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
How it’s Done
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.
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.
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).
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
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.
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!
for voltage switch