My dealership tells me I need a new MAF for my 1998 ML320. They want to charge a fortune - they tell me the part has to be "installed" and the transfer function reset in the computer to make it work correctly. Is this true? My guts are - this is a snap in part - installation time near zero, and that the MAF will reset the computer itself once it starts providing the correct readings. That is - it starts open loop ever time with the same transfer function and that the MAF tweaks it as it runs and will re-normalize itself. Am I wrong? Does this transfer function actually have to be reset properly once the MAF has gone bad?

 

resetting the adaptation value after changing the airmass sensor is good.

you may also replace it without doing the reset.

if the fuel consumption is high or you notice any unusual behaviour on the engine, then you can have it resetted.

just replace it yourself.

 

Sleepwalker,
I go with Myran.

I changed my MAF (m112 motor) due to it being out of range CEL illuminated + P0170 at bootup/start.
STFT's (Short Term Fuel Trim) were < +/- 3.
LTFT's (Long Term Fuel Trim) were >30 which I understand is the trigger point for the boot up check.
Normal LTFT's on good motor should be near 0.
On restart LTFT's remained at >30 for about 5 mins driving and then tumbled to 1.8 on both banks. Performance improved noticeably.
Since I have a OBD scanner I reset the CEL and fault code P0170 so I don't know whether the CEL would have reset automatically. I guess it would have since on the next boot the P0170 would no longer have been present.
You refer to adaptation? Is this the same as LTFT? I'm not that familiar with MB speak. I call it fuel trim zero offset.
If so why does this need to be reset with the computer?

Actually on my R170/m112 I didn't change the MAF. I just cleaned the hot film sensor using Toluene which removes the baked on atmospheric deposits. Unfortunately with identical symptoms and readings cleaning didn't work on my w220/m112.

Your views and MB experience please?
Stuart

 

yes. it's the same.

it will delete the adjustment values.

 

Thank you for responses - Yes - Long Term Fuel Trim - this is what I mean. I think because the MAF was faulty for a period the settings get off, and stored in a lookup table in the computer. When you put a new MAF in it automatically adjusts back to normal over some driving time. I think MB is giving me BS when they tell me this has to be reset by them - as though there is some magic to that to scare me away from buying the part for 250 bucks and snapping it in myself. Well they also tried the warranty void if I do it myself trick...I'm surprised my warranty doesnt void if I dont fill up with gas with them....

 

Hi.
I also believe they are BS you!
I bought my MAF off eBay for £50 (80USD!)
Collected it myself so that iIcould be sure it was OK. It was advertised as new but you know what some people call new. Make sure it is a genuine Bosch, not Pierburg etc., and then you can be sure it is excatly as original equipment. 250 USD is about the same price as they are here from MB. Outrageous. Just go and see how much a Bosch MAF is on a German Ford to see what is going on.
Stuart

 

Yes, the price is outrageous - Volkswagen charges 40 bucks for them - but I read that is because they got class actioned on them and so Bosch provides them to VW on the cheap. Out of curiousity I bought a Volkswagen one - it looks identical - BUT - they put different screws and fitting shape on it so that you cant swap the electronics package into an MB housing. I am sure it is the same part packaged with a different bow. In the end MB installed the part at a "reasonable" price for me, and I didnt have to drive a rental for the weekend.

This little detour has been interesting. I think that anyone who has an MB 6 cyl like in my ML320 should get a spare MAF on the cheap for back up. Ebay sounds like a good idea. I suspect you can clean the MAF too by soaking it in alcohol or toluene for a half an hour to clean off the accumulated grime. When MB gave me back my car they gave me the readings on the old MAF and the new one at various RPMs - here they are for reference:

Old - Engine off, 1.04V, no airflow; Idle, 1.51v, 16kg/h; 3000rpm, 2.12V, 56kg/h
New - Engine off, 1.01V, no airflow; Idle, 1.51v, 16kg/h; 3000rpm, 2.31V, 72kg/h

So, as you can see the new one has higher voltages, meaning it shows higher flow rate, at 3000 rpms. I guess this shows a "dirty" MAF. I think you can get these voltages out of your ODB if you have a scantool. I am amazed how little a variance can cause such a large predicted airflow difference - no wonder they are wonky. This also explains why my car ran fine in the city but only "crashed" on a highway acceleration. I hope people can use these values fortheir own testing if they ahve the same problem...

 

If you plot the voltages you will see that the gradient of the MAF plots are different.
What really matters is what fuel trim offset the computer uses since this is a reflection of whether the predicted air / fuel ratio gives the correct emmission readings. Since the MAF is sending the incorrect signal the computer, the computer has to apply an offset to get the correct oxygen sensor readings.
Voltages are not held in the OBD memory. However the LTFT is.
Stuart.

 

OK - I believe you - I think it is certainly a complicated feedback loop. Presumably the kg of air per hour has to do with the pressure, tempertaure and humidity of the air and other factors - but the volume of air should be relatively constant at 3000rpms since the volume of the pistons is constant. PV=nRT. I guess that matters because optimal combustion is going to be a specific fuel/oxygen ratio that the car engine needs to estimate. Then it checks this post combustion at the oxygen sensors in front of and behind the cat conv. I dont know if the gradient is relevent because the sensor might be non-linear in volts to implied mass flow; i suspect that it is. Maybe you cant use the info I provided to see if your MAF is OK - but for reference they did the testing on a day that was about 70 degrees and 80% rel humidity. Your comments are very interesting though...you just cant take the engineer out of some guys like me...heck, I try to just let guys fix the darn car but I just gotta know....

 

So when replacing this yourself, is it necessary to disconnect the battery or is it simply unplug, and re-plug?