Hi Guys,

Recently, our two W221s both developed an almost similar ABS, ESP, Distronic etc. inoperable issue even though the vehicles are quite different, 2009 S600 vs. 2007 320CDI, and are located very far from each other.

When the cars have been sitting over night, they start and drive just fine. However, if the car has been parked for a couple of minutes after it had reached normal operating temperature, as soon as the door is opened there is that dreaded warning gong and Release Parking Brake is displayed in red letters.

Switching on the ignition makes all the usual vehicle dynamics faults come up in the display, like ABS, ESP, Distronic etc. inoperable. When the engine is started and the car driven for a couple of 100 feet or more, then stopped, the engine turned off and then directly on again, the warnings disappear and everything is fine for the rest of that drive!

So, while the the faults where displayed, I connected the SD laptop (engine not started, only ignition on) and found out that one of the wheel speed sensors (right front S600, left front S320CDI) was reading random crazy speeds (and in the instrument cluster speedometer, I could see up to 320km/h ). So, I ordered a new original sensor directly from Mercedes, installed it, cleared all faults and ...... NO difference unfortunately! Exactly the same weird behavior with the new sensor.

What on earth could cause this issue in those two cars? N47/5 connector corroded? Cable problem?

I guess the Release Parking Brake warning is because the car thinks it is moving due to the crazy speed sensor readings. But where can these faulty speeds come from instead of the sensor itself?

Thanks for any help!

Very weird. Can you pull codes? Do you have a gravel drive-way?

the front hub inner seal is part of the sensor / reluctor relationship - it can get all the rust and disc wear filth clog it up, as somehow its magnetic

on the rear wheels its a different design and the ring corrodes and falls off

any build up will, alongside the designed to fail sensors cause issues

Yes, interesting. Maybe, all the filth can trigger the sensor to fire at a high chaotic rate while nothing is spinning....

Actually, I did try to clean through the hole with a paper towel wrapped around a stick and turning the brake disc and with compressed air while turning the disc. Maybe it was not enough to clean it...

I can try to work a little more with Xentry SD, but I think it was all "!" instead of "F" and then I found the rogue sensor readings, which thought could be rectified by a new sensor.

Another weird thing was IIRC that Xentry at first was unable to initiate/find the ABR module, but succeeded when I tried once more.

A big Mercedes Center suggested it might be a low voltage problem, but that was before I had found the crazy speed readings from one of the sensors. I mean if the N47/5 thinks one wheel is spinning that way nothing can work correctly...

And yes, there is some gravel on the paved roads here once in a while...

ABS modules like to die on ALL BRANDS OF MOTOR VEHICLE - they designed them to die - but merc ones are specced to last at least 10 times the length of the BM trash

often no comms with the module is a normal consequence of the failure modes built in.... lots of places can fix the deigned in faults and offer life time warranty and then no coding fun refitting your old unit

a UK place - as its just for reference....

https://www.ecutesting.com/product-c...dule-combined/

Our W221s hate low voltage. I would check the dates on the batteries and recharge them fully and test with a voltmeter. I suppose you could also have a bad "new" wheel sensor or a bad connection. I would recheck your work on the replacement sensor. I have heard of gravel roads causing damage to sensors, but usually it is on an unpaved driveway gravel covered and the driver was going fast (he shall remain nameless but he drives an S600).

Yes, there is a company near me that fixed the amplifier that goes bad sooner or later. Has worked great since...

Is it possible to just unbolt the N47/5 ABS module from the hydraulic block or does one have to mess with break fluid and stuff with cleanliness issues etc.?

The battery is original Mercedes AGM and quite, plus kept in perfect condition by a very advanced charger when the car is not driven. And when the car has been sitting a while, the cold start does not show this weird fault.

Maybe a ground point has gone bad and causes voltage issues for the ABS module under certain conditions...

yes it just unbolts - just be careful with the multiple dangly bits that are left floating

in the old days the failure was just a link wire that "accidentally" fell off between the circuit board and the mutliplug - just re-solder
but today the ESP pump motor brushes gum up with carbon dust - meaning the brushes wear funny or don't make contact

It sounds like a weak AUX battery problem. At first start you get the problems because the battery is low but then when you drive just a little and re-start it is fine because engine starts just slightly easier and don’t pull AUX voltage too low.

Also, you mention that when car is not used it has a charger connected and at cold start from this the “Christmas tree” does not light up. It would be because the AUX battery is well charged.

This happened to me on the 2010 E550 and the fix was to replace the very small AUX battery. Weird thing is the car did not give any message for low battery…?

The AUX battery in my cars is at the left end of the dash and is only like 3” x 4” x 1” in physical size.

Crazy speed reading may be another issue though, but perhaps low voltage at start can somehow mess up the speed module too…

Good point! Yes, the car had all sorts of fault messages about two years ago. I changed the AUX battery and everything worked perfectly. Hmmm, this battery should last longer than two years, especially if the car is always kept well charged. But maybe this particular battery had already been sitting on the shelf for a long time and was not super fit to begin with...

Will investigate further, but still, even when the car has been sitting for a couple of days without charger, cold start is just fine. And after just like 2 minutes of parking at working temperature the speed readings from the front right sensor go berserk. But very interesting that your AUX battery was the problem even though you did not get any messages (I got the AUX batt message displayed two years ago)!

this is where Merc's bad naming doesn't add clarity

the consumer battery in the boot of early cars is often referred to by owners as aux (did you mean this one?), I don't think safety systems should be on anything other than the main drive set up - so I would hope / expect if the rear battery is missing dead etc.. the ABS and ESP works correctly regardless


the 211 aux battery is servo back up for the SBC madness - a 221 should not have this one....

My 2007 S600 early W221 has a very large consumer battery in the trunk and a starter battery up front under the hood. If these batteries show less than 12 volts at rest, you need to fully recharge, test the battery for voltage, and replace it if it doesn't hold a strong charge. Any major voltage fluctuations in the S600 due to failing battery causes the car's computer to shut systems down with unpredictable results. I would start with fresh well charged batteries and then re-inspect the work you did on the wheel speed sensor. That was the weird part of this story and may well be the final culprit. As BOTUS has indicated, the useful life of a speed sensor is less than 100K miles. Also not outrageously expensive. What my indy calls the price of admission into the S600 Club.....

price of admission

Thing is that our 2009 S600 has the later one battery setup with the small AUX battery and our S320CDI from 2007 has the two battery setup with the large battery in the trunk. And both developed this almost identical fault within a couple of weeks in totally different locations. Talk about weird!

It is getting weirder folks. So, I was able to hook up Xentry again to both cars after the disappointment of the immediately reappearing ABS etc. faults despite the sensor replacements.

Oh well, NO fault stored from the speed sensors I replaced .... but fault stored from the OTHER SIDES (but still front wheels at least ) that NEVER before had thrown a fault code!

I mean what are the odds on two different cars??? Is there a precisely defined best before date (not age obviously, more than 2 years and 100,000 miles apart) for those sensors?

Could not find any other faults. So, I am going to order one more (for now) speed sensor to see if it finally helps getting rid of the faults.

My theory is the temperature (and maybe humidity).
My S500 has all sorts of weird electrical issues on hot days + hot car.
And then i drive 2 seconds and the issues just disappear.

The odds are indeed long on those speed sensors. After you replace the second sensor, go buy a lottery ticket. If you win, we can split the results!

Speed sensor is much cheaper than an ABS unit. Ask me how I know.

Yup, had the same thought actually!

merc literature says ALWAYS replace ABS sensors in pairs (aka across the axle)

don't believe its essential in all circumstances - but having done wheel bearings on the front this year I know there is NO WAY u can clean the reluctor ring without pulling the hubs off the car

I agree. I would never change the brake pads on only one side. Some things just need to be maintained properly in pairs. Still not outrageously expensive.

Hi again,

Today I finally had some time to replace the speed sensor on the other side of our S600.

Cleared the codes, took a quick test drive and parked at operating temperature in relatively hot weather for a couple of minutes. Restarted it, and BINGO, no new ABS etc. inoperative fault! Hope this has fixed the annoying issue. We'll see after a longer drive. Maybe there is secret hidden software in the car that punishes people who do not replace both sides.

I did one more thing though: I pushed the big connector to the N47/5 ABS module out half way and then in again a couple of times. That could have helped too if there was bad contact somewhere. We will see with S320CDI when I have had a chance to replace the other side too...

Very happy it cleared the lunacy on the V12. I love mine.

As a general principle, always replace stuff in pairs. I wouldn't do brakes on only one side nor any suspension stuff on only one side. Also with most parts, go for OEM and not Chinese pattern parts. The Chinese make great products when they have a detailed spec, but if you tell them to just make it cheap, they won't even lubricate internal gears in things like water pumps. And a year later, you know what happens.

Post again after you do the other car, and do both sides.

Absolutely love our V12 too. Mercedes' goal with it was to make the best car in the world (for its purpose) with the top of the line W221 S-Class. Even today one can feel it, especially if it is fully loaded with all the options.

And yes, I drive and have driven current generation Mercedes models including W223 (did not feel like the best car in the world was the goal)...