How to Electronically Modify Your Car, Part 6

This article was first published in 2009.

Last week in How to Electronically Modify Your Car, Part 5 we looked at how a cheap resistor or pot can be used to trick the engine management system into running more advanced ignition timing. This week we’re going to look in more detail at how pots can be used.

Resistors vs Pots

As we have already described in this series, a pot can be wired as a variable resistor. In this circuit diagram, when the pot’s wiper (indicated by the arrow-head) is at the bottom, no resistance is introduced into the circuit. When the wiper is at the top, maximum resistance is introduced into the circuit. If the pot had the right values of resistance and power dissipation, you could vary the brightness of the light by changing pot position.

A pot wired as a variable resistor is good when you want to add resistance to a sensor circuit. (The example we used last week was of an intake air temp sensor, where the resistance went up as intake air temp went down. Adding a resistance meant that the ECU thought the intake air temp was colder than it really was).

But that’s all a variable resistor can do – add resistance, or, if the wiring is done differently (ie in parallel), subtract resistance.

But most sensors in cars don’t use a variable resistance designs. Instead, they have inbuilt electronics that causes them to output a varying voltage. For example, most airflow meters, MAP (manifold absolute pressure) sensors, accelerometers, yaw sensors and throttle position sensors have an output voltage that varies across the range of about 1-5 volts. If you just insert a series variable resistor in their circuit, things are liable not to work in the way you might expect!*

But by using a pot in a different way, very good modification results can be gained with voltage outputting sensors. Before we look at how to do it, let’s examine the type of sensors that we’ll probably be applying this modification to.

(*Strictly speaking, the variable resistance sensors are actually variable voltage outputting sensors, but I am trying to keep things as simple as possible!)

Sensor Outputs

Many voltage-outputting sensors have three connections. Here is a diagram of a MAP sensor. One wire (here it is red) is a regulated 5V supply from the ECU. (Regulated means it is held at a fixed voltage, irrespective of battery voltage.) There is also a ground wire (black) that somewhere is connected to the car’s body – probably back at the ECU. Finally, there is another wire (green here) that is the signal output.

If you connect a multimeter to the signal and ground wires, and then drive the car, you’ll be able to see on the multimeter how the signal varies in different driving conditions. For example, you might find that the lowest reading is 0.8V and the highest is 4.6V. You might also be able to see that the highest occurs at full throttle and the lowest occurs on the engine over-run.

The ECU uses this signal to detect what is occurring – in this case, what the intake manifold pressure is at any given moment.

OK, let’s go back to the MAP sensor. (Remember that the MAP sensor is connected to the ECU, although I haven’t bothered showing all the wiring.) The highest that the MAP sensor output can go is 5V, and the lowest it can go is 0V. But this sort of sensor is rarely holding a steady signal output – it’s up and down as the driver moves their foot.

So if we wanted the ECU to see a higher voltage coming from this sensor, we can’t just cut off the sensor wire and connect it to 5V, as shown here. Sure, that would mean that the signal voltage the ECU sees from the sensor is higher – but it’s also now fixed, not varying with engine conditions. For the same reason, we can’t just connect the signal wire to ground.

If we want to raise the sensor output voltage (so for example, the ECU thinks that manifold pressure is higher than it really is), we need to add a bit of voltage to the signal. So, how do we do that?

What we have done here is wire a 10 kilo-ohm pot between the signal wire and the 5V wire. Note how the connections have been made to each end of the pot’s resistance track. (This might look like we’re connecting the signal wire straight to 5V, but we’re not – the resistance of the pot is so high that almost no current flows through this connection.)

Let’s put a multimeter in the circuit, measuring the voltage on the wiper of the pot. (Note how the other probe of the multimeter still goes to ground - voltages are almost always measured with respect to ground.) The pot has been set so that its wiper is at the top – close to the 5V supply. This is the same as connecting the multimeter probe to the 5V supply, so the meter reads 5.0V.

No we’ve moved the pot wiper so that it’s down the bottom – effectively connecting it straight to the signal wire. Since the signal is at 1.6V, that’s also what the meter reads.

But now we’ve moved the wiper up just a bit. The multimeter is reading a combined signal – the signal voltage plus the little voltage we’ve added to it. So the meter reads 3.3V – we’ve added 1.7 volts to the signal.

There’s just one step left. If we connect the ECU signal wire to the pot wiper rather than to the MAP sensor, we can dial-in any addition to the sensor signal that we want. The signal will still rise and fall as it did before, but with an additional voltage on top. By changing the position of the pot, we can change how much voltage we add to the signal. (See later in this story for the way that this addition actually occurs.)

If we want to subtract a voltage, we just connect the pot between ground and the sensor signal, like this.

Uses

This modification is a very powerful one that can be applied in many different circumstances. We’ll show in detail a modification in a moment, but the first way I ever used this mod remains perhaps the most impressive.

I had a small 3-cyliner Daihatsu Mira turbo (660cc!) to which I’d fitted a bigger turbo and water/air intercooling. The injectors were running out of flow capability so I fitted larger injectors from a Charade GTti. But the ECU didn’t know that larger injectors were being used, so over-fuelled the car.

The ECU uses as its main load input a MAP sensor, much like the one described above. By using a pot on the output of the sensor, I was able to lower the voltage (and so the load) that the ECU thought it was experiencing. This resulted in the ECU pulsing the larger injectors at a reduced rate (technically speaking, at a reduced duty cycle) and so the fuelling was able to be adjusted so that it was again correct. And, at the top end, where the engine previously ran out of injector flow, the larger injectors could keep up!

The driveability of the car was perfect.

Example Car Modification – Increased EGR on Honda Insight It’s not generally realised, but Exhaust Gas Recirculation (EGR) can be beneficial for part-throttle economy. (This article would grow too long if we went into detail on why this is so – see EGR Comeback for more on this topic.) I therefore decided to increase the amount of EGR occurring on my car, a hybrid petrol/electric Honda Insight. The Insight uses an EGR valve that is electronically-controlled by the ECU. This valve, normally held shut by a spring, is opened by the action of electrical current through a coil. The amount that the valve opens is monitored by a lift sensor. The ECU monitors this lift sensor and alters the opening of the EGR valve to give the required ECU-mapped EGR flow for the driving conditions. A multimeter was used to measure the lift sensor output when the car was being driven. This showed that the voltage from this sensor rose as the EGR valve opened by greater amounts, being around 1.2V with the valve shut and rising to about 2.5V at its peak. The values aren’t very important – what is important, is that voltage rose with greater valve opening. If this sensor signal could be altered so that the ECU was told that EGR valve opening was less than it actually was, the ECU would compensate by opening the valve more – that is, EGR would increase. To increase the amount of EGR that occurred, this circuit was used to reduce the sensor’s output voltage – note how it is identical to one of the diagrams already discussed. The car was then driven in a variety of situations, with the increase in the amount of EGR being finely adjusted by turning the pot positioned in the car. (I used a full-size 10-turn pot, which allowed very fine adjustment to be easily carried out while on the move.) With too much EGR flowing, in some driving situations the car could lurch and stumble, eg when passing over the crest of a slight rise and lifting-off a little. Fine tuning of the increase in EGR consisted of adjusting the pot to the point where driving behaviour was virtually identical to standard, with perhaps just a fraction more throttle needed when full EGR was occurring. (That’s as you’d expect.) The final configuration runs a lot more EGR than standard. In carefully tested urban conditions, the result was a small but measurable fuel economy improvement of 3 per cent. (In highway conditions there was no change.) This might not sound like much of an improvement, but the Honda Insight hybrid is amongst the most fuel-efficient cars in the world – so to be able to make any improvement is impressive. Secondly, the modification cost almost nothing and was easily installed.

Implications

A 10 kilo-ohm pot can be used in nearly all cases without upsetting the original sensor behaviour (note: an exception is a narrow band oxygen sensor, where even a 10 k pot is too great a load.). By using a multi-turn pot, fine adjustments can be made.

As with any modifications that alter a sensor’s output, you need to be aware that the ECU will change all of its outputs that rely on that sensor’s input. In the case of the Honda Insight’s EGR valve, the lift sensor is used only for EGR feedback. However, other sensors (eg a MAP sensor) are used by the ECU for lots of things. For example, reducing the output of the MAP sensor will not only alter the pulsing of the injectors, but will also advance the ignition timing.

Another point to keep in mind is that the technique does not add a constant voltage to the signal. Instead, it does better than this by adding (or subtracting) a constant percentage. For example, if you set the pot to add 20 per cent to the sensor voltage, this percentage stays the same across the whole sensor output range. This is one reason that the pot approach works so well in practice.

Multi-turn Pots? Both in this week’s article and also in Part 5 of this series, reference has been made to ‘multi-turn pots’. So what are they? Most pots rotate through only about 270 degrees - less than a single turn. However, multi-turn posts can have 10 or even 15 whole turns to cover their full range. In a car modification, this makes it much easier to dial-up the exact effect you want. Many multi-turn pots are designed to be soldered to a printed circuit board – they are very small and are adjusted by a fine screwdriver. These are available from hobby electronics suppliers. Large multi-turn pots that use a knob are available from industrial electronic supply sources. It’s worth buying one - eg a 10 turn, 10 kilo-ohm unit.

Conclusion

The use of a pot to tweak the signal of a voltage outputting sensor is very powerful. It is also very simple to install, extremely cheap, and allows fine tuning of the modification.

Next week we’ll cover another overlooked but effective way of modifying cars – relays.

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