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How to Electronically Modify Your Car, Part 8

Using an electronic voltage switch module

by Julian Edgar

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At a glance...

  • Using an off-the-shelf voltage switch module
  • Wiring and adjustments
  • Automatically switching the air conditioner
  • Improved performance and fuel economy, especially on small engine cars
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This article was first published in 2009.

Last week in How to Electronically Modify Your Car, Part 7 we looked at how relays can be used for a lot more than just operating driving lights and similar uses. This week, we’re going to look at how to use an off-the-shelf pre-built electronics module, the eLabtronics Voltage Switch.

Two points need to be stated at the outset.

So far in this series, we’ve used components that are very cheap indeed – resistors, pots and relays. The electronic module covered here (available from the eLabtronics) is much more expensive – although in terms of most car modifications, it’s still very cheap.

Secondly, this module looks a lot more complex than the modifications we’ve so far covered – there are lots of complicated electronic parts on the printed circuit board! However, working with a module like this is actually very simple - if you have been reading each part of this series, you’ll have no problem.

So what does the module do? The Voltage Switch monitors a signal voltage and when an adjustable trip-point is reached, it lights the on-board LED and turns on its output. Let’s look at that in more detail.

Voltage Switch Module

Click for larger image

As can be seen here, there are only four wiring connections. These are:

  • +12V (ie power)

  • Ground

  • Input

  • Output

There is also a multi-position option switch and two adjustable pots. One pot sets the trip-point and the other pot sets the difference in voltage between the turn-on and turn-off values.

Click for larger image

Here’s a wiring diagram showing one way that the module can be used. In this case, we want to turn on a fan when the engine load is low. For example, that could be an intercooler fan that switches on when the car is at idle, reducing heat soak at traffic lights. Engine load is sensed from the engine management's system airlow meter, and the fan is directly driven by the module.

Starting from the bottom of the diagram, the module is connected to 12V and ground.

The next connection is to the airflow meter output signal. This connection is made in parallel with the existing connection between the airflow meter and the engine management ECU – we just tap into this wire. Because the module takes almost no current on this input, the extra monitoring of the signal doesn’t change the signal that the ECU sees. In other words, not only does the airflow meter feed information to the ECU, it also now feeds information to our Voltage Switch.

Finally, here the output of the Voltage Switch is connected to a fan, with the other side of the fan grounded. This fan wiring is the same as we’ve shown previously in this series - one side of the component is grounded (and so connected through the car body to the negative terminal of the battery) and the other side is fed +12V.

Now that’s the wiring finished! As you can see, despite is looking like a complex module, with only four connections (and two of those just power and ground), it’s actually very easy to wire into place.

Let’s now look at the adjustments, starting with the multi-position switch.

DIP Switch Positions

Click for larger image

The Voltage Switch has a four-position DIP option switch. When setting switch positions, the board is positioned so that the terminal strip is on the right.

So what are the different switch positions for?

If you think about the example we gave above of turning on an intercooler fan when the airflow meter signal was low, you’ll realise that in that case we want the Voltage Switch to trip when the signal falls below a certain point. For example, if the airflow meter output is 1.2V at idle and 4.6 volts at full power, we might want the intercooler fan to come on when the airflow meter signal drops below 1.3V.

But a much more common requirement is to trip the switch when the monitored voltage rises above a certain level. For, example, you might want to turn on an intercooler water spray pump when the engine load (and so airflow meter voltage) are high. Therefore, you might want the Voltage Switch to trip when the monitored voltage rises above 3.8 volts.

Clearly then, two different modes need to be provided – one that trips when voltage falls below a certain level, and one that trips when voltage rises above a certain level.





With the DIP switches set in the pattern shown above, the Voltage Switch trips as the monitored voltage rises above the set-point, causing the on-board LED to illuminate and output to turn on. The LED and output then switch off when input voltage falls below set-point.





With the switches set in this pattern, the Voltage Switch trips as the monitored voltage falls below the set-point, causing the on-board LED to illuminate and the output to turn on. The LED and output switch off when input voltage rises above set-point.

(There are other DIP switch positions that allow the output to flash just a few pulses when the trip-point is met, or alternatively to continuously pulse the output. The eLabtronics Voltage Switch, Part 1 describes all the switch functions.)

Pot Positions

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Two adjustable pots are provided on the module.

The first is Set-Point. I’ve been using this term without having first defined it – but that’s because it’s pretty obvious. The set-point is the signal level at which the switch trips or switches. You adjust this pot to set the voltage level at which you want the output to turn on. Rotating this pot clockwise increases the input voltage level at which the module trips.

The other pot has a name that looks a lot more daunting – Hysteresis. Hysteresis is the difference between the switch-on and switch-off values.

In one of the above examples, we had an intercooler water spray turn on when the airflow meter signal rose to 3.8V. But at what voltage does the water spray switch back off again? If it switches on at 3.8V, and switches off at 3.8V, there’s clearly a problem. In fact, at 3.8V the pump would chatter on and off.

But what if we have the pump switch on at 3.8V and switch off when the voltage drops back to 3.4V? That way, the pump will be decisively on or off – not chattering on and off. In this case, the hysteresis has been set to 0.4V (3.8 – 3.4V).

With the module, hysteresis is adjustable over a wide range - rotating the hysteresis pot clockwise increases the hysteresis.


Before we look at an on-car modification using the Voltage Switch, consider some of its uses.

You can monitor any sensor that uses a changing voltage output – not just an airflow meter but also a MAP sensor, coolant temperature sensor, intake air temp sensor, fuel gauge, oil pressure gauge, yaw sensor, throttle position sensor and so on. So you can switch devices on and off on the basis of temperature, engine airflow, intake manifold pressure, oil pressure, fuel level – whatever is being measured by the sensor.

You can even monitor battery voltage itself, eg to sound an alarm if the voltage gets too low, or to disconnect a battery charger if the voltage gets too high.

Let’s take a look in more detail at one application of the Voltage Switch module.

Example Car Modification – Auto Air Conditioning Controller

If you have a small engine car, or one where you want to chase every possible ounce of power and fuel economy, automatically switching off the air conditioner at high throttle angles is very effective. You get more power when you put your foot down, and – because when accelerating and climbing hills, you can hold taller gears and so use less engine revs – better fuel economy as well.

To achieve this outcome, the Voltage Switch monitors the output of the throttle position sensor. These sensors invariably have a voltage output that increases as the throttle is opened wider. By monitoring this voltage, the Voltage Switch can be set to trip when a certain throttle angle is exceeded. And, by setting an appropriate hysteresis, the point at which air con turns back on can also be set.

So how do you turn off the air con when the Voltage Switch trips?

All air conditioner compressors use an electro-magnetic clutch. This means that when power is fed to the clutch, the compressor is able to be driven by the engine. When power is cut off, the compressor drive wheel just spins freely. This compressor clutch power feed is the single wire that you can see going to the front of the compressor. To disable the compressor, all that we need to do is to put a relay in this circuit, so that when the Voltage Switch trips, the power feed to the compressor clutch is turned off. (See How to Electronically Modify Your Car, Part 7 for coverage of relays.)

Click for larger image

Here is the wiring diagram. When looking at a diagram like this, mentally break it down into sections.

We know from earlier that the Voltage Switch needs power and ground connections. At the bottom, we can see that the power connection is fed from the air conditioner switch – that is, the switch on the dash that turns on the air con. Wiring the 12V supply in this way means the Voltage Switch is powered-up only when the air con is running. The negative (-) connection of the module connects to Ground. OK, so that’s power and ground connections.

The input of the module connects to the throttle position sensor.

The output of the module connects to a relay. From Part 7 in this series we know that connection must be to one side of the coil of the relay (ie pin 85) and the other side of the coil must connect to ground (pin 86 – and yes, it does go to ground).

The Voltage Switch is set to trip when the throttle position sensor voltage rises above a certain level, at which point the relay is activated. Now we want the relay to break the circuit to the air con compressor (ie turn it off) so we connect the module to the relay contacts that open when power is applied (87A and 30).

Click for larger image

Here is the modification wired into place. The green arrow points to the Voltage Switch module mounted inside a box, and the red arrow points to the relay mounted alongside.

Click for larger image

And here’s the view with the box open and the mounting bracket not yet in place.

I fitted the Auto Air Conditioner Controller to my Honda Insight and the results have exceeded my expectations. On the small engine Honda, the ability to hold higher gears up hills and accelerate more easily at small throttle angles is excellent.

An unexpected benefit – and probably the thing I notice the most – is the absence of the air con cutting-in when climbing a long hill. Previously, you’d have the car set to work with a fairly large throttle angle at low-ish revs in a high gear (the best approach for fuel economy) and then part way up the hill, the air con would cut-in and all the balance would be lost.

With the Auto Air Conditioner Controller in place, time after time it’s like you have a really attentive passenger with their finger hovering over the air con button, ready to turn the air con off at the first sign you can benefit from their doing so.

Fuel economy has also clearly improved – previously, running the air con would increase fuel consumption by about 20 per cent. With the air con controller in operation, this is reduced to about 10 per cent.


The availability of prebuilt electronic modules like the eLabtronics Voltage Switch allows very effective and easy modifications to be made, utilising the sensors that are already present in the car. This reduces cost (no need for new sensors), makes it easier to wire into place (no need for new wires from the sensor), and better integrates the modification into the car.

Next week, we’ll look at building your own electronic modification module – that is, constructing a kit.

The parts in this series:

Part 1 - background and tools

Part 2 - understanding electrical circuits.

Part 3 - volts, amps and ohms

Part 4 - using a multimeter

Part 5 - modifying car systems with resistors and pots

Part 6 - shifting input signals using pots

Part 7 - using relays

Part 8 - using pre-built electronic modules

Part 9 - building electronic kits

Part 10 - understanding analog and digital signals

Part 11 - measuring analog and digital signals

Part 12 - intercepting analog and digital signals

Part 13 - the best approaches to modifying car electronics ? and the series conclusion

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