W211 rear ABS sensor fault

[selphy]

Evening,

My 2008 S211 came up with a ABS Inoperative warning message the other day. I borrowed a code reader and it showed a couple of old stored codes for front right ABS sensor distance too far from sensor and a current fault code for rear left ABS sensor check wiring/sensor. The old codes I know about - I recently had trouble with my front right wheel bearing but that's fixed now so I think the problem is now the rear left sensor. I cleared all the codes and when I switched the ignition on all seemed OK but the fault came back up when I reached 10mph.

Armed with a new ABS sensor I removed the connector from the old one and checked it with my DVM. It measures 4 megohms or 6 megohms depending on the polarity and on diode test mode it measures 0.6V or 1.5V depending on the polarity. Those values match the new sensor exactly and also match the right rear wheel sensor so I'm not sure the sensor is the problem. I tried plugging in the new sensor without actually installing it and the ABS warning message still came up immediately. Unfortunately I don't have the code reader with me now so I haven't been able to try clearing it again but I believe that because it comes up immediately (i.e. even before reaching 5 or 10mph) it is more likely a sensor or wiring fault that has been detected than a bad/poor quality signal.

When I had trouble with my front wheel bearing if I unplugged the sensor the ABS fault came up as soon as the ignition was turned on whereas if there was a poor quality signal due to the gap being too large between the sensor and the rotating magnets in the wheel bearing seal the fault would clear every time the ignition was cycled and only re-assert itself when the car got up to 5 or 10mph.

Not sure what my next move is. I don't want to throw parts at the car and am fairly sure there's nothing wrong with the sensor so I'd like to return it. Should I now be looking at the ABS reluctor ring instead? I'll see if I can borrow the code reader again and see what it shows now just to make sure the fault is still showing as the rear left sensor.

Thanks for reading...

 

[selphy]

anyone?

I need to fix this and I'm not wanting to throw parts at it randomly so some guidance would be much appreciated. Do I have to pull the whole rear hub apart to inspect the impulse ring?

 

[selphy]

Answering my own questions here again but it might be useful for someone else.

I found that you can get fairly easy access to the ABS sensor connections by lifting away the C-pillar door seal and some trims. This exposes the bulkhead connector for the ABS sensors:

You can run a couple of test wires into a sensitive AC voltmeter (ideally an oscilloscope) and refit the trim so that you can go for a drive and monitor the output of the ABS sensor.

I researched a bit more and found that the ABS sensors are 2-wire hall sensors which have a 12V connection and a current output which generates a voltage across a sense resistor in the ABS ECU. By measuring AC voltage accurately as you drive you should be able to observe so that you can connect a test instrument to the inside of the car and go for a drive. Something to take note of: the left hand ABS sensor connects to the top plug on the bulkhead connector whereas the right hand one connects to the bottom plug- I checked this by tracing the wires back from the sensors themselves. Presumably the same on all S211s.

Results:

On my suspect (left hand) ABS sensor the AC voltage bounced around between 30 and 60mV irrespective of speed.

On the right hand side sensor the AC voltage started at about 30mV when the car was stationary and went up steadily with speed. At 35mph I was reading around 220mV.

I conclude that either my left ABS sensor is faulty (even though it measures the same resistance and diode volt drop as a new one) or the impulse ring is broken or damaged. I think I'm going to change the sensor and hope the impulse ring is not damaged. I expect I'll have to destroy the old sensor to get it out so I may not be able to see if the end of it has been damaged by a rusty split impulse ring.

Any thoughts on how to check the impulse ring without pulling the whole hub apart?

Thanks!

 

[pmcgsmurf]

Have a look at my thread from recently where I had a similar problem.

In the end I just bit the bullet and stripped it all down, impulse ring looked terrible on mine and sensor was corroded in.

 

[selphy]

thanks pmcgsmurf.

Mine is as estate (S211) so it has Airmatic at the back. I looked at the Haynes manual and it says I will probably damage the air springs if they aren't depressurised with Star before doing this job. It also mentions several special tools. I will try to have a look at WIS as well to see what it has to say. This might be one for the garage...

 

[sceh]

I doubt you need to depressurise the system frankly. I have the electronic workshop manual so if you give me your VIN I can tell you how Mercedes do it. It is generally very simple

 

[sceh]

I have a w123 with hydropneumatic at the back and also a Landrover with air suspension and never had to depressurise either for anything. I have various Haynes manuals and sometimes their advice is a bit iffy

 

[selphy]

Thanks,
I took it into the garage in the end and it came back all magically fixed for the price of a couple of hours labour and an ABS impulse ring. The sensor itself was fine.