Hi everyone,

I'm hoping someone can point me in the right direction because I'm starting to run out of ideas.

I have a UK-spec 2019 Mercedes S350Ld. I've owned it for the last 9 months and it's been absolutely faultless until recently. A few strange issues have started appearing:

1. Speedometer keeps switching from MPH to KM/H

Originally this happened maybe once a month, but it's getting much worse. Now I can park the car, go into a shop for 30 minutes, come back out, and it's switched from miles to kilometres again. There are no other driver profiles on the car - only mine.


2. Random memory settings are being forgotten

The car is also forgetting other settings occasionally. For example, I keep the seat side bolster set to Level 1, but every now and then it resets itself back to Level 0.


3. DPF pressure sensor fault keeps returning

About a month ago, engine light, I got a fault relating to the DPF differential pressure sensor.

I initially had the sensor replaced with an aftermarket part, but the car didn't seem happy with it. I then bought a brand-new genuine Mercedes sensor and had it fitted.

The garage cleared all fault codes and carried out the teach-in/adaptation process. Unfortunately, the fault has returned and the engine management light comes back on.

When the fault is present and I view the live data, the sensor offset value shows **4,095 hPa**, which seems completely wrong. The DPF itself doesn't appear to be blocked (around 11-20% according to diagnostics).

The car also drives badly when the fault is active. It's almost like a strange limp mode where fuel consumption becomes terrible and the car feels inefficient. However, when I clear the ECM fault, and do a new teach-in process to reset the offset value, it drives perfectly for 2-3 separate trips before coming back again.


Fault Codes

ECM (Engine Control Module):
"Differential pressure sensor 1 for exhaust gas recirculation (low pressure) has a malfunction. The signal offset is outside the permissible range."

SAM Front (Signal Acquisition Module):
"The starter battery has excessive resistance."

Has anyone experienced anything similar on a W222 S-Class, or have any suggestions on where to start looking?

Thanks in advance for any help.

 

Replace both batteries.

 

Hi VII, thanks for your reply. I am happy to try anything at this point, but can you elaborate a little when you have time? I guess you've experienced these problems before? I cannot seem to find anything about it anywhere. And battery hasn't "appeared" to be an issue before, as it always starts, etc.

I guess I'd just like to know more about why you think both batteries need replacing before I embark upon such a cost.

I appreciate your time and help!

 

@VII is spot on. Intermittent and/or erratic unrelated electrical error codes are normally a sign at least the aux battery and sometimes both. Your car is displaying classic signs of the problem. All those sensors need stable voltage to work properly. When they don't work properly they send error codes to the ECU. The fact@ t hat you have multiple is a clear sign. Your car's battery is at least 7 years old at this point so it's about the right time. But check the production dates on your batteries to verify age, my guess is they are the original batteries. You have a small auxiliary battery in the rear side compartment of the trunk/boot. Not sure where main battery is located because your car is a diesel and Euro spec. Don't want to give you bad info.

 

Faulty batteries and in particular high discrepancy between batteries will cause random electrical issues that coupled with diagnostics indicating high resistance means I would start there.

 

I see, does it make sense that it only appears to affect the DPF sensor? And give no low battery warnings?

As much as I love me W222, it seems the W221 was easier to fix stuff on, or at least understand what was going on!

It happened again this evening, drove well, then gears held on longer than they should, causing low mpg, and then engine light on. Check the codes, same as detailed in my first post, and the offset value for the sensor was again 4095.5 hPa.

 

I've seen vehicles burn sensors due to old batteries (although usually wheel speed sensors). When diagnostics indicates high resistance the battery is past service.

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Unless your diagnostic has been run through an updated Mercedes Xentry system your analysis may not be complete. There are modules that cannot be “seen” by 3rd party scanners and detecting electrical faults is one of those. Now, as to the batteries, my protocol for 50 years of owing Mercedes, besides the obvious, is to replace batteries and tires at 5 years regardless of mileage. This has the wonderful benefit of avoiding the “big surprise” which usually happens at the least opportune time. And, just another thought, on a number of occasions on friends cars I have found that loose or corroded battery terminals were the culprit. Hard to believe that such a minor thing could cause cascading “gremlins” but it happens, so just humor me and check those connections. Oh, and be sure to check the ground connection to the chassis … another favorite “gremlins” hideout of intermittent electrical issues.

 

OP connect with Peter AKA Benzninja. WWW.Benzninja.com Buy the equipment and a membership with Peter. Money well spent. Peter is a wealth of knowledge and will help with many to most issues.

Having Xentry means you have real MB kit. You will get to see the list of everything MB would see. Want to change software? With the equipment software can be changed.. updated per your wishes.

For less than a visit to the dealership you will be able to help yourself in the future and Peter will be there to help when needed.