What kind of sensor you're dealing with
Most modern ABS sensors are two-wire Hall effect types. Unlike a reluctor (variable reluctance) sensor, which makes its own AC sine wave as teeth pass, a Hall sensor needs power and switches its output as each tooth goes by, giving a square-wave style signal. Some cars do still use reluctor ABS sensors, so confirm what you have before you wire anything. Our article on Hall effect or reluctor explains how to tell them apart.
Getting the polarity right
With only two wires, one carries 12V in and the other carries the signal out. These sensors are usually polarity sensitive, and they'll only work with 12V on the correct pin. A factory service manual will tell you which is which. Without one, look at how the factory harness is wired, or simply try it: there are only two ways to connect it, so you'll know quickly.
Why you need a pull-down resistor
A two-wire Hall sensor changes how much current it draws as teeth pass, rather than switching a clean voltage. To turn that current change into a voltage the ECU can read, wire a resistor from the signal wire to signal ground. As a starting point, a 270 ohm, 0.5 W resistor usually works.
The result won't be a perfect 0-5V square wave. The low side often sits above zero and the high side below 5V. That's fine, as long as the edges are sharp and the high and low levels are clearly separated.
Setting the ECU input
- Set the sensor type to Custom. The edge doesn't matter much, and Falling is the default.
- Spin the wheel and view the signal with the Haltech ECU oscilloscope.
- Note the high and low voltages, then set the arming and triggering voltages between them.
These are values that have been used successfully on two platforms:
| Car | Supply | Pull-down resistor | Arming voltage | Triggering voltage |
|---|---|---|---|---|
| Holden VE Commodore | 12V | 150 ohm | 2.0 | 1.5 |
| Nissan 350Z | 12V | 220 ohm | 2.3 | 1.9 |
For anything else, start with a resistor near the values above, scope the signal and set the thresholds from what you actually see.
Roll the car slowly while watching the wheel speed channel. If it drops out or jumps at walking pace, the thresholds are too close to the signal edges.
More background is in How wheel speed sensors work and Vehicle Speed Sensors. Stuck on a particular car? Contact us.
