This article was first published in 2009.
Last week we looked at using a multimeter to
measure volts, ohms and amps. This week we get into real world modification.
While an electronic modification might look
ridiculously simple and be incredibly cheap, the results don’t match this. One
potentiometer or one fixed resistor – either component tiny and costing under a
few dollars - can make a dramatic change to your car. It might look as absurd as
changing a wheel nut and then expecting the suspension to be revolutionised, but
strange as it seems, just a tiny, low cost component can often achieve the
results you’re after.
So let’s get into a modification – but first, how
do you even start tackling a car electronics system?
Electronic Car Systems
All electronic cars systems – engine management,
climate control, electric power steering, auto trans control, stability control
and so on – are made up of these parts:
In an engine management system, the input
sensors might include the airflow meter (measuring how much air is going
into the engine), throttle position sensor (measuring how much the throttle is
open), camshaft position sensor (measuring the rotational angle of the
camshaft), and intake air temperature sensor. These are the ‘eyes and ears’ of
the system – without these inputs, the system doesn’t know what is going on.
Output actuators in an engine management
system might include the fuel injectors (they regulate how much fuel is added to
the intake), idle speed control solenoid (it regulates how much air bypasses the
throttle butterfly), and camshaft position control (it determines how advanced
or retarded the camshaft timing is).
The Electronic Control Unit (ECU) uses
inbuilt software to determine from the input sensors what conditions the engine
is undergoing, and then what signals should be sent to the output actuators.
All electronic car systems use this fundamental
operating logic. That means that when learning about a new car system, or
working out how to modify it, you should always think: inputs, outputs,
So if you are working on an electric power
steering system, and want to alter steering weight, you must first determine
what the inputs are (perhaps road speed and steering wheel input torque) and
what the outputs are (perhaps the amount of current
to electric steering
motor and the direction of that current flow).
If you are working on modifying an electronic
stability control system, what are the inputs (perhaps steering wheel angle, a
yaw sensor and road speed) and the outputs (perhaps electronic throttle control
and individual wheel braking through the ABS).
Now this might all seem like a huge jump from the
previous parts in this series – suddenly we’re dealing with complex car systems,
not just measuring volts with a multimeter! However, it’s a lot easier to go
from one to the other than you might think. In fact let’s look at one
modification right now.
Intake Air Temp Modification
It needs to be stated upfront that the following
modification has gained a poor reputation through claims it will
revolutionise the performance of your car.
For example, I’ve seen people claiming that on one
particular car, you can gain 30 horsepower – or, if you want, 20 per cent better
economy! These figures are completely and utterly absurd.
However, in terms of bang for your buck,
the results on some cars can be absolutely outstanding. So, what modification is
being talked about?
As we mentioned above, the engine management
systems (in most cars) uses an inlet air temperature sensor. As the name
suggests, the ECU uses this to determine the temperature of the inlet air. But
it’s what it does with this information that’s interesting. In addition to
working out the density of the air (ie how heavy it is, which indicates how much
oxygen there is in a given volume), the ECU uses inlet air temp as an important
input in determining what ignition timing to use.
If the inlet air is colder, the ECU can
advance the ignition timing.
If the inlet air is hotter, the ECU will
retard the ignition timing.
Advanced ignition timing, especially when using
higher octane fuel, will result in more power. In many cars, this will be felt
as better light-throttle response, for example, the ability to use a higher gear
in a given light-load cruise situation. This makes the car sweeter to drive and
can improve fuel economy.
So, if we can make the ECU think that the
intake air is colder than it really is, the ECU will advance the ignition
timing. And this can be achieved very simply!
Intake air temp sensors use variable resistance
designs – that is, they vary in their resistance to current flow with changes in
temperature. Normally, they use a design where resistance (measured in ohms)
increases as the temperature decreases.
If you want the ECU to think that the temperature
is colder, you add extra resistance in series with the sensor.
Resistors and Pots
To change the resistance of the intake air temp
sensor, you need to use a resistor. This is an electronic component
available in a range of resistance (ohms) and power (watts) values. (In this
application the lowest power rating – ¼ watt – is fine.) Resistors are available
from electronics parts stores for literally a few cents each. For example, you
might buy a ¼ watt, 3.2 kilo-ohm resistor (3.2 kilo-ohms = 3200 ohms).
But in this modification (and many others), it’s
easiest if you start off with a variable resistor. This allows you to
adjust the modification until you get the results you’re after. A variable
resistor sounds expensive but again the component is very cheap. In electronic
terms, a variable resistor is called a potentiometer, or pot. Pots cost
about $2 each.
Let’s have a closer look at these two components.
A resistor poses a resistance to the flow of
electricity. It doesn’t have any polarity (ie it can go into the circuit either
way around) and its resistance is marked with colour-coded bands on the body of
the resistor. Don’t bother trying to learn the colours – just buy the right
value and then check it with your multimeter. In this series we will show
resistors on circuit diagrams as rectangles.
So here is a resistor in a circuit – its presence
would cause the light to glow less brightly. (That’s because, as described in
How to Electronically Modify Your Car, Part 3, there’s a voltage
drop across the resistor.)
A pot has three connections. Two of the
connections are to the two ends of the internal resistor, while the third
connection is for a wiper that can be moved along the resistance track, allowing the resistance to be chaged. When you
use a pot as a variable resistor, just two connections are used – one end of the
track, and the wiper. On a pot the connections are usually physically like
they’re shown on this circuit diagram.
When you get a pot – any pot – always check its
resistance. Set the multimeter to ‘resistance’ (or ‘ohms’) and then connect the
probes to the pot as shown. The meter should show the maximum value of the pot.
So a 10 kilo-ohm pot should have a value close to 10 kilo-ohm (10,000 ohms). It
doesn’t matter if it isn’t exact, but if the measurement shows (say) 4.8
kilo-ohms, it’s a 5k pot not a 10k pot!
Next connect the multimeter to the pot like this.
Rotate the shaft of the pot and you will see the ohms read-out on the multimeter
increase. (If it decreases, swap the outer connection to the other side of the
pot – ie to the unused pin.)
It is always good to test pots like this. The
other day, I was doing a car modification with an unusual design, multi-turn
pot. I figured the pins were like any other pot pins and didn’t check them
thoroughly with the multimeter. Result? I wired the pot into the car incorrectly
– and then spent a few hours chasing my tail, wondering why the mod didn’t
Another way of showing a pot in a circuit is like
this. The arrow shows the wiper (the moveable arm) that can go from one end of
the resistance track to the other. Showing the pot like this is easier to
understand – when the wiper 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. In this series, pots will be shown on circuit diagrams like this.
might be wondering why a pot has three connections, when here we’re using only
two. The answer is that a pot can also be used in a completely different way to
being just a variable resistor. The other use (as a true potentiometer) is also
extremely useful in electronic car modification - we’ll talk more about this
Back to the Modification
OK, so now we understand fixed value resistors, and
variable resistors (pots). Now, how do we apply them to the intake air temp
sensor modification? Well, if we start off with a variable resistor (pot) we can
easily adjust the value.
Set the pot to zero resistance (as measured by the
multimeter) and then wire it into the circuit for the intake air temp sensor
like this. Right, that’s the pot installed (easy, huh?). Simply turn the pot and the ECU will 'think' that the intake air temp is colder than it really is.
Now what? Here’s how I did this mod on a hybrid
petrol /electric Honda Insight, and what the results were.
Car Modification – Intake Air Temp Sensor on Honda Insight
hybrid Honda Insight is in many ways a simple car – certainly, much simpler than
the Toyota Prius. The engine, a 1-litre 3 cylinder, uses conventional engine
management – not even electronic throttle control.
(Australian-delivered) car is designed to run on 95 RON fuel but I normally use
98 RON. The RON value is purely a measure of the fuel’s resistance to
detonation; nothing else. Higher octane fuel therefore has a higher resistance
to detonation, so can tolerate a higher engine compression ratio and/or more
ignition timing advance.
an extent, the ECU will automatically advance timing when running on higher
octane fuel – but only to an extent. It expects 95 RON fuel, so it’s never going
to advance timing to the degree it would if originally calibrated for 98 fuel.
what, I wondered, would happen if I altered the signal the ECU saw from the
intake air temp sensor? Since the lower the intake air temp, the greater is the
engine’s resistance to detonation, if the ECU was convinced that the intake air
temp was actually lower than it really was, it could be expected to run more
ignition timing advance. That could in turn well suit the higher octane
Honda workshop manual provides no real detail on the intake air temp sensor, but
it is easily removed and tested. At about 35 degrees C the resistance was 1600
ohms, at about 20 degrees C it was 2000 ohms, and when packed briefly in ice it
increased to 5000 ohms. All measurements were done with a multimeter.
(to reiterate), higher resistances equal lower temperatures.
snipped the signal feed near the sensor itself (this could have been done at the
ECU but it was simpler to do it under the bonnet) and wired-in a 5 kilo-ohm (ie
5000 ohms) pot as a series variable resistor (ie as shown above).
if neither wire is connected to the sensor body ground, the pot can be inserted
in either wire. If one side of the sensor wiring is grounded at the sensor, then
the pot must go in the signal wire. On the Insight the signal wire is
used a 10-turn pot so that changes could be made very gradually, but a normal
pot could be used if care was taken with rotation. (10-turn pots are much rarer
than normal rotation pots – I bought mine on eBay.) Note that as can be seen in the pic below, this particular pot uses an unusual terminal configuration - always check unknown pots with a multimeter before wiring them into place!
turning the pot, I could therefore make the ECU think the intake air temp was
colder than it really was. I turned the pot and noted that idle sped rose
slightly before then falling. This is indicative of an advance in ignition
timing (what was wanted) followed by an idle speed correction. (Note that this
change in rpm didn’t always occur – it depended on other parameters like engine
wound in about 3000 ohms of extra series resistance and went for a drive.
greatest care should now be taken to listen for detonation. If you don’t
have an acute ear for it, do an AutoSpeed site search for some of the electronic
detonation detectors we have described. Of course, high octane fuel should also
be being used.
the road the Honda was clearly far more driveable. In light load driving, gear
changes could be made earlier, a characteristic of increased light load torque.
The earlier up-changes also suggest that in urban driving, the fuel consumption
might be a little improved.
5th gear could be used up slight rises at 60 km/h, something the car
was reluctant to do previously.
on a highway fuel economy test, there was no discernible change.
detonation could be heard and no check engine light appeared.
using a pot in the initial configuration, you can
twiddle it to your heart’s content. When you have decided on the value that is
required, the pot can be removed and its resistance measured with a multimeter.
fixed value resistor of the same value can then be bought and wired in series
with the temp sensor, the lot then covered in heatshrink or tape. And that’s
just what I did, using a 3.2 kilo-ohm ohms resistor (that actually measured
closer to 2.9 kilo-ohm!).
Part of the above article was written when I first
did the modification and wrote about it in AutoSpeed. In the months since, I
have been able to enjoy the way the car now drifts along on tiny throttle
openings, tractable and responsive even in high gears at low revs. Personally
(and of course you may not feel the same way), I’d value the results of the modification
at about $500.
And the wonderful thing is that if you try it on
your own car – and it doesn’t work – you’ve spent only an hour and $5. Can’t
beat those electronic mods, can you?!
Next week, we’ll look at other uses for pots.
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