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
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!)
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
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
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.
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
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
The driveability of the car was perfect.
Car Modification – Increased EGR on Honda Insight
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.
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.
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.
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.
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
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.)
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
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.)
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.
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.
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?
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
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.
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