My 1999 modell E 240 have a bad air mass sensor. I have read somplace the are possible to repair the sensor, but I can't find it. Can someone help?

The problem with most AMMs is that they work by tranfering heat across a surface to the air that passes. Any form of covering insulates that activity. Thus in many manufacturers there is some evidence that cleaning the surface restores sensitivity.

I have never heard a competent reproducable technique used on the style AMM that is failing on MBs (yours in particular).

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

I am NOT a Tech Expert, but for what it's worth, I have had some success in cleaning the MAF on our '98 E320.

I removed the whole unit from the duct , then removed the sensor from the housing (needs special screwdriver or ..) . I blasted the flow through surface first with brake cleaner (I have lot's and it gives a good blast) and then all metal parts with electrical contact cleaner. I then made a wad of paper towel to fit in the flow thru opening, soaked it with contact cleaner had pulled it back & forward a few times. Also cleaned MAF plug contacts and put in a little dielectric grease.

First time I did this, it lasted for 3 months. have just it again and will see if it works this time!

Wonder if the high current cleaning cycle that is supposed to actuate each time the car is shut down may not be working, but don't know how to test it.

I would have more confidence in your cleaning ability if you had numbers. Three months is not the number I'm talking about. If your adaptation numbers changed that would be significant. If you take a sensor running at 1.30 setting a code only when the conditions are right (wrong) and you change the adaptation to 1.29 and the condition is a little rarer I'm not sure I'd consider that much affect. Nor would I infer a three month delay much of a fix if it really did make a substantial change that went away that fast.

BTW, hot film air mass meters (which is what we are talking about) do not have a cleaning cycle. The hot wire sensors do.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve - I don't have a scanner that provides that sort of data - Just an Innova (equus?) code reader that does show which of the monitors have run and cleared, but this not work with continuous monitors.

I had not realised that the film type of MAS did not have a cleaning cycle. (BTW, my car is a '98 E320). The sensor has a flow through section with a flat shiny metal piece inside - presumably this is the hot film? Then below that, there is a dead ended cavity with some type of element inside - what is that?

I did check the resistance and voltage at my MAS - 4060 ohms at 8C and 5.02v supply voltage.

In industry, flowmeters of this type are commonly used - these units are in the $6000 range, so they don't throw them away - They do maintain them by removing on a regular basis, cleaning and recalibrating. This is what led me to try cleaning. Some type of ultrasonic cleaning may be what is really needed. Presumably Bosch have a method?

The key is the value/cost. I would imagine part of that cost is the designed capability to service. The concept of "recalibrating" undoubtably plays a large part. In this system the system calibrates (adapts) to the change in the part. it has limits. If the device were to recalibrate itself more would be capable.

It is conceivable that you are improving (cleaning the surface). With the numbers we could tell. Spread over 70k or more whether servicing the device for minimal extention of life is a completely economic equation. Three months extention might be justafiable with your own labor. If done professionally I'm guessing that with proper evaluation and adaption resetting the cost would be a significant portion of a real solution. That is the economic equation.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Three months extention might be justafiable with your own labor. If done professionally I'm guessing that with proper evaluation and adaption resetting the cost would be a significant portion of a real solution. That is the economic equation.


I can see that - definitely only a DIY proposition unless Bosch were able to provide dealers with a reliable cleaning method.

Any idea what the bottom part of the sensor is for (see previous message)?