ABS sensor lead 'Open Circuit'

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ABS sensor lead 'Open Circuit'

Post by Spooky » Wed Sep 05, 2012 8:59 am

I can't believe just how many bongs and lights this brings up. The fault occurred as I went through the Billing mud run on Sat (not at high speed, either); the car lit up like an Xmas tree, still managed to get out under its own steam, but then sank to Access height!!!

After about an hour's fun with the Faultmate, I managed to isolate the loss of the NSR ABS sensor lead. Looking under the car, I could see some large clumps of clay in the vicinity, so presumably that's what did the damage ...

Whilst I can understand the loss of the ETC and the EPB fault message, why on earth does it also take away the diff controls and the air suspension? It almost seems as though the car was designed to allow an unscrupulous dealer to print money, as they try and locate the fault!!! Fortunately, I have the LLAMS system, or it would have been a job for the Big Yellow Taxi company to get me back to Pompey (complete with Tintent).

Anyway, replacement sensor fitted yesterday (about 20 mins, including jacking up & removing the wheel), and all was fixed. Incidentally, whatever had done the damage had also managed to bend the brake line on the wishbone!!!


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Re: ABS sensor lead 'Open Circuit'

Post by Bigcarpchaser » Wed Sep 05, 2012 10:35 am

Had this twice Mick, both sides about 2 months apart although mine wasn't cured with new sensor, had to run a new cable back to the ECU...pretty dumb design IMHO

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Re: ABS sensor lead 'Open Circuit'

Post by PillowSmuggler » Wed Sep 05, 2012 10:44 am

I asked the same question after my failed 2007MY EGR blanking experiment, and my local tech had a plausible explanation.

I can understand part of the reasoning, though not all. In simplistic terms it comes down to whether one or more of the ECUs can be trusted by the others. If an ECU relies on another one, and that other one is either unavailable or thinks it's not working correctly, then this ECU also reports itself as not working, and a cascade of ECU failures results (which is why the order that obd2 codes are logged is important). How the cascade builds up depends on what software you have.

I've made this up as an illustration as I do not know the actual logic LR coded in, but it shows the principal:

If the ABS sensor has failed, the the ABS ecu flags as failed. The transfer case ecu then shuts down as it no longer knows how much slip is occuring and therefore doesn't know how much centre diff loading to use. It also needs a speed signal to know whether a range change is safe; it cannot get this from the now untrusted ABS ecu so Special Programs Unavailable, Range Change Unavailable are possible messages. The failure of the transfer case module and ABS modules also means that the dynamic stability and wheel based braking for traction control are gone. The suspension ecu also needs a speed signal to know when enforced height changes will be imposed, but as the ABS ecu is down, it too drops to a (stupid!) safe mode and marks itself as failed. And so on and so on.

In real life, my egr blanking resulted in a cascade that put the following out of commission:
Range Change unavailable
Special Programs unavailable
Gearbox stuck in 5th gear ( "F" in dash display)
No emergency handbrake mode
Suspension on bump stops.
Max engine rpm of approx 2000.
Fault light lit
And they were just the bits I could see - for all I know half the other ecus could have kicked their toys out too.
All these cleared with an ignition cycle (thankfully that even works at 60+mph so you don't crash on the motorway :shock: )

Again in reality my flooded transfer case ecu brought about:
Range Change unavailable
Special Programs unavailable
No traction control.
Suspension not lowered however it felt much bumpier/harsher - not really figured the why of this yet but did happen each time.
No fault light
Traction control inop / harsh suspension remained after ignition cycle though range change/special programs returned to work.
Took me years and lots of fault code reading to figure that out, but this was the reason I couldn't get up a grass slope after the abingdon 4x4 lake - no traction control though no reported faults on the dash. I finally figured this out by putting just one wheel in a muddy clay puddle and three wheels on dry tarmac and gunning it to get free. All power went to the slipping wheel. Sure, I could drive off slowly as there was then traction in the puddle, but the centre diff and individual wheel braking aspects of traction control were offline. Resetting the fault code in the transfer box with the faultmate straight after a failed test let me drive away harshly without issue.

Personally I think that if the suspension ecu has good data from the height sensors then it should fail to normal height not the bump stops and that would solve 90% of our issues after a component failure as it'd leave the car drivable in all but the most demanding offroad situations.

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Re: ABS sensor lead 'Open Circuit'

Post by skinnersplace » Mon Mar 18, 2019 8:32 am

Hello pillow smuggler - old post I know (but that the power of the internet)

Can I ask what your EGR blanking experiment was that resulted in the above cascade failure ?

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Re: ABS sensor lead 'Open Circuit'

Post by Bodsy » Tue Mar 19, 2019 10:42 pm

Result is that you need a software update if your vehicle is 07/08/09 D3.
otherwise you can get this random fault.

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Re: ABS sensor lead 'Open Circuit'

Post by Globetrotter » Wed Mar 20, 2019 6:22 am

A good explanation of the fault system. The LR3 uses a "can bus" system which means they are all chained together in a loop (resistors are up in the instrument panel if I remember correctly) and the signals from one ecu pass through the other ecu's. Like has been said, once one ecu stops working the signals cannot pass along the "can bus" so the destination ecu does not receive the correct information and shows a fault and then the faults cascade down.

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