Is this a bad MAF?

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Is this a bad MAF?

Postby steevegt » Thu Oct 06, 2016 3:14 pm

Hi,

I would like to hear your opinion on this, as this is a first for me...

The following capture was made on a MAF from a 2008 Citroen Berlingo 1.6 HDI.
Don't have the engine code at hand, but I can write it later in here...

Blue channel (A) is the scoped signal wire from the MAF. Inverted frequency type signal, as I have seen in other PSA engines.
Red channel (B) represents the current on the 12 volt feed wire, using a low current probe (450 mA).
Black is a math channel that represents the negative value of the frequency of channel A. I use the negative value of the frequency in here, just for simplicity. The point is to have an increase in airflow to be represented as an increase in frequency.

maf.png
MAF 1.6 HDI 2008


As you can see clearly, there are some high frequency events that are very strange (at least for me). Completely non linear to the direct current measurement. This high frequency peaks, seems to happen at the same time that theoretically we should have the lowest airflow.
There is a strange increase in current in that event, that I'm not sure if it is related or not to the current of the hot wire. Don't know...

maf zoom 2.png
MAF 1.6 HDI 2008 (Zoom)


This events are not present at higher airflow levels.

If you want to look at the actual signal when the high frequency event is present:
maf zoom.png
MAF 1.6 HDI 2008 (Zoom 2)


So, my question is, can this be normal? Have you seen it before?
The MAF electronics could be designed in a way, that it could produce a non linear frequency measurement, at lower airflow. I believe that is possible...

In the waveform library I could only found one MAF measurement that is likely to be the same engine as this.
The thing is that it doesn't contain a measurement at idle. The lowest RPM in the waveform is 1500, and the high frequency event is not present.
I could compare the frequency at the highest RPM, 1.045 khz, very close to the value I'm getting.

The owner of the vehicle only says that it kinda looks like the engine is a little bit out of performance, but he admits that he may be wrong, because he only drives that van once per week, and switching from one car to another could give that impression.

I didn't do a serial data reading of the MAF g/s PID, but most likely I will when I get the chance. Unfortunately Pico software doesn't allow me to that (yet) :wink:
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Re: Is this a bad MAF?

Postby volrem » Thu Oct 06, 2016 4:30 pm

Hi

Volvo uses same engine and two different MAF types.

Newer design can be measured (one where MAF is in intake tube with two screws).

Older design is with measuring tube and I have been unable to interpretate measurement results (not that I have tried too much). So far used only serial data for evaluation.

New design came on MY2008

New design:
new.jpg
New design MAF
new.jpg (6.61 KiB) Viewed 852 times

new design maf.png
New design MAF


Old design:
old.jpg
Old design MAF
old.jpg (7.12 KiB) Viewed 852 times

old design maf.png
Old design MAF


And a file to play with. Snap with brand new Pierburg (OE) sensor.

2006 S40 1.6D MAF new SNAP.psdata
Old design MAF (new sensor)
(4.13 MiB) Downloaded 60 times


I haven't spent enough time to understand how to interpretate this old type MAF.
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Re: Is this a bad MAF?

Postby STC » Fri Oct 07, 2016 2:28 pm

Steve, Volrem

I was taught that on the newer digital MAF sensors (Bosch) that the Intake Air Temp is riding on the same wire. So Air Mass being the Frequency and Air Temp being the Duty Within the Frequency all the way along the wave form, or that's what I assumed ??

Looking at the waveform, it is deliberate and not just noise, I think ?

I'm guessing now but could it be that they are sending the IAT in 12ms blocks every 24ms (as measured on Volrem's File) then at WOT there is some constraint (probably financial or speed) and IAT is not transmitted.

Makes some sense as IAT failures rarely cause driveability issues at WOT, not fast changing or as critical as MAF but important for emissions. At WOT there is less regard for emissions anyway.
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Re: Is this a bad MAF?

Postby volrem » Fri Oct 07, 2016 2:48 pm

It's a 4-wire MAF.

12V supply
GND from ECM
MAF signal to ECM
IAT signal to ECM

So it has it's own wire for air temperature. This of course doesn't mean that there isn't some secret message decoded into that MAF signal.

And no it's not noise and is repeating.
Attachments
maf2.png
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Re: Is this a bad MAF?

Postby STC » Fri Oct 07, 2016 3:51 pm

This of course doesn't mean that there isn't some secret message decoded into that MAF signal.


Could it be airflow direction ???
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Re: Is this a bad MAF?

Postby steevegt » Fri Oct 07, 2016 7:37 pm

First, thanks volrem and SCM for your time and help

volrem,

The MAF is exactly like the old design you point. 4 wire also.

SCM,

I remember reading that in digital type MAF's (not sure if Bosch, other, or all) that the flow indication to the computer includes the air temperature in it.
I made the assumption that this means that the voltage type MAF's represent directly the current through the hot wire, and that in the frequency type, the frequency represent directly the air flow, taking into account the temperature. In the first type of MAF's (voltage) the computer calculates the air flow from the voltage of the MAF and the temperature of the air. In the digital MAF, the computer does not need to do this, as this is made inside the MAF electronics...
I may be wrong in here, it's just an assumption that I made...

The airflow direction makes sense to me. It really looks like that the high frequency events happens when the airflow is minimum, and maybe reversed, as this only happens at idle...

I may get the chance to play with this MAF tomorrow, and I will try to confirm this.
If I got any news I will post in here.

Thank you volrem and SCM once again...
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Re: Is this a bad MAF?

Postby STC » Fri Oct 07, 2016 8:50 pm

the computer calculates the air flow from the voltage of the MAF and the temperature of the air. In the digital MAF, the computer does not need to do this, as this is made inside the MAF electronics...
I may be wrong in here, it's just an assumption that I made...


Hi Steve
I understand it differently to that. The Digital MAF does not do those calculations within its electronics. The ECU will still require raw data separately for both and will make the calculation within the ECU.

The difference is that the older ones had two signal wires, one for Air Mass the other for temp - Just as Volrem has pointed out in this thread.

The later ones use only one signal wire for IAT and MAF. MAF is the Frequency and IAT will be the Duty Cycle. Not the easiest thing to explain or understand from just written text, Need Pictures.

Frequency is how many times it happens in a second, but within that frequency can be a duty cycle.

so a duty cycle of say 20, 30, 40 ...... % can be at any frequency you like.
equally a frequency of 20, 30, 40..... Hz can have any duty of between 1 & 99% duty

you can adjust one and keep the other the same or adjust both together.

Very Cunning / Clever way to save one length of wire in the car. Of course, Voltage drop across that wire will not effect either Frequency or Duty so accuracy would have been the priority factor.

that the flow indication to the computer includes the air temperature in it.


Again I am guessing, We know that your MAF has a separate signal wire for IAT so we are left guessing that the "odd" bit is Airflow Direction based only on "What else could it be ???" Equally it could be Fault detection or what ever until we know for sure ??

If that is the case the it can be valid that they can put all 3, MAF, IAT & Direction of Flow on one wire. I would have thought LIN or CAN would have been the method of choice but then what fun could we have with the Pico trying to analyse what is going on.

I digress, VAG use LIN for the AC Pressure & Temp Sensor. Just 2 signals.

Indeed this does need more experimentation to Confirm or Deny our theories and guesses.
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Re: Is this a bad MAF?

Postby Steve Smith » Sat Aug 19, 2017 10:18 am

Many thanks for the posts above as this thread came to the rescue with a recent "Support Request" at Pico.

What has become apparent with this style of MAF meter is the Digital MAF Guided Test is not applicable to this design.

I have been looking into the characteristics of this airflow meter and have found very little in the way of a conclusive explanation to the varying frequency evident during engine on at light/medium load.

POSS EGR AT IDLE SPEED.jpg
EGR Operation?


One thing we can be sure of is the frequency of the output signal decreases in proportion to an increase in airflow. With the Key on Engine off (No Airflow) we have approx. 5 kHz and at WOT Max load we should see approx. 1 kHz

Intake air temperature is also taken care of via a Thermistor hardwired to the PCM and so I am not sure this MAF digital signal incorporates temperature values (Here we would duplicate what we already know from the thermistor)

What struck me when looking at Pierburgs MAF meter data sheet was how the frequency plummets from 5 kHz to approx. 2.4 kHz with minimal airflow?

I guess what I am trying to say is there appears to be very little airflow required in order to witness a large change in frequency (they seem disproportionate)

AIR FLOW CURVE.jpg
Flow Curve


Given the sensitivity of the MAF meter under low airflow conditions could variation in the frequency at low to medium engine speeds be the result of EGR operation?

If we think this through, we know that introducing EGR has the effect of lowering MAF (The PCM uses this characteristic to qualify EGR operation)

With this style MAF meter, lowering MAF (during EGR operation) would increase the frequency of the output signal. If the EGR is pulsed or the intake manifold subjected to pulses from the exhaust gases I could see how this MAF meter would behave looking at the curve above.

Looking at the waveform under WOT full load we can see how the frequency does become stable and fixed at approx. 1 kHz where EGR would be off.

I guess the only way to verify would be to find such a vehicle and inhibit EGR operation.

I will carry on chasing this one as I may be able to dig a little deeper with Volvo who have used this style of MAF meter

Once again, food for thought and thank you for posting as I could use what we have here to assist our customer

Take care……Steve
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