Is this a bad MAF?

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steevegt
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Is this a bad MAF?

Post by steevegt »

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 1.6 HDI 2008
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 1.6 HDI 2008 (Zoom)
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 1.6 HDI 2008 (Zoom 2)
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:

volrem
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Re: Is this a bad MAF?

Post by volrem »

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 design MAF
New design MAF
new.jpg (6.61 KiB) Viewed 16952 times
New design MAF
New design MAF
Old design:
Old design MAF
Old design MAF
old.jpg (7.12 KiB) Viewed 16952 times
Old design MAF
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 769 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?

Post by STC »

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.

volrem
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Re: Is this a bad MAF?

Post by volrem »

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?

Post by STC »

This of course doesn't mean that there isn't some secret message decoded into that MAF signal.
Could it be airflow direction ???

steevegt
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Re: Is this a bad MAF?

Post by steevegt »

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?

Post by STC »

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?

Post by Steve Smith »

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.
EGR Operation?
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)
Flow Curve
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

KimAndersen
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Re: Is this a bad MAF?

Post by KimAndersen »

It's odd to look at this MAF curve from a Pierburg with the decreasing frequency at a higher air flow rate. If you compare an Pierburg with a Bosch HFM 6 with a digital output signal - the frequency increases with a higher flow rate - completely the opposite as seen in this picture.
BOSCH HFM 6 - 0 281 002 764
BOSCH HFM 6 - 0 281 002 764
Now that the automotive manufactors have changed there MAF sensor from simply frequency digital output to SENT protocol, I guess it will become even more difficult to interprete those data coming a MAF sensor !!!.

I know that Bosch HFM 7/8 uses the SENT protocol, where a single data message can transmit both temperature pressure - airflow and humidity on the same signal line - so are we prepared to diagnose these MAF sensors !

I know that picoscope can decode a large number of different protocols including the SENT protocol, but how do we really interpret those data !!


Regards
Kim :wink:

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Re: Is this a bad MAF?

Post by STC »

I know that picoscope can decode a large number of different protocols including the SENT protocol, but how do we really interpret those data !!
Bosch official line will be
1. That they do not go wrong.
2. Use KTS or VM Serial Data to Evaluate them.

Bosch as a company consider that their oscilloscope(s) are sufficient to measure and diagnose any motor vehicle issue on planet earth. Period.


Our friend Volrem recently wrote, (I do not recall the thread) The VM's think we are mere mechanics and don't do not need to know. Perhaps they think we would not understand.

This is a global forum - what percentage of car fixers are here 0.0001 % at best ????

If I may rattle on ? VM's, Aftermarket tool suppliers had no clue ??

How and What possessed Alan @ Pico ??? To think that a market existed and we are not all wrench and hammer monkeys.

I have had the good fortune and pleasure of meeting and shaking hands with him... A well versed & knowledged individual, Acutely Articulate, Firm but still a Humble man.

He has given us the EYE's to see what no man has seen before - WOW !!!

He should be recognised and Knighted for services to the Motor Trade !!! A true and worthy patron saint !!! An Investor in people !!

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