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.

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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.

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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)?

Not sure what you are refering to. The external resistor is the air temp sensor. Other than the surface the rest of the device is the electronics to interpret the small changes in current it takes to maintain the exact temp of the surface. The variations in current are the mechanism of measurement converted to a voltage signal.

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

Not sure what you are refering to.

Tried to find a picture on web but could not - must take one next time I clean unit!

Basically, I see three parts on the actual sensor after removing from housing:

- Nearest to flange there is an exposed transistor or resistor - probably thermistor?

- Further down, there is a rectangular cavity that passes right through plastic sensor - inside is a mirror like small plate. There is a small hole on one side.

- Even further down, near bottom of sensor, there is a hole on front side of sensor facing air flow - it does not pass all the way through. There is some type of element or device inside - hard to get at and clean.

It is the last one that I was asking about.

Tried to find technical data from Bosch sites, but they only describe a different (newer?) type with a method of separating particles and water droplets ahead of the sensor.

Not to worry - just trying to understand better how cleaning may help and on which parts .

It's great boys. Hope one of you can find the picture, and some info. I have seen it some place for a year or two, but I can't find it when I need it.
I think I most buy a new sensor, but if the old one is repairable ,I will have the it as a spare parts. I have been in the MB shop this week,and the error code are PO170 and PO173 = air mass sensor
From a chilly Norway Arvid

Today, I decided to READ the manual for my Equus 3100 OBDII code reader http://www.iequus.com/item.asp?cid=10&pid=3100 and learn more about how the monitors work.

When I turn the unit on, it reads any trouble codes and displays them. An icon indicates if the MIL (Check Engine) light is on or if the DTC is a pending code.

What I found was that I have pending codes for P0170 and P0173 - this is what I had before cleaning the MAF and resetting the CE light. They are still in memory, so either CE light will come back on or eventually they will clear. Manual says it will take 80 warm up periods to erase the pending codes - is this correct?

As I drove, the monitors almost all changed from flashing to solid which indicates that the monitor has completed it's diagnostic checking . One (EV ) stayed on - this is evaporative emissions control. I fiddled with gas cap and a few miles later that icon went solid - may have a leak in gas cap gasket? Would that cause the P0170/P0173 codes?

I drove the car quite a distance around town today with a number of stop starts and so far no CE light - Muche better than before I did the cleaning. ( decided to check out the new 2005's at the local car lots!) Want a diesel Liberty!

If you cleared the codes & then show them pending again, the same problem still exists.
Also, if you cleared the memory ( reset the light ), the 80 warm-up cycles do not apply, as THEY would reset your light, if the problem was truly rectified.
It takes a fair bit of different driving, to complete ALL the requirements, to show solid icons on your reader.
From my experience the " EV " icon can take a VERY long time to be happy again.
I suspect thightening your gas filler cap may have been just a fluke.

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2007 C 230 Sport.

I drove around town for about 1-1/2 hours with a stop for lunch and at every car dealer I came to Sometimes shut down engine sometimes not. All icons eventually went solid.

You may be right about the gas cap. Can't see anything wrong with it, but have had a gas smell in car on occasion.

So far, I must have about 3 hours driving since I last set light and cleaned MAF - better than before, but if you are right, should get CE light again sooner than later.

Trying to find an ultrasonic bath to really clean sensor. Still may not be the problem. Vacuum is OK, but I don't know how to check fuel pressure - need special gauge?

Thanks to all - this is a great site

Boy, I've seen some really nice ultra-sonic cleaners at the SEMA show in Las Vegas, a couple of weeks ago.
If you are bound & determined to go with ultra-sonic cleaning, you should contact some local engine rebuilders.
If they don't have one, they could possibly suggest someone to you.

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2007 C 230 Sport.

Manny,
I could not recall whether I had zapped the CE light before or after I cleaned the MAF, so I decided to ZAP the pending codes and start again.

So far, no more pending codes but still a few flashing icons - need to drive a few more miles to clear these.

No matter what, I have not had a CE light since I cleaned the MAF (prior to cleaning it came on within 15-20km). This is what happened before and it lasted 3 months. So, cleaning DOES seem to work, even if not for long term. Not worthwhile for the pros unless Bosch gives them a method and equipment. However for DIY, why pay $300 if all it needs is a clean! Bad design anyway - Bosch website shows a new design that separates particles and droplets.!!!

BTW - I found some ultrasonic cleaning baths on eBay - meant for jewelery, Dentures, etc! I think they use water. Cost is only about $29.00. If my light comes back on, I might try one of these and then wash in alcohol afterwards to make sure no water is left. Maybesomeone else would like to try this first