimum (ie fully anticlockwise), the LED will go off quickly once the temp starts to drop. If the LED goes off too early, adjust this pot a little clockwise. Note that the set-up process can require some trial-and-error changes, so leave the pots accessible for a day or two of normal driving so that the fine-tuning of the switch behaviour can be adjusted. Make sure that you don’t set the temp threshold too low or the fans will be on all the time – in most cars you want them trip when the temp needle gets to say 60 or 65 per cent of full movement. Well, over half way, anyway.

Connecting the Fan

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The next step is to connect up the fan. While the SVS has an existing on-board relay, all radiator fans should be driven using a separate, heavy duty automotive relay. Connect it up as shown here.

If the SVS is being used to replace an existing temp switch, you can use the relay that probably already exists in the car’s wiring. In that case, just wire the NO (normally open) and COM (common) terminals of the SVS’s relay to the connections that previously led to the temp switch. In fact, that’s exactly what we did on the Maxima, an approach which saved having to buy a new relay and run all its associated wiring.


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With surprisingly little set-up time we soon had the radiator fans on the Maxima working as we wanted. We left the hysteresis at its minimum level and found that we could soon tell by closely watching the temp gauge when the fans were about to come on, and when they were about to go off again.

In short, the system works perfectly!

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