DS4 (B7) abs problem

[Mrh]

Hello!

I have problems with my abs-system. The fault messages is:

"front lh speed sensor fault signal fault, coherence" and "front lh speed sensor fault, short circuit to positive or short circuit to earth or open circuit".

I have already replaced the sensor, but the problems remains...

Any sugesstions, what to troubleshoot next?

[Hell Razor5543]

Hi, and to the forum.

Before changing any more parts I would suggest arranging a diagnostics session. Although this will cost (I don't know how much it would be in Stockholm) it could save you money changing parts to see if they will fix it. Failing the diagnostics I would then check the wiring (where you can) for any signs of damage, as the second error suggests they system cannot 'see' the sensor properly.

[Mrh]

Thanks for your reply I own a iCarsoft diagnose tool. Isnt that good enough? Do I need a more specific diagnose?

What wiring can I check? To the ecu-box?

Can corrosion where the sensor head is fitted, cause the problem?

[GiveMeABreak]

Did you follow the wires from the sensor back - sometimes there could be a short in the wiring form the sensor to the ABS / ESp ECU, or the reluctor ring could be corroded - but as you have a short circuit fault, this is likely to be electrical. So double check all the wiring for splits and corrosion.

[Mrh]

Thanks for your reply. I appreciate it. Have not followed the wires yet, but I'm going to do that.

I am kind of novice in this type of troubleshooting...

Can you tell me a good way to inspect the reluctor ring.? Is it located next to the wheel bearing?

[GiveMeABreak]

The rear reluctor rings are incorporated into the rear Disk hubs.
The front reluctor rings are on the front hubs - both the rings are available separately - but be careful to to damage or touch them with anything magnetic or can affect their polarity and cause a misreading. But with a short circuit, this does point to an electrical issue to be honest. If it was the reluctor ring deteriorating, it would probably throw the sensor fault without the short circuit details you describe.

The reluctor rings are (5) on the diagram below, but where you have replaced the speed sensor and the shorter cable that plugs into the harness - I would check that the plugs are clean with some electrical contact cleaner and then follow the wires back a little further to make sure there is no visible damage.

[Mrh]

Ok, great Marc. Than you. I will follow your advice. Do you think that corrosion on the "magnetic area" in the housing, where the sensor head is attached, can cause the problem? I found corrosion on the magnet... What tool and chemicals can I use for cleaning there? WD40?? Is the magnet fragile?

[GiveMeABreak]

The magnetic ring is very sensitive - and fragile. You have a Hall type of sensor that uses alternating pairs of north / south poles within the ring. The speed sensor reacts to the changes in the magnetic fields and converts these pulses into electronic signals that get sent to the ECU.

So on your ring there are no 'teeth' to rust. So unless the magnetic ring itself is badly deteriorated - keep checking the wring.

ABS.PNG (26.93 KiB) Viewed 136 times

Don't put anything oil or grease based near the discs! Don't use WD40 either. Just brush away any debris with a non-metallic brush so you don't damage it.

[Mrh]

Thank you for your explenation and knowledge I will troubleshoot the wring tomorrow. Hope that I can find any faulty part.