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 inputs.
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 problems:
But both of these problems are easily overcome!
RelaysA 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 following carefully!
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 disconnected.
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 standard.
To provide a manual Traction Control on/off switch, all that’s needed is a switch positioned as shown here.
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. Did you enjoy this article? Please consider supporting AutoSpeed with a small contribution. More Info...
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