ABS (Wheel Speed Sensor) Waveforms and Testing - Drive Accord Honda Forums | radio-pro.ru
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post #1 of 4 Old 01-26-2017, 04:53 PM Thread Starter
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ABS (Wheel Speed Sensor) Waveforms and Testing

Given that there are many different kinds of ABS sensors (2 wire passive, 3 wire active, 2 wire active) in use on different vehicles, I thought it would be interesting to figure out what type the 7th gen Accords use and what the expected output signal is.

The front ABS sensors can be accessed at their connectors under the hood:



On the sensor side, for the front sensors, the pink wire is the supply wire and the white wire is the output signal/ground wire (Wiring Diagram thanks to BBB Industries):



Backprobing the Pink wire should show a constant 11V with the key on. Notice that this not the battery voltage and appears to be a regulated voltage rail generated in the PCM:



The white wire is the combined sensor “ground” and output signal. Backprobing the white sensor wire shows the following square wave signal as the wheel is spun by hand. The lower level is approximately 0.7V and the upper level is approximately 1.4V. The frequency of the square wave is proportional to the speed at which the wheel is spun:



Here is a zoomed out view showing how the signal changes with wheel speed:



Notice that each time the wheel is spun it ends up remaining in a random state (either high at 1.4V or low at 0.7V) depending on which pole of the bearing ring magnet the wheel came to rest at. As a result, a simple ABS sensor test can be performed using only a DMM (voltmeter) if no scope is available, by simply measuring the sensor return voltage after spinning the wheel several times. Sometimes you should see 1.4V when the wheel comes to rest, and other times you should see approximately 0.7V. If you ever see 0V there is probably a wiring problem (open circuit) or sensor failure.



To test the ABS sensor output with a DMM (Voltmeter) only, connect the negative (COM) terminal of the voltmeter to chassis ground and connect the positive terminal (V) to the WHITE ABS sensor wire by backprobing at the connector. Then, spin the wheel several times. Sometimes the output should settle at approximately 1.4V and other times it should stop at approximately 0.7V. (This is not a complete test, as there still could be a problem on a particular portion of the ring magnet, or a dynamic problem, but at least it does provide a basic functionality check of the sensor, verifying that it is detecting both poles of the ring magnet and that there is no problem in the wiring harness).



There are several types of ABS sensors that have been used on different vehicles including 2 wire passive sensors (older), 3 wire active sensors and 2 wire active sensors. Active sensors are used on most newer vehicles due to their ability to sense down to very low wheel speeds. The wide variety of sensor types can lead to quite a bit of confusion as to how to test the different ABS sensors and what type of signal to expect. On this 2007 Accord, the ABS sensor is a 2 wire active sensor. The 2 wires serve two different purposes at the same time: (1) they provide the voltage to power the circuitry in the ABS sensor and (2) they simultaneously provide a path for sending back the return signal indicating wheel speed. This is accomplished as shown in the following diagram:



As the alternating North and South poles of the ring magnet (integrated into the wheel bearing) pass by the Hall Sensor (a semiconductor magnetic field sensor), the electronic circuit in the ABS sensor opens and closes a switch inside the ABS sensor which changes the overall resistance through the sensor. This results in two different levels of current (red arrow) returned to the PCM, a higher current when the switch is open due to being over one magnetic pole and a lower current when the switch closes when the sensor is over the other magnetic pole. These two current levels are reflected as two different voltages (0.7V or 1.4V) as determined by the resistance inside the PCM relative to the two resistance values in the ABS sensor. These two signal voltage levels are designed to both be relatively low to ensure that in either state, the voltage across the ABS sensor is maintained at a high enough level to power its internal circuitry. The benefits of an active sensor are that it can sense at very low wheel speeds and it is more immune to electrical interference (since they output a digital signal rather than an analog sinusoidal signal). The advantages of the two wire active sensor over the three wire active sensor are twofold: (1) one fewer wire is needed to the ABS sensor and (2) since there is a current flowing in both ABS sensor states, there is continuous feedback regarding the health of the ABS sensor and the state of the connection. In comparison, in a three wire sensor an open-circuit or short on the signal wire might not be immediately differentiable from the sensor just being in a low state. In the 2 wire sensor, if there is ever zero current (or current higher than the expected level in the closed switch state ) then the PCM knows immediately that there is a problem with either the ABS sensor or the wiring to it.

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post #2 of 4 Old 01-26-2017, 07:54 PM
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I don't understand all you said but I'm getting some stuff. Thank you, will study this a bit more.

One question: why would they use regulated 11V for this? Is it to give some sort of safety margin so the voltage stays constant in case alternator / battery have issues (and voltage drops) so the system still operates while the car runs?

Are there other systems in the car that use lower voltage like this? I did not actually consider this.


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post #3 of 4 Old 01-26-2017, 08:12 PM
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The last diagram showing that there are multiple magnets in the wheel bearing assembly explains alot. I always wondered how speedometers registered accurately at low speeds. I thought they only had one magnet.

I feel like you would get a kick out of autospeed.com, many of the articles there analyze car electronic systems and components.

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Are there other systems in the car that use lower voltage like this? I did not actually consider this.
The ECU gives 5V to many engine parameter sensors like the intake air temperature sensor, etc.


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post #4 of 4 Old 01-27-2017, 03:58 AM Thread Starter
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Quote:
Originally Posted by rookie View Post
One question: why would they use regulated 11V for this?
I don't know for sure, but at least one benefit is that this stabilizes the voltage of the upper and lower logic states (the 0.7V and 1.4V levels). In this particular setup, the two voltage levels are created by a voltage divider (so input voltage, 11V, is simply scaled down by the resistors). Therefore, the 0.7V and 1.4V levels will vary proportional to the input voltage (the 11V). If they didn't use a regulated 11V and instead used the battery voltage directly, this can vary from something like 11V to 14.5V or so which is about a 30% variation. That means that there could be a 30% variation in the 0.7V and 1.4V levels as well (so the 1.4V level might be as high as 1.8V or so and the 0.7V might be as high as 0.9V). The variation in these levels could make it harder on the PCM to reliably detect between the two threshold and lead to less "noise margin".

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I feel like you would get a kick out of autospeed.com, many of the articles there analyze car electronic systems and components.
Looks interesting! Thanks for the tip, I'll check that out!
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