E220 MAF Sensor reference voltage query

[brooksie]

Hello everyone, I hope that one of you specialists can answer my question.
My problem started when the intercooler hose split, up until then the car worked fine, power was good, kickdown worked, the engine ran well and performed as expected.
Once the hose split, the engine management light came on, and the car ran in limp mode.
I had trouble getting a replacement hose, so for a while the car ran in limp with a temporary repair to the hose.
As the engine was not being pushed, I think the DPF filter became clogged causing secondary problems.
The hose was replaced, fault code reset, the engine light out but the performance was low with no kickdown.
The MAF sensor was blamed, and replaced with another, not new one, and eng mgmt. reset. Still poor performance.
The DPF showed signs of being clogged on the diagnostics, so was cleaned and a regen run. Still poor performance, but eng mgmt. light out. DPF readings now acceptable.
After some while, the eng mgmt. light returned, and the fault pointed to MAF sensor. A pattern Ridex sensor was sourced and fitted, eng light reset. Still poor performance, and after a while the light was back on.
Time to dig deeper. With the engine running, voltages on the MAF sensor were checked.
12V-OK, Gnd-OK, Output-around 1.2v to 1.5v, 5V Reference voltage-low @2.5v.
Mercedes cct diagram for ECU obtained, and established the source of the 5v, this comes from the ECU internally.
The schematic diagram of the ECU does not show the internal components, only the internal block diagram.
When the MAF sensor is unplugged, the 5V returns to 5V as if there is a load on the 5v output supply,(measured on the ECU output side) but drops when the plug is refitted, engine running or just ign on. As the MAF has been replaced, this now points to the internal voltage regulator in the ECU being faulty.
The garage that has been investigating for me advised that maybe Mercedes should look into this, so as the car had to go in for an Airbag replacement, I asked them to reset the system at the same time which they did FOC. After the 10 mile drive home, and a restart, on came the light again. They did not look any deeper into the problem as they thought the software update they carried out and the airbag reset may have fixed it, although the power was low still with no kickdown when I picked the car up.

I now believe the problem lies within the ECU, but understand this is probably very costly and may not be the fix.

So, my question to you all is, has anyone had problems with this fault on the E220s, and in your experience what would you suggest the problem may be, and the likely fix?
Has anyone actually measured the reference 5v on a working car, is it actually 5V or is this a red herring?

I look forward to your reply…hoping you may be able to help…

 

[Dodipops]

It’s always 5v but the 5v reference voltage is to more than one sensor.

Although the wiring diagram will show straight to ecu the PIN number might be the same as say 5v reference for EGR or something.

Could be something else on the circuit pulling it down??

 

[brooksie]

Thanks Dodipops, the 5v comes back up to 5v when the MAF is unplugged, so I don't think there's a problem somewhere else. I will put the OBD2 analyser back on to check the fault code now to make sure nothing else is reporting as a problem.
Does anyone know who to contact or how to get into mercedes for technical info ie schematic diagram for the ECU internals? I am thinking of making another 5v regulator cct which is fed from the 12v and connecting that to the MAF only to see if this makes a difference, it all seems so odd that the car was fine until the hose split...

 

[Supernoodle]

Is that a MAF with an intake air temp sensor? On the diagram you have how are the pins exactly labelled?
I can't believe that a 5v sensor supply is low without an error. The sensor supplies are monitored.
MAF sensors normally are variable frequency with a fixed duty cycle of 50%, so there isn't a reference voltage. The frequency varies with airflow. If you measured this voltage it would be 2.5v.

If you have an IAT in the MAF that will connect to a pin on the ECU that will be pulled up internally to 5v via a resistor. That forms a potential divider. Sensor disconnected you would see 5v, connected something between 0.5v and 4.5v.

So not really sure what you're measuring.

Also worth pointing out some variamts use air mass adaption, ie MAF drift compensation. If you have that, running with a leak can mean that it learns an offset. Just fixing the leak doesn't reset that. That needs star/Xentry for the filter replacement function.

 

[brooksie]

Thanks for the info Supernoodle, the MAF is marked on the diagram as a Hot Film MAF (see screenshot) , and the diagram shows the MAF as B2/5, 4 PINS, Pin 1 /Violet output to ecu, pin 2 rd/wh (GND) ,pin 3 BN 12v, pin 4 Blu 5V Ref. When you say the MAFs are variable frequency, I guess you are referring to the output? I could hook an oscilloscope onto this the look at whats happening on this connection.. What you say about the air mass adaption makes sense, its just like its trying to run (as it did do a few miles with a leaky repaired hose) and has learnt an offset, and as you say needs STAR/Xentry to go to the filter replacement function. Logically I cannot see why I would have other issues (ie 5V supply fault) after the hose split, but I think a reset of the system is more likely to be the answer, there is an MB/tuning specialist locally, I think he may be my next port of call..

 

[Supernoodle]

From what I can see those pins from to the MAF to ECU both say Sig. So I guess they are signal pins rather than just a supply. So to really know what sort of signal you need to put them on scope.
From the key b1 implies that has the IAT incorporated, so I guess that is one of the signals? IAT would be analog, but expect the MAF to be variable freq. Could be analogue but most manufacturers switched away from as they aren't accurate enough.


I agree to much of a coincidence that you have a sudden electrical issue. Checking the signals is worthwhile if you have the kit so you know what is normal for the next time there is an issue.

 

[brooksie]

From what I can see those pins from to the MAF to ECU both say Sig. So I guess they are signal pins rather than just a supply. So to really know what sort of signal you need to put them on scope.
From the key b1 implies that has the IAT incorporated, so I guess that is one of the signals? IAT would be analog, but expect the MAF to be variable freq. Could be analogue but most manufacturers switched away from as they aren't accurate enough.


I agree to much of a coincidence that you have a sudden electrical issue. Checking the signals is worthwhile if you have the kit so you know what is normal for the next time there is an issue.

It looks like that was a total bum steer, I decided to ask a local specialist to have a look, they charged quite a high diagnostic price, but I guess the equipment and training of the staff all has to be paid for somehow... anyways they first found the exhaust pressure sensor was giving some odd readings, so it was replaced, the kickdown was still wrong so looking closer they found the DPF Back pressure sensor was also giving odd readings, this was substituted with a known working sensor, problem cleared, the car now runs as it should with full boost and kickdown, it's lovely to have normal power and performance back after a lot of headscratching...
The car also has given a new error code which is intake air leak, I have a feeling the boost charge hose may be going the same way as the intercooler inlet hose went...need to monitor this as it has not reappeared since resetting...