This article was first published in 2004.
This is Part 3 in this series. In Modifying Electronic Car Handling Systems, Part 2
, we covered how it is
possible to disable traction control while still leaving stability control
working. So why would you want to do that? Well, it lets you have a lot more fun
in a car – you’re able to spin the wheels and have much better throttle-steering
through corners, while at the same time the safety net of stability control is
still there if you get right out of shape.
And don’t think that modifying the systems like this is very complex – it
isn’t. Basically, all that we do is tell the ECU that the driven wheels aren’t
spinning any faster than the undriven wheels. That way, the ECU doesn’t know
when the driven wheels are slipping! It’s easy to achieve this by disconnecting
the speed inputs from the driven wheels and replacing these signals with the
ones from the undriven wheels. In effect, the undriven wheels (eg the front
wheels in a rear-wheel drive car) feeds both the front and rear speed
This diagram shows how it’s done in a front-wheel drive car. Because wheel
speed sensors are inductive, they have enough grunt to drive the extra inputs
without any problems.
However, making this wiring change permanent as is shown here has two
ABS won’t work
difficult to quickly swap back to standard when you want full traction
But both of these problems are easily overcome!
If you have a car with just traction control, you might be puzzling of the
need for a system like this. After all, there’s an ‘off’ switch on the dash and
when you don’t want traction control, you just press it, right? However, all
cars with stability control also have traction control – and a single dashboard
switch that turns off both systems.
The approach covers here allows you to retain stability control (and ABS) while
dispensing with traction control.
A relay is just a remote-controlled switch. In this application, that sounds
pretty good – you can remotely change whether you have the traction control
working or not. And by using two DPDT relays and another SPST design, it’s even
easy to have the sensor operation return to normal whenever you put your foot on
the brakes – ie ABS operates as normal.
However, experience has taught us that when we mention relays – especially
non-automotive relays – many readers get confused pretty fast. So, unless you’re
right up to speed with relays, their functions and types, please read the
Lots of automotive enthusiasts are as happy as Larry in wiring up relays to
control heavy loads - lights, fans and so on. After all, there are just two
wires that supply power to the coil to switch on the relay, and two other
terminals that get connected together when the relay is on. But it's when relays
have five and eight connections that some start to sweat. But if a few overall
ideas are kept in mind, there's no need for hassle.
Firstly, just as with those simple car relays, all relays have two terminals
for the coil. A relay is just an electromagnet-based switch, and the coil makes
the electromagnet happen.
A normal car relay is a single pole, single throw (SPST) design. This
designation refers to the switching part of the relay where when it's activated,
one wire (a "single pole") can be connected only one way (a "single throw").
Just like a simple on/off switch, when you power up the relay's coil, the
connection is made; when you unpower the coil, the connection is broken. In this
diagram the relay's coil is yellow. Near to the coil you can see a switch, which
is open. This is called a Normally Open contact - it's open when there's no
power being applied to the relay. When power is applied to the relay, the single
contact closes. This is a Single Pole Single Throw relay - SPST.
But wouldn't it be good if we had another contact that was broken as the
relay closed? That's what happens in this design. Another contact has been added
that is Normally Closed. When the relay is energised, this contact is broken and
the other one (the Normally Open contact) is closed. We still have only a single
pole to be switched, but now it can be connected two ways - a double throw
design. This type of relay is therefore called a Single Pole, Double Throw
relay. As you can see, it has both Normally Open (NO) and Normally Closed (NC)
contacts. Some people call this a changeover relay.
In the above design a single circuit could be switched in two different
directions. But why not switch more circuits at the same time? That's what this
design does - it's a Double Pole, Double Throw relay. The 'Double Pole' bit just
means that it has two separate inputs that can be switched - and we now know
what the 'double throw' stuff means.
With this type of relay when the relay is activated you can:
- turn on two completely independent circuits
- turn one off and one on
- turn off two completely independent circuits
Even more complex designs of relay exist, but if you analyse their specs on
the basis of how many poles and throws they have, it becomes a helluva lot
easier to understand!
Relays are really good items to get your head around because once you
understand them, you can do many things with them. They’re also rugged (eg
unlike transistors and integrated circuits, you can’t very easily blow up a
relay), easy to drive, cheap, widely available and versatile.
Ok, so take a look at what we need to do with the relays. This time we’ve
halved the car – we’re looking at only one side. As you can see, when we want
the traction control to stop working on a rear-wheel drive car, we need to
change the input pair of wires normally connecting the ECU to the Rear Left
wheel to the Front Left wheel.
Or, to show it another way, here you can see that the ECU input needs to
either connect to the rear wheel sped sensor or the front wheel speed sensor.
Think about the relay labels used above and you’ll realise that what we’re
talking about here is a double pole (the two wires that go into the ECU), double
throw (they either go to the front wheel sensor or rear wheel sensor). Hmm – and
a Double Pole Double Throw 12V relay costs about AUD$6!
Here’s the relay in place. It initially looks confusing but let’s take it
step by step. The two wires going to the Rear Left Wheel speed input on the ECU
connect to the two Common (“C”) terminals of the relay. As is shown by the two
arrows, these wires can then be switched two ways. At the moment they’re shown
connected to the Normally Closed (“NC”) terminals, which are wired to the Rear
Left speed sensor. In other words, when there is no power applied to the relay’s
coil, the system works completely as normal – all the correct wheel speed
sensors are connected to the correct inputs.
But when power is applied to the relay coil, the Commons are switched to the
Normally Opens, which in this case connects the Rear Left speed input on the ECU
to the Front Left sensor. At the same time, the Rear Left sensor is
OK, but how do we make the relay change? That’s as easy as applying 12V to
the relay’s coil. As shown here, a switch, 12V supply and earth connection are
all that’s needed. Close the switch and traction control disappears.
The next step is to add the final relay so that when the brake lights come
on, the system returns to normal wheelspeed sensing. This requires just a Single
Pole Double Throw design. As you can see, the coil of the main DPDT changeover
relay is fed through the Common and Normally Closed contacts of the SPDT relay.
This means that, as shown, the Changeover Relay is normally in the ‘no traction
control’ position, with the front speed sensors feeding both front and rear ECU
inputs. However, when the brakelights come on and 12V is made available to the
Switching Relay, power is turned off to the Changeover Relay so the contacts
drop back to their Normally Closed positions, returning the traction control to
To provide a manual Traction Control on/off switch, all that’s needed is a
switch positioned as shown here.
The circuit has been designed so that if power is lost to the system, the
traction control returns to standard.
These diagrams show only half of the system. To complete the system, just
mirror-image the wiring for the other side of the car. (Note you can use the one
Switching Relay to operate both Changeover Relays.)
Yes there are lots of wires that need to be connected, but yes, it’s just a
bunch of switches being made....
Next week we’ll show you step by step
how to make it happen.