ABS MRE Sensor By-Pass

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ABS MRE Sensor By-Pass

Postby Steve Smith » Thu Mar 01, 2018 7:15 pm

I thought this may be of interest following on from ABS the diagnosis here:
post87051.html?hilit=AC%20coupling#p87051

I am not sure if we already use this technique (I haven’t) but I thought it should be shared for sure

Given the MRE style ABS sensors predominately result in a current flow of approx. 5 to 15 mA this generated a discussion of how could we use a “by-pass” technique in order to qualify the wiring harness and ABS ECU from a sensor “feed and return” point of view.

Thinking this through, at the point of ignition on (vehicle stationary) the current flow through these ABS sensors will be in one of two states (high or low / 5 or 15 mA)

If we can simulate this current flow then perhaps we can by-pass an MRE ABS speed sensor

Below I have momentarily shorted together the ABS sensor wires with the ignition on and vehicle stationary.

SHORT CIRCUIT TEST.jpg
Short circuit test


With the ignition on (no short) the ABS light extinguishes as normal, at the point of the short circuit the current flow increases to 70 mA and the ABS warning light illuminates to indicate a fault.

N.B The waveform indicates 704 mA as I have used a test lead wrapped 10 times through the jaws of a current clamp in order to improve vertical resolution and capture the mA values
See this technique here:
https://www.picoauto.com/library/case-s ... ing-lights

Notice how the supply voltage drops to 0 V whist the ABS sensor wires are shorted together.
This is most probably because the supply voltage passes through a resister or an elaborate control circuit (within the ABS controller) preventing high current flow in the event of a short to sensor ground

So, using ohms law if we have a supply voltage of 10 V, then how do we obtain and simulate 5 to 15 mA?

Using a 1000 Ohm resister we have 10 V / 1000 Ohm = 0.01 mA (10 mA)
Below we have the ABS sensor disconnected and a 1000 ohm resister inserted between the ABS feed and return wires.

1000 Ohm Inserted.jpg
1000 Ohm Inserted


With the 1000 Ohm resister inserted and ignition turned on (vehicle stationary) the ABS warning light extinguished as normal. It would appear the ABS controller was happy with approx. 10 mA current flow.

In an attempt to fine tune this procedure the capture below replaced the 1000 Ohm resister with a 750 Ohm resister where the current flow increased to approx. 15 mA.
Again, with the ignition turned on (vehicle stationary) the ABS warning light extinguished as normal. It would appear the ABS controller is also happy with approx. 15 mA current flow.

750 OHM.jpg
750 Ohm Inserted


IMG_2392.JPG
Image of By-PAss


I think if nothing else, this highlights another rapid test we can carry out to confirm the integrity of the ABS sensor wiring and the ABS Control units internal “feed and return” circuitry

A point to note here was the ABS system on the vehicle under test (2013 BMW 320 d F31) would activate the ABS controller at the point of “driver’s door open” before ignition on

I guess this is some form of “prepare state” ready for a seamless start up and go.

I hope this helps, take care……..Steve
Steve Smith
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Re: ABS MRE Sensor By-Pass

Postby Shooter » Sat Mar 03, 2018 1:53 am

https://www.ebay.co.uk/itm/ZX21-Variabl ... SwZaNaBUb0

Ideal tool for emulating a broad spectrum of resistance values in a circuit, enough wattage for most actuators.
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