This article explains how to set up and use wheel speed
inputs on the Modular ECUs.
Most cars have a tailshaft speed sensor mounted in the
gearbox. This either connects to the ECU, or to the dash which
then outputs a signal to the ECU. We need to be able to read
road speed to perform functions such as idle control on some
cars, launch control, gear detection and traction control.
Almost all the speed sensors you will find are magnetic in
nature, but the technology varies between different sensors. As
far as the ECU is concerned, there are two kinds of inputs it
can accept; a digital signal that is pulsed to ground with a
varying frequency, or a variable reluctance sensor which gives
approximately a sine wave output whose frequency again varies
with speed.
Both of these input types can be wired into the VSS input
on the ECU, and this is done in the factory wiring of plug and
play ECUs (assuming the car has a VSS from factory).
For the digital switch type sensor, you must select the
type as digital in the wheel speed input settings. For the sine
wave analogue type, you must select reluctor as the type.
Go
to (a) Inputs tab (b) select wheel speed (c) tick the check
box to enable wheel speed sensor (d) then select for the
sensor type (digital/reluctor).
On some older cars, this digital signal is generated using
a magnetic reed switch. A magnet on a rotating shaft causes the
reed switch to close as the magnet passes it, which pulls the
signal to ground. Because it uses mechanical contacts, they tend
to bounce when they close instead of making a clean square wave.
Therefore to use this kind of sensor, you may need to run a high
value on the filter setting to filter out this contact bounce.
Normally much lower filter settings can be used on Hall effect
sensors.
Once you have wired in the wheel speed sensor, you can
verify that it’s working by checking the ECU data window in the
software. As the wheel rotates, the VSS input should change
state continuously, and it should keep the same state when the
car is no longer moving.
Assuming that this is working, the next step is to
calibrate the road speed. If you have selected km/h as your
speed units, the software will give you an option to learn the
50 km/h calibration value. In miles per hour mode, the target
speed is 30 mph. To learn the speed, drive the car to this
speed, and hit the “learn” button in the software. After this,
the road speed should read accurately.
In a manual transmission car, you can now go through the
different gears and learn them in turn. This does not require
driving at any specific speed; simply drive in each gear with
the clutch fully engaged (ie not slipping), and hit the “learn”
button for that gear. The ECU then learns the ratio between the
road speed and RPM and uses that to detect which gear is
engaged. After this, the gear number in the gauges window should
also read correctly when driving in that gear. This is useful in
reviewing data logs, to gain context and driver training, but
also to select different boost pressure in different gears.
On systems with individual wheel speed sensors, as required
for traction control, the process is the same but the wheel
speeds must all be learnt at the same time. If you have a two
wheel drive car and you’re using a dyno to do the learning, then
you can manually copy the calibration from the two driven wheels
to the two passive wheels, assuming the front and rear wheels
have the same type of tyre.