Triggering an Intercooler Fan

In turbo intercooled cars, it’s of critical importance to keep the off-boost temperature of the intercooler core low. That’s the case because an intercooler that’s kept colder can absorb more heat before passing that heat on to the engine. Basically, the intercooler works more effectively as a heat-sink, smoothing out the intake air temp spikes that otherwise occur in turbo road cars. But when people are making decisions about intercooler size and placement, most think about only getting rid of the heat being created when the car’s on boost – making sure that there’s adequate airflow through the intercooler, and so on.

And hey, that’s fine – except when you’re stuck at the traffic lights and your underbonnet intercooler is acting as a pre-heater, funnelling hot air through it and out the bonnet scoop. Or your front-mount intercooler is absorbing a heap of heat from the radiator which is only a few centimetres from it... These are major reasons why in many turbo cars, the maximum intake air temps are recorded when the car is actually stopped, idling in hot traffic. Yes, that’s right – not on full boost at all! (And the significance of that is when you cane it off the line, well....)

So how do you keep the intercooler temps down when you’re stationary? The best way is with a fan. That could be the existing electric radiator fan (or the ancillary one often factory-fitted for the air con condenser) or a purpose-fitted fan designed just for intercooler temp reduction. In the case of the rad or air con fan, operating it sucks air not only through the radiator but also through the front-mount intercooler core. That’s especially the case if you use intelligent sealing (eg by black-painted foam rubber) around the core edges. Fed a supply of outside air, the intercooler is much less likely to get heated by the close proximity of the radiator. In the case of underbonnet intercoolers, the dedicated fan will stop the chimney effect - reverse airflow from the engine bay out through the core.

So easy, huh? Just fit a fan (or trigger one already fitted) whenever the intercooler would otherwise be heat soaking. But how do you automatically trigger it?

Triggering Intercooler Fans

In AutoSpeed we’ve previously covered a technique (Our New Pressure Switch!) that used a very sensitive switch to detect the aerodynamic pressures created when the car is moving. However, the hysteresis of that system (ie the difference between the ‘on’ and ‘off’ pressures) was fixed and an anti-chatter relay and timer needed to be added.

This time we’re doing it differently. 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 Jaycar stores or the AutoSpeed Shop.) By connecting the input of the Voltage Switch to the airflow meter, the fan can automatically be triggered at idle loads. Furthermore, because the module has adjustable trip-point and hysteresis, you can also make it so that the fan keeps running until the load has exceeded a preset amount.

Let’s look at what happens with this system. (The voltages used here are examples only – different cars will have different voltages.) The Simple Voltage Switch (SVS) is configured to react to a falling voltage, switching on the fan at airflow meter signal voltages of 1.5V and lower. When you’re driving along, the airflow meter is spitting out a voltage signal of 2.5V, so the fan remains switched off. But as you lift the throttle approaching a set of traffic lights, the airflow drops back to a low value, the voltage output of the airflow meter decreases to below 1.5V – and the fan is triggered. At the traffic lights the fan keeps on running, then when the light turns green, you move off. Because of the adjustable hysteresis, the fan can be set so that it won’t switch off as the voltage rises past 1.5V. Instead, you can make the ‘off’ voltage 1.7V, which has the benefit of keeping the fan running if you are just moving along slowly in a traffic jam.

By adjusting the setpoint (ie the trigger point) and the hysteresis (ie the difference between on and off voltages) it’s possible to set the SVS so that the intercooler fan doesn’t even come on with gear-changes – the car actually has to come to a stop and the airflow drop right back to idle before the fan is triggered. Alternatively – especially in an auto trans car where the load stays high on gear-changes - you can set the system up to trigger the fan earlier.

Fitting and Set-Up

The first step is to measure the output voltage of the airflow meter. Use the workshop manual to locate the output signal wire, or if you don’t have one, back-probe the airflow meter until you can find a signal that varies in voltage with engine load. The SVS is designed to work with voltage signals varying within a 0-5V range, so if you can’t find such a signal (because the airflow meter outputs a frequency signal, for example), you won’t be able to use this approach.

The second step is to build the kit and then 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 High Performance Electronics for Cars book which contains a range of projects (including this one), covers the background in engine management, and shows how to build electronic kits.

With the kit built and tested, install it in the car. Connect 12V and earth wires correctly, and tap into the airflow meter output for the signal feed to the SVS. (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 a pilot light in place if you can’t see the SVS’s LED.

Turn the hysteresis pot fully anticlockwise, then start the car and turn the voltage adjustment pot until the SVS’s LED is on at idle but goes off immediately you blip the throttle. Then go for a drive. Try some starts and stops, making appropriate adjustments to the setpoint adjustment pot until the LED behaves as you want the fan to. Then adjust the hysteresis pot clockwise until the right on/off behaviour is found.

(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.)

The guinea pig car was a 1988 Maxima V6 Turbo with an underbonnet intercooler equipped with its own dedicated cooling fan (see DIY Budget Intercooler Fitment). In this car it was possible to adjust the SVS so finely that the LED came on only when the car had come to a complete stop and the idle had settled. (However, the final setting had the LED lighting much earlier than this.)

The next step is to connect up the fan. (Because it’s useful to see when the fan is on, we’ve also included a dash-mounted pilot light in all these diagrams.) Many fans will able to be driven directly from the SVS’s on-board relay. However, big radiator fans and the like should use an additional heavy duty relay.

If you are using the on-board relay (rated at 5 amps max) to switch the fan, the wiring will look like this. The Maxima’s modified Daihatsu radiator fan drew a peak current of 3 amps at switch-on, so it was able to be driven directly from the SVS.

If you are switching an automotive type heavy duty relay (able to switch up to 20 amps or more), connect it up like this. Note that this circuit can be connected to the fan in parallel with the existing factory radiator (or air con fan) relay and control system – the standard operation of the fan won’t be affected; it’ll just be on more often.

The Results

Testing of the system showed that it was very effective. Because of the Maxima’s small intercooler, the fan was set to come on fairly early and switch off fairly late, ie to be running more often than would be the case in some cars. On the road the car felt much sharper off the line after it had been idling in traffic – in fact you really don’t realise how much a typical intercooler gets heat-soaked until it’s not happening!

Temperature measurements clearly showed the reasons for this change. The temperature of the intercooler core was measured for 4 minutes after the warmed-up vehicle had come to a halt. With the fan running, over this period the intercooler core temp rose from 16.4 degrees C to 19.2 degrees – just a 2.8-degree increase.

But without the fan running, in the 4 minutes the core temp rose from 17.4 to 44 degrees – an increase of 26.6 degrees C! No wonder the car feels sharper off the line when the fan is drawing cool air through the intercooler. Note also that these measurements were made with an ambient of only 11.1 degrees C... the story is likely to be much worse in hot weather.

Conclusion

By using the voltage switch to control the operation of an intercooler fan, it’s possible to very effectively fight intercooler heat soak. The wide range of adjustment also allows you to customise the fan trip point to your particular requirements.

AutoSpeed Shop , Simple Voltage Switch