This article was first published in 2008.
Last week in
we covered how to use a centrifugal blower
(aka a high power cabin ventilation fan) to force air through an intercooler.
This approach is especially good at low road speeds, when heat-soak otherwise
occurs. With the intercooler fully or partly shrouded, it’s possible to move a
LOT of air through the ‘cooler, keeping it much lower in
temp than it would otherwise be. However, oftentimes the full power of the
blower isn’t needed – instead it’d be nice to have it vary in speed to suit the
conditions. The easiest approach is
to have two speeds – which is what we cover in detail here.
But first, when should the fan speed be fast and when should it be slow?
Heatsink vs Heat Dissipation
As we have covered many times, most intercoolers on road cars work primarily
as heatsinks (see Intelligent Intercooler Water Spray - Part 1
for a detailed discussion of this). In the case of these cars, it is best to
have the forced-air fan working hardest when the likelihood of heat soak is
greatest – that is, when the car is stopped or travelling only slowly.
However, in a car like the example Maxima, which runs a small Mazda
intercooler with little thermal mass, the heatsink capability of the intercooler
is quite small. IOTW, you want it cool before boost hits – and then you want
the fan working flat-out at full load to better get rid of the heat!
The techniques covered in this article can be used to run the fan flat-out at
idle and at a slower speed in other conditions, or conversely flat-out at high
loads and at a slower speed at other times.
So which way do you set the system up for your car? The best way of getting a
feel for what’s going on is to fit a digital intake air temp gauge – then the
guesswork is gone. The display we covered at LCD Temp Display!
is an excellent, cheap bit of gear – and is the one used in the
There are two primary ways of altering the speed of the fan.
One of the most elegant techniques is to use a PWM controller motor speed
controller kit. This approach allows variation of the speed manually by a knob
or automatically with engine load, and is a fairly straightforward electronic
kit to build. And one is covered in detail in three articles in AutoSpeed,
starting at Motor Speed Control Module - Part 1.
However, make sure that the current draw of your blower motor doesn’t exceed the
max capability of the kit, which is 10 amps in normal configuration and 20 amps
when extra components are added.
If you’re comfortable with simple electronic kits, the PWM controller is the
best approach. However, if you don’t want to get into diodes and resistors and
capacitors and all those things, there is a simpler technique. It will give you
only high and low speeds and you’ll still have to do some wiring, but it’s much
easier and quicker.
So what’s that way then? It uses a dropping resistor so that in slow mode the
voltage that the fan sees is much lower. This approach is often used by OE
manufacturers - for example, the old Subaru Liberty RS has a two-speed control
of its water/air intercooler water pump, with dropping resistors used to vary
Using a Dropping Resistor
So how does it all work?
Here we have the starting point – the fan is connected to a 12V and earth. It
runs at full speed all of the time.
Now we place a resistor in the circuit. This lowers the voltage that the fan
sees, which slows it down.
Placing a switch in parallel with the resistor bypasses it – so this becomes
the fast/slow speed switch. Close the switch and the fan speed is fast. Open it
and it’s slow.
In essence that’s the speed control. However, in this design the fan would be
running all of the time – even with the car off! That’s because the current draw
of the fan is so great that it should be fed straight from the battery. Because
the currents are so high, we also need to use some relays – these are magnetic
switches that allow a small current to control the larger one being drawn by the
So let’s take a step back and revert to a simple single speed fan.
In this circuit we’ve used a relay. The electromagnetic coil of the relay
(green) is excited when power flows through it, which happens when the on/off
switch is closed. This pulls-in the relay contact (directly above the coil in
the diagram), so switching on the fan. The relay coil draws only very little
current, so the on/off switch can be a light duty design and the wires
connecting it to the relay can also be light duty.
So that’s good – but how about some speed control as well? This requires
Looked at in isolation this looks very complicated – but as a continuation of
the other diagrams, it’s simple. Relay 2 is turned on and off by the Fast/Slow
switch. When this relay is activated, it bypasses the resistor and so the fan
gets fed full voltage (assuming that the on/off switch is closed!).
So in summary, two relays, two switches and a dropping resistor are used.
Relay 1 is controlled by the on/off switch and turns the fan on and off. Relay 2
is controlled by the fast/slow switch and regulates fan speed.
The relays are all normal automotive relays – the numbers on the circuit
diagrams match the numbers universally used on the different connections of the
automotive accessory relays. The switches – which handle only small currents –
can be any toggle or rocker switch. But what about the dropping resistor? Can
that be any old resistor? The answer to that is an emphatic no!
The resistor needs to absorb the power that’s no longer being fed to the
motor – the power has to go somewhere and it is dissipated by the resistor as
heat. This means that high power resistors like those shown here will be needed.
It’s difficult to be specific about the values (these will depend on the exact
blower that you are using and what speed you want the slow setting to be), but
as a guide in the Maxima application, two 0.5 ohm 50 watt resistors were mounted
in series, giving a total resistance of 1 ohm, with 100 watts power dissipation.
This decreased the voltage that the fan motor saw to about 7V, which slowed it
down considerably. (However, even at the slow speed, the airflow through the
intercooler could still be easily felt by hand.)
Series and Parallel Resistors
The total value of resistors mounted in series is as simple as adding their
resistance values together – so two 1-ohm, 20 watt resistors gives a total
resistance of 2 ohms with 40 watts power dissipation.
When two resistors are wired in parallel, the total resistance is halved – in
this case the two 1-ohm resistors would give a total resistance of 0.5 ohms, but
still with 40 watts of power dissipation available.
By using multiple resistors in series and parallel arrangements it’s possible
to gain different values of total resistance quickly and easily.
High power resistors tend to be rated so that they are BLOODY hot when
working anywhere near maximum power. This means two things: (1), you should use
resistors with lots and lots of power handling, and (2), you will probably also
want to mount the resistors on a heatsink.
One approach to sourcing the resistors is to use the injector dropping
resistors used on some older cars. These comprise a ceramic and aluminium
package mounted in the engine bay. Older Nissans and Mazdas, especially, often
used these. By wiring the resistors in different series/parallel arrangements,
it’s possible to end up with different total resistances. Then it’s as easy as
temporarily wiring these in series with the fan motor to see how fast it runs.
However, if you have to buy new, electronics component suppliers (eg Jaycar
Electronics) sell resistors in a variety of values and powers. For example,
10-watt resistors are sold by that company in values from 1-ohm and upwards.
In the case of the Maxima, we used resistors we’d previously bought from Rockby Electronics. These came
pre-packaged in finned alloy cases but for additional heat dissipation, we
mounted them on two heatsinks (salvaged from a defective PC power supply) and
then used an aluminium bracket to locate the complete assembly in the airstream in front of the
As briefly indicated above, it’s important that a high current load like the
blower fan is fed directly from the battery. You should always use a fuse
mounted close to the battery to give protection in case of a short circuit. In
the Maxima the battery has been relocated to the boot and has its own dedicated
circuit breaker (see Relocating the Battery)
and so power was picked up from the connection under the bonnet between the old
positive terminal and the new cable that runs to the battery.
Rather than use an inline fuse, this fuse box and its associated loom were
used. The fuse box was salvaged from a Holden Commodore loom that had been in a
(electrical?) fire – this part of the wiring was still in brand new condition.
Inside the box are two 30-amp fuses, which in this application are ideal. (The
other fuse isn’t used – but it is very handy to have an extra power lead and
associated fuse available under the bonnet.) The complete burnt Holden loom cost
$1 from the shop at a municipal tip.
The fuse box and the two relays were mounted on a bracket used to hold the
intercooler in place. To keep the high current wiring short, the relays should
always be mounted as close as possible to the source of power and to the
The simplest way of operating the fan is to have the two switches shown in
the diagram above mounted on the dash. One is a master on/off switch while the
other gives slow/fast speeds. By watching an intake air temp display you can
easily decide when to switch on the fan, and whether to have it working at high
or low speed. If you want to get trickier, you can replace the switches with
thermostats of the sort shown in DIY Adjustable Temp Switches
and you can even use a multi-coloured LED to show the status of the fan speed -
The Multicolour Dash Indicator.
It makes a radical difference to intake air temps having a good centrifugal
intercooler blower working at even low speeds – at high fan speeds, the temp
drop can be astounding. Now there’s no excuse to have doughy performance off the