This article was first published in 2005.
More and more cars are now
being fitted with fully electric power steering. In these systems, hoses, pumps
and reservoirs are dispensed with – instead, an electric motor attached to the
steering column does all the work. The advantages to the manufacturer include
lower servicing and assembly costs, and to the consumer, less likelihood of
failure through power steering fluid leaks. Fuel consumption is also improved.
But there’s another
advantage to the modifier, one which so far has been completely overlooked.
Because it’s an electronically-controlled system, it’s easy to alter the
characteristics of electric power steering to suit individual preferences.
Specifically, you can alter the steering weight to radically improve steering
feel and high speed stability.
And you want the good news?
You can give your electric power steer car user–adjustable control over the
steering assistance for under ten bucks.
The modification covered in
this story was carried out on a Toyota. However, we’d expect that very similar
changes would be possible on any car with electric power steering that uses a
torque-based measuring system to determine the amount of electric
Electric Power Steering Systems
It’s now over four years
since we covered the basics of electric power steering (see Electric Power Steering)
but what do most electric power steering systems look like today? Well, some are
simpler than covered in that story.
Typically, an electric
power steering system consists of:
- - a powerful electric motor geared to
the steering shaft
- - torque sensor(s) that detect how
much effort is being put into the steering
- - an electric power steering Electronic
Control Unit (ECU)
- - a road speed input to the
The ECU looks at the
steering torque and steering direction being applied by the driver, and at the
road speed, and directs the electric motor to provide the required amount of
assistance in the correct direction.
Torque refers to the
strength of twist being applied to a shaft. The higher the twisting force, the
higher the torque.
Since the key ingredient in
modification is the torque sensor, let’s take a closer look at it.
As with some conventional
hydraulic power-assisted steering systems, a torsion bar is used measure the
relationship between the torque being applied to the steering wheel by the
driver and the resistance being posed by the tyres.
It’s important to realize
that this measured torque is a two-way process – if the front wheels are on wet
grass they’ll turn very easily, so despite the driver turning the steering wheel
hard, not much torque will need to be applied to alter the steering angle of the
tyres. However, if the front tyres are on coarse bitumen, they will resist
turning and so the amount of torque needing to be applied by the driver will be
much higher to get the tyres to turn.
In other words, the torque
sensor indicates both the driver’s input of torque and the torque reaction of
The use of the torsion bar
therefore takes into account the real steering effort needing to be applied –
irrespective of road surfaces, tyre inflation pressures, and road speed.
So how does this torsion
bar system work? The torsion bar forms part of the steering column – it twists
when subjected both to high input torque and high tyre reaction torque. Two
sensors are used. Each measures the amount of twist and outputs a voltage that
is proportional to this. When no twist is occurring, the voltage output of each
sensor is in the middle of its range. So, with sensors with an output range of
0-5V, each sensor reads close to 2.5V when there’s no steering torque being
However, when subjected to
torsion, the sensors’ output voltages change. When there’s increasing left-turn
steering torque being applied, one sensor increases in its output voltage while
the other sensor decreases in its output. The opposite occurs on right-hand
Therefore, the larger the difference
between the output voltages of the two sensors, the more steering effort that is
This graph shows the output
of the two torque sensors and how this relates to the amount of steering effort.
It can be seen that at the point where there is no steering torque being
applied, the sensor voltage output lines cross.
One of the hard points to
grasp about these torque-measuring systems is that the output difference between
the two sensors is not proportional
to the amount of steering lock applied. This is because the tyres mostly resist being turned when they are being turned – once a certain
amount of tyre angle has been adopted, the effort required to maintain that
steering lock is much less than the effort required to first gain it. Instead,
the greatest difference between the two sensor outputs occurs when steering
input is being rapidly applied on a grippy surface at low speed... which is fine,
because that’s when you most need the assistance!
Because of the way the
output signals of the torque sensors are configured, the ECU knows both the
direction that the torque is being applied in and how great it is. The ECU then
instructs the electric motor to assist appropriately, and as a result, the
required steering torque effort by the driver decreases, resulting in a lower
difference in the output voltages of the torque sensors. The assistance provided
by the motor is therefore reduced.
Modifying the System
So to summarise the above
paras for those just skipping along: the greater the difference in the output
voltages of the two torque sensors, the greater the amount of steering torque
that the ECU knows is being applied to the steering.
In nearly all cases, the
desired outcome of modified electric power steering will be more steering feel –
or in other words, you want less power assist. At higher speeds this results in
better turn-in cornering feel, better straightline stability and a far more
secure on-road feel. Sure, there will be slightly heavier steering when parking,
but unless you’re very frail, that’s unlikely to be a problem.
So to achieve the outcome
of less power assistance, the ECU needs to be fooled into thinking that there is
less steering input effort than is really occurring. To achieve this, all that
we need to do is reduce the difference
between the voltages of the torque sensors.
This can be achieved very
simply by the use of just two multi-turn potentiometers (pots). Even including
the cost of a box to mount the pots in, the total bill will be under 10
So, how do you know if your
late model car has electric power steer? The easiest way is to look for the
underbonnet presence of a hydraulic power steering fluid reservoir. If the car
has power steering and there’s no reservoir, it must be electric – or the car
uses a combined hydraulic system that powers the steering and brakes.
How to Do It
The first step with any
electric power steering system is to disable the system and go for a drive.
Usually, switching off the system is just a case of pulling the electric power
steering fuse or relay. Of course the steering will be much heavier when moving
slowly, but the car will still be driveable. What you are looking for is the
change in steering weight at speed – say 80 km/h.
Is it much heavier, or the
same as usual?
If it’s the same as usual,
the amount of power assistance being applied at this speed must normally be
zero. (In that case, you’re not going to be able to improve steering weight by
modifying the system!) However, if you notice a firmer, meatier steering weight,
you can be sure that there’s too much assistance normally being given at this
speed – and so there’s room to make improvements.
The next step is to find
some of the functions of the power steering ECU pins. At a pinch you can get
away without a workshop manual but it’s always best to have one. Earth one lead
of a multimeter and then use the other to backprobe the plugged-in power
steering ECU. Have the car running and use an assistant to waggle the steering
while you’re taking the measurements.
On the Toyota Prius on
which this modification was performed, the following important voltages were
- Torque sensor #1 – 2.5V output with
no torque input, varying downwards with left-hand torque and upwards with
- Torque sensor #2 - 2.5V output with
no torque input, varying upwards with left-hand torque and downwards with
- 5V regulated output
Either of the two sensors
can be intercepted – the ECU is just looking for the difference between the
output voltages. So how is the modification done? The following diagram shows
just how easy it is.
Let’s take it step by step.
Pot 1 is placed across the 5V-to-earth connections. If this pot is set to its
middle position, 2.5V will be available on its wiper.
Pot 2 is wired with one end
connecting to this 2.5V supply and the other to the sensor output. This pot’s
wiper goes to the ECU.
If the wiper of Pot 2 is
placed closer towards Pot 1, the signal the ECU sees will be held more and more
at 2.5V – that is, no torque change. On the other hand, if the wiper of Pot 2 is
placed closer to its other end, the ECU will see more and more of the unaltered
So with Pot 1 set to
provide 2.5V on its output, by adjusting Pot 1 you can alter the signal from
being always held at 2.5V at one extreme, to being dead standard at the other
extreme. Set Pot 2 to ‘in-between’ positions and you can get ‘in-between’
The two pots used are 10
kilo-ohm multi-turn designs. If you use small trimpots these are very cheap, or
if you use full-size multi-turn units, more expensive. We set the system up with
the latter, simply because we had them already on the shelf. (Always use
multi-turn – eg 10-turn - pots as this makes the setting-up much
Install Pot 1 - it goes
between the 5V regulated supply and earth. Use a multimeter to measure the
voltage on the central wiper terminal (the meter connected with one probe to the
wiper and the other to earth) and then adjust the pot so that its output voltage
is the same as the ‘at rest’ sensor output voltage. In this case, that was 2.5V.
Then cut the signal wire
between the sensor and the ECU. Connect the sensor end of this wire to one end
of Pot 2, and connect the other end of Pot 2 to the wiper of Pot 1. The wire to
the ECU then connects to the wiper of Pot 2.
When doing the wiring it’s
easiest to ignore the description and simply look at the diagram.
Adjust Pot 2 so that its
wiper fully at the end closest to the signal input. Start the car and drive it –
it should drive normally. If it doesn’t, check your wiring.
Then adjust Pot 2 so that
the wiper starts to move towards the other end. The steering should now get
heavier. If you go too far, it’s likely that you’ll trigger a fault condition –
when setting this pot, drive the car lots to make sure that (a) the weight is
good across a variety of driving situation, and (b) no fault condition is
Using a Fluke 123
Scopemeter to data-log both the input signal from the sensor and the modified
output shows the changes that have been made.
As can be seen, the input
trace (bottom) and the output trace (top) appear to have the same shape.
However, close inspection shows that the upper trace always moves less distance
from the midpoint of about 2.5V. In fact the recorded minima (circled) show that
the output dropped only as low as 1.853V, compared with 1.102V for the input.
Other data (not shown here) indicates that the maximum voltage recorded on this
drive from the sensor was 3.673V, versus 3.029V on the modified output.
In other words, the output
voltage holds closer to the ‘no-torque’ value of about 2.5V, telling the ECU
that there was less steering torque being input than there really
The result is less power
assist and so greater road feel.
Because the ECU has a road
speed input, the steering still alters in weight with speed. In this car, the
parking weight is slightly increased – which is neither here nor there – but
from about 60+ km/h there is noticeably better road feel than standard. Turning
into a high speed corner gives far more reassuring feedback as to what the front
tyres are doing, in addition to giving weight against which the steering is
worked – allowing more precise inputs of lock.
As we said with the last
car where we modified the steering weight (Modifying Speed-Sensitive Power Steering),
when you have the ability to alter this characteristic, you suddenly realise
with startling clarity that the amount of steering weight makes a huge and
instant difference to how the car feels on the road.
Thanks to Silicon Chip magazine’s John Clarke for
technical help during the development of this modification.
A potentiometer (pot) is a
simple electronic component. Most pots are rotary designs - like the volume
control on an older radio, as you turn the shaft, the internal wiper moves along
a resistance track.
There are only three
terminals – shown here as A, B and W. Most pots have clearly laid-out terminals
but if the pot you are using is confusing, a simple check with a multimeter (set
to resistance) will show you what’s what. Between terminals A and B should be
the full value of the pot, eg with a 10 kilo-ohm pot, around 10K resistance.
Measuring between either A and B and W (the wiper) will give a resistance that
alters as you adjust the pot.
In the application shown in
this story, the A and B terminals of the pot can be connected either way around
– this will just alter the direction that you turn the pot to go up or down in