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Modifying Electronic Car Handling Systems, Part 2

Switching off traction control while leaving the stability control working!

by Julian Edgar

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At a glance...

  • Separating the handling effects of traction and stability control
  • Turning off traction control without changing stability control
  • Zero cost approach
  • Part 2 of a 4-part series
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This article was first published in 2004.

In this story we’ll show how you can disable traction control while still leaving stability control operating. This will allow you to spin the drive wheels in a straight line as much as you want, and will also allow you to move the car around with power when cornering - without the traction control system always intervening. However, the ‘safety net’ of stability control will remain – that is, if you get too sideways, the stability control will still reassert itself, helping to prevent the car leaving the road. It’s a brilliantly simple way of reinvesting your car with fun while still having the electronics available to help in times of major crisis.

WARNING!

As with any car modifications, altering the electronics of traction control and stability control systems can be dangerous. If you make incorrect wiring connections, your car may cease to have ABS, traction control and/or stability control. When modifications have been undertaken, the car may handle in a very different way to standard and the driver may need higher skills.  As with any modification of the brakes and suspension, good quality workmanship must always be used. Further, very careful test driving should always be undertaken until you are confident with the altered handling characteristics of the car.

In Part 1 of this series (Modifying Electronic Car Handling Systems, Part 1) we looked at how traction control and stability control systems work. In that story we covered how - in many respects - they’re really separate systems.

The traction control system reduces power (and/or brakes wheels) to prevent wheelspin. It doesn’t matter if the car is cornering or moving straight ahead; all that the traction control wants to do is have the driven and undriven wheels rotate at much the same rate.

Stability control is quite different. Stability control systems watch the steering angle and how much the car is yawing (rotating around a vertical axis) and then attempt to pivot the car back onto course by braking individual wheels. Unlike traction control, the individual braking isn’t occurring because that particular wheel is spinning; instead it’s being slowed so that the car will partly pivot around, pulling the car back on course.

The two systems are likely to be controlled by the one user-adjustable ‘off’ switch and have the same electronic control unit (ABS is also usually included in this electronic box), but conceptually, traction control and stability control are really different animals.

And as such, they can be modified in different ways.

In this story we’ll show how you can disable traction control while still leaving stability control operating. The modification requires no difficult electronics construction, although it must be said there are plenty of wires to connect up. This makes having access to a factory workshop manual of the traction control ECU very useful – although not vital.

Click for larger image

The test car on which the modification was performed was a 1998 Lexus LS400. Obviously, we can guarantee that the modification works well on this car, and we can’t think of any reason why it wouldn’t work on other cars as well. (In fact, as you’ll read later in this story, you can test if the mod works on your car without having to spend a single cent!) But traction and stability control systems operate in different ways on different cars, so we can’t make a blanket statement that this approach will definitely work on every car.

But we think it’s likely it will....

Sensors

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The traction control system determines if wheelspin is occurring if one or other of the driven wheels is rotating faster than the undriven wheels. Wheel speeds are sensed by using the sensors that are more commonly known as ABS sensors. These almost always consist of a magnet surrounded by a coil of wire. A toothed cog, which is attached to the hub, rotates past this sensor. This type of sensor is called ‘inductive’.

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The output of an inductive sensor is a voltage that rises and falls as a sine wave – it’s like an AC (alternating current) waveform because basically the sensor is like a little alternator.

As the wheel rotates faster, the frequency (the number of up/down movements of the voltage per second) also becomes faster. The Electronic Control Unit (ECU) is looking at this frequency, and from it the ECU can calculate the speeds of each wheel equipped with a sensor (usually all four wheels).

An important point about an inductive sensor is that it has plenty of grunt: because you effectively have four little AC generators going – one in each wheel – there is plenty of signal strength available. This makes doing what we’re about to do really easy.

Telling the ECU Lies

Let’s concentrate on a rear-wheel-drive car. (For a front-wheel-drive, just reverse the obvious in the following discussion.) The traction control system knows that the rear-left wheel is slipping when it is spinning fast than the front wheel – let’s say, faster than the front-left wheel. And the traction control system knows that the rear-left wheel is slipping when it is spinning fast than the front-right wheel.

It stands to reason, then, that if we tell the ECU that the front-left and rear-left wheels are spinning at the same rate, and that the front-right and rear-right wheels are spinning at the same rate, the ECU will see no wheelspin by the drive wheels. It doesn’t matter if the rear drive wheels are smoking up a storm, if the ECU thinks that each of the rear wheels is rotating at the same rate as the wheel directly ahead of it, the traction control system won’t intervene. After all, it can’t know that anything odd is happening (unless the system measures road speed via the undriven wheels and then matches that against engine revs – fairly unlikely).

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And it’s very easy to make the ECU think that the rear wheels are spinning just as fast as the front wheels. All that we do is disconnect the rear-left wheel ECU input from the rear-left speed sensor and instead connect it to the front-left sensor. And disconnect the rear-right wheel ECU input from the rear-right speed sensor and instead connect it to the front-right sensor. So both left-hand wheel speed inputs get their info from the front-left wheel; and both right-hand wheel speed inputs get their info from the front-right wheel.

If we use a couple of electrical relays to do this, we can easily switch between the standard wheel speed inputs and the no-traction-control set-up. (You could of course just use manual switches, but this would make on-the-go changes difficult and would also prevent the ABS working.)

The ABS? OK, take a deep breath.

If the system is set up so that the ECU cannot detect wheelspin, it also won’t be able to detect wheel lock-up. So the ABS system will turn off at the same time as you switch the system into no-traction-control mode. Since we’re believers in having ABS operating all of the time, that outcome needs to be got around. However, if you use relays to do the switching, it’s easy to automatically switch back into normal mode whenever you hit the brake pedal, using the signal input from the brake lights.

But let’s take a few steps back.

Testing

As mentioned earlier, it’s easy to see if the approach will work before actually starting to install switches and relays. You will need to find (with complete certainty!) the pairs of wires that come from each wheel speed sensor. (At a pinch this may be able to be done at the wheel speed sensors themselves... which would mean you could do it without a workshop manual!) Cut the wires leading to the driven wheels and connect each of these pairs to the wheel at the opposite end of the car.

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In a rear wheel drive car, the rear-right and rear-left wheels.

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In a front-wheel drive, the front-right and front-left.

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Note that there’s one other tricky thing to know about. You’ll find that one side of each of the sensors is earthed inside the ECU. This wire can be found by using a multimeter to measure for continuity between each ECU wheelspeed sensor input and ground. The sensor input that has continuity to ground is the earth wire – so the other wire in the pair must be the positive, or signal wire. Make sure that you connect negative to negative and positive to positive, as shown in this example.

Once you have made these connections, start the car and go for a gentle drive.

Note that during this testing your car will NOT have ABS and will NOT have traction control!

However, no warning lights should come up on the dash (eg the ABS, traction control and stability control warning lights shouldn’t glow). Additionally, the stability control system should still operate while the traction control doesn’t. Now obviously take great care when finding out what the car’s behaviour is – a large expanse of grass is perfect, as you can do the testing at very slow speeds. Wheelspin in a straight line should be uncontrolled (except by your right foot!), while you should be able to push the car more into a corner under power without the traction control intervening. However, when the car is sliding in a direction different to where you’re pointing the steering, the stability control system should still operate much as it did previously.

In other words, you should be able to throttle-steer much more than the standard system let you.

If warning lights come up when you first start the car, either you have made a wiring error (check polarity as mentioned above) or the ECU is smart enough to detect that there is one sensor connected to two inputs. If you can spin the drive wheels in a straight line but the stability control comes in really early when you’re sliding the car around a corner, you’ll need to wait for a technique that we plan to cover in a later article – it will allow you to control the action of the stability control as well.

In the guinea pig Lexus, the (non-LSD) rear-end would spin as much as you liked in a straight line, and would also allow far more cornering attitude changes with the accelerator than could be achieved in unmodified form. However, start whipping-on opposite lock to control the rear-end powerslide and the stability control would rapidly calm the situation. In short, it reduced the early intervention of the traction control so you could have a lot of fun without having to switch off all the electronic assists...

Next Week...

That’s probably enough for this week – next week we’ll show how to use relays to allow you to change between traction control and non-traction control while on the fly and also automatically bring the ABS back to functioning whenever you brake.

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