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The New Intelligent Intercooler Water Spray Controller, Part 2

Building the very cheap and effective design

by Julian Edgar

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At a glance...

  • Cheap DIY intercooler water spray controller
  • LED bar graph of intercooler temp
  • Triggers pump only when intercooler core is hot and...
  • ...Engine load is high
  • Incorporates delayed off function
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As we covered last week in The New Intelligent Intercooler Water Spray Controller, Part 1, our new intercooler water spray controller has sufficient intelligence that it only sprays when it’s actually needed. That saves a heap of water and provides far better cooling than a dumb boost switch or a manual dashboard mounted control. In addition, the new controller has a 10 LED bargraph display of intercooler temp which makes it much easier to set up.


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The genesis for the new design was a Mixture Meter project completed for Silicon Chip electronics magazine. Also covered in AutoSpeed at Smart Mixture Meter, Part 1, the design used the ten LEDs to display the output of the oxygen sensor. The project also included an alarm that sounded if the mixtures were lean and the engine load was high – ie, a lean-out warning.

It was when we were musing about intercooler water spray controllers that it suddenly occurred: as with the Smart Mixture Meter, a water spray controller needs a load input with an adjustable switching threshold. If the oxygen sensor input is connected to a temperature sensing thermistor, the voltage received at this input can be configured to drop with increasing temperature. Just as the voltage dropped when the air/fuel ratio was running lean.

Therefore, with very little change to the existing kit, the piezo alarm will sound when the intercooler temp and engine load are both high. Hmmm, but in that situation we want to operate a spray pump – not turn on a buzzer. The answer is to replace the buzzer with a heavy duty automotive relay (protecting the switching transistor with a diode) and then replace that switching transistor with a Darlington design that will cope with the new, higher, current demand.

Finally, so that the intercooler water spray won’t turn off on gear-changes or when the load is fluctuating rapidly, we add a 1-second switch-on delay and a 2-second switch-off delay.

In other words, with just a few simple modifications to an existing kit, we could have a good water spray controller. In fact, that approach is the exact one I took to control the intercooler water spray in my turbo Prius – and it’s been doing an excellent job for the last 12 months or so.

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A story was run in Silicon Chip covering how the Smart Mixture Meter could be modified to be an Intercooler Water Spray Controller (see Low-Cost Intercooler Water Spray Controller) - and then there was another interesting development.

Printed circuit board manufacturer Bob Barnes saw the article and decided to produce a printed circuit board (PCB) dedicated to the Intercooler Water Spray function. The PCB is coded SC 05103061 and is available for AUD$17.05 from the contact listed at the end of this story. In addition to mounting all the components on the PCB, the new PCB adds a remote mount LED output to show the driver if the spray is operating. This is a worthwhile feature as in most cases, the unit will be tucked away out of sight after it has been appropriately calibrated.

So now we have a PCB with the full functionality of an intercooler water spray controller. The PCB is available only from RCS, while you'll need to buy the electronic components from an electronics store like Jaycar. All are generic, off the shelf components – there are no programmed PICs or anything like that. (If you want to make things easier, you can get most of the components in one go by buying the Jaycar Smart Fuel Mixture Display kit – cat no KC5374.)

Design Summary

So in summary the design uses a 10 LED coloured bargraph display to show actual intercooler core temperature, as sensed by a remote mount thermistor. When the intercooler temp exceeds a user-adjustable level, a monitoring LED comes on. Engine load is sensed by an input from (usually) the airflow meter. When the airflow meter output signal exceeds a user-adjustable level, another monitoring LED comes on. When both the temp and load monitoring LEDs are on, an externally mounted relay is tripped, so operating the water spray pump. A delay is introduced by a capacitor so that the pump doesn’t rapidly turn on and off with gear-changes and the like.


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Because a complete off the shelf kit of the parts is not available, this project is best suited to those that have built electronic kits before. Follow this overlay diagram and the photos for the component placement. Don’t forget to click on the pics to enlarge them.

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The thermistor connects across the ‘thermistor’ inputs and the spray monitoring LED connects between ‘Ext LED’ and GND, with the flat of the LED going to ground.

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The output to the external relay coil is from these two points – it’s easiest to make the solder connections under the PCB.

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Note that on-car experience has shown that the 2200uF capacitor might need to be increased in value to provide the best delayed on-time period, causing the spray to continue to run for a short period after the load or temperature has dropped. However, initially test the system with the 2200uF value.

Using It

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LEDs 1-10 show intercooler core temperature based on thermistor input

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LED 11 indicates the trip point for intercooler temperature has been reached and VR4 sets this trip point.

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LED 12 indicates the trip point for load has been reached and VR5 sets this trip point.

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Ext LED indicates that the spray relay output has been turned on (ie both Load and Temperature Trip-Points have been exceeded)

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VR1 sets the cold end of the LED bargraph scale

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VR2 sets the hot end of the LED bargraph scale

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VR3 sets the night dimmer sensitivity

Bench Calibration

Connect 12V and ground. Set both VR1 and VR2 to their centre positions. Remember the LEDs will show sensed intercooler temperature – green is cool, yellow is warmer, orange is warmer still and red is hot.

At room temperature (~20 degrees C) turn VR1 until the second from lowest LED (ie LED9 - green) lights, then adjust a little further anti-clockwise until the lowest LED (LED10 - green) is just lit. Warming the thermistor with your hand should then move the illuminated LED from LED10 to LED9. Use a heat-gun to further warm the thermistor. When it is just too hot to touch (eg 55 degrees C) turn VR2 clockwise until LED1 (the ‘hottest’ LED - red) lights. Make sure you don’t turn it so far the LED goes out. When the thermistor is again cold you may need to tweak VR1, as to some extent the pots influence each other.

When calibrated in this manner, the bar-graph display is configured to show intercooler temperatures over approximately a 20-55 degree C range. (Note that because of the thermistor’s characteristics, the bar-graph isn’t linear – it needs a bigger change in temperature to move a LED at the hot end than it does at the cold end.)

On-Car Installation

For remote installation, the thermistor should be soldered to some twin-core flex and the joins insulated with heatshrink. The thermistor should then be pushed well into the fins of the intercooler, inserting it from the rear of the core. This location will give the best indication of intercooler temperature. The thermistor may need to be held in place with a dob of silicone.

The load input for the Intercooler Water Spray Controller can be taken from the airflow meter or MAP sensor outputs, or where these are not available, from the throttle position sensor. This can be done at the ECU or the sensor itself. Use the workshop manual wiring diagram to find the load sensor output and then use your multimeter to check that it’s correct. The measured voltage on the load sensor output should rise with engine load. (Some cars use a frequency output on the airflow meter – in that case, use the throttle position sensor output.)

The intercooler water spray pump relay is best located under the bonnet, near to the pump. The relay is wired to operate the pump.

Test that when VR4 and VR5 are adjusted so that their adjacent LEDs are on, the pump turns on and the water spray operates.

Install an assistant in the car and then drive the car and monitor the displayed temperature on the bar-graph. The lit LED should move up and down the display with the variation in intercooler temperature. Adjust VR4 until its adjoining LED lights when the intercooler is becoming warm. Then adjust VR5 until its adjoining LED lights when the car is coming onto boost.

Drive the car so that the intercooler gets warm and the car is under load and check that both LED11 (intercooler temp) and LED12 (load) light. When both have been lit for more than about a second, the water spray will operate, as shown by Ext LED.

After monitoring the displayed intercooler temperature, you may decide that you want to make further adjustments to the load or temperature trip points. Another important aspect to keep in mind when setting these thresholds is water consumption – check this over a few weeks to make sure that you haven’t got the system tripping too early.

The Intercooler Water Spray Controller can be mounted so that the bar-graph LEDs can be seen by the driver, or alternatively, the whole device can be located out of sight.


Intercooler water sprays are a very effective and cheap way of upgrading intercooler capabilities. However, using just a ‘dumb’ system to trigger the spray often results in the need for frequent water top-ups. Compared with such an approach, the Intercooler Water Spray Controller is likely to reduce water consumption by up to two-thirds with no loss in cooling efficiency.

Over the 12 months that we’ve been using the system we’ve been enormously impressed with its operation. With the spray set to come on at about half full load and the temperature trip set at about 35 degrees C, in winter the spray will stay off for months at a time. But then, as soon as the days start warming-up, on it comes. Driving through a series of steep hills and valleys on a country road on a hot day, you can clearly see the actual intercooler temp getting lower after the spray has been working – the system trips less after the core has been well soaked, even when climbing a subsequent hill on boost. Furthermore, if it’s raining (it rains in hot weather here), the cooling effect of the evaporating rain can clearly be seen - the spray doesn’t trip in these conditions.

In short it’s proved to be a set and forget controller. All you do is keep an eye on the ‘spray-on’ LED and when you see it’s often working, don’t forget to fill the tank!


RCS Design – +612 9738-0330

Jaycar Electronics –

Note: As stated in the main text, because it is not available as an off the shelf kit, this is a project for those experienced in electronic construction. We will not be providing technical advice on fault-finding.

Parts List

1x single pole single throw (SPST) 30A automotive relay (Jaycar Cat. SY-4068 or equivalent)

3x PC mount 2-way screw terminals with 5mm pin spacing

1x Light Dependent Resistor (Jaycar RD3485 or equiv) (LDR1)

1x 100mm length of 0.8mm tinned copper wire


1x LM3914 LED bargraph driver (IC1)

1x LM358 dual op amp (IC2)

1x BD682 PNP Darlington transistor (Q2)

1x BC327 PNP transistor (Q1)

3x 16V 1W zener diodes (ZD1-ZD3)

1x 5.6V 400mW zener diode (ZD4)

2x 1N4004 1A diodes (D1)

2x 1N914 diodes (D2, D3)

3x green, 4x yellow, 2x orange, 1x red 5mm LEDs


1x 470uF 16VW PC electrolytic

2x 10nF MKT polyester

1x 2200ìF 16V electrolytic capacitor

Resistors (0.25W, 1%)

2x 1Mohm

1x 220kohm

4x 10kohm

4x 2.2kohm

1x 10ohm

1x 220kohm horizontal trim pot (VR3)

2x 100kohm horizontal trim pots (VR4, VR5)

2x 5kohm horizontal trim pot (VR1, VR2)

1x 4.7kohm 0.25W resistor

1x1.8kohm 0.25W resistor

1x 4.7kohm thermistor (Jaycar RN3438)

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