A4 abs sensor

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A4 abs sensor

Post by Burt_100 »

I have a 2012 A4 that I am trying to check the lhr abs sensor on but for some reason I am struggling, unplugged it only has like 300mv on both wires and plugged in is the same, the trace is very noisy, I have tried various earths and I am now direct to the battery but nothing, I have tried the rhr and it seams to be the same unplugged and plugged in, I did once get a waveform from it but I can’t get it since, has anyone any idea what I am doing wrong? I can’t understand it I’ve checked a number in the past like this and the test worked everytime

Steve Smith
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Re: A4 abs sensor

Post by Steve Smith »

Hello and thank you for the post

You are doing nothing wrong here, we just need to be aware of the style of system we are working with.

Unplugging the ABS sensor may well add a fault code to the system and prevent a power supply being sent to the sensor under test (assuming this is not an inductive sensor)
With no power supply, we will have no signal.

Back pining is quick, but if you can add break out leads, that will help with varying style measurements.

I am going to assume we are measuring MRE style ABS sensors.
We have a Guided Test that covers these measurements here using PicoScope 7, Select Sensors> Wheel speed>Wheel speed sensor (magneto-resistive) Note: the signal is AC coupled
You can also find this test with PicoScope 6, Automotive>Sensors>ABS>MRE Magneto-resistive element

Could we ensure no codes are stored and then spin the wheel using the Guided Tests above to capture a signal?

If no signal is obtained, could we add a second test lead to our break out leads, so we are now measuring the signals present on both wires at the ABS sensor?

One of these wires should display 10 to 12 V and so you will have to convert both scope channels to DC coupled +- 20 V

Could we momentarily clear the fault codes whilst monitoring the signals on the break out leads connected to the ABS sensors.(this may also require cycling the ignition)

What I hope to see is the power supply be momentarily restored to the sensor under test on one of the test leads. This supply may only be present for a second or so and then fall back to 0 V because the ABS ECU has detected a fault with the sensor/circuit under test.

If this is the case, we need to understand what has happened to the circuit such that the ABS control unit is preventing a supply being sent to the ABS sensor under test

We have discussed MRE Sensor by pass testing here topic19571.html and this may help as we monitor both voltage and current via such sensors

I hope this helps and please feedback if you have no joy, take care…..Steve

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Re: A4 abs sensor

Post by leonardorj »

Maravilha o Suporte técnico ai !
Vou dizer como eu costumo lidar com estes diagnostico. Eu meço o chicote desconectado do carro e meço a resistência dos dois lados . Assim eu tenho uma noção se eu tenho defeito na fiação.
Como o suporte disse a primeira coisa é saber em qual sistema você está trabalhando e um esquema elétrico pode te ajudar. Particularmente eu ainda prefiro usar um scanner em casos menos complexos. Eu postei uma esquema de uma audi a3 2012 2.0

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