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  #1  
Old 07-30-2004, 11:40 PM
zorin's Avatar
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Question ?Air Flow Meter or just the Pot?

Hello,

I had taken out the Air Flow Meter pot out of my 300E '86 to test my wife's 260E. But ever since I put it back it has not been working properly. I noticed this when there was high idle at start up and then RPM reducing to the point of stalling.

I checked the diagnostic plug, it shows 40% duty cycle (which indicates a problem with AFM in the manual). I also measured the voltage between pin 1,3 - it's about 4.5 V which is OK. However the voltage between pin 1,2 is close to zero. The manual says it should be around 0.5 - 0.7 Volts.

I had marked to correct position of the pot before I take it out, so it is sitting at its original position.

I am not sure what I have done to cause this malfunction but do I need to replace the whole AFM or just the pot?

Thanks.
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'88 Benz 420 SEL
'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery

86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)
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  #2  
Old 07-31-2004, 03:22 AM
MARIO FARIAS's Avatar
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ROTATE THE POT TILL .8V

pLEASE, I think that you have to rotate the POT till 700- 800 mV.

between pin 1 and 2 . Try to use a better preccision Multimeter.

You can note the RPM increase at 800 mv.



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  #3  
Old 07-31-2004, 10:48 AM
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Thanks Mario,

Can you be more specific please? What do I rotate? I have place it in the exact same position as before I take it out.

Thanks.
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Zorin

'88 Benz 420 SEL
'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery

86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)
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  #4  
Old 07-31-2004, 11:37 AM
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Tighten three of the four screws snug and lightly tap on the body to rotate the pot position on the AFM. I would try and achieve less than .7v at idle with engine running. The adjustment has to be with the engine running. The amount of movement required can not be seen, as it it too small. It is so sensitive that MB does not sell the pots separately (though they are available through Bosch).

If around this voltage a small movement makes the voltage go the wrong way, replace the pot. The spot of contact at idle is where they wear out.
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  #5  
Old 07-31-2004, 04:24 PM
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Yes, marking the position will not be nearly accurate enough to put it back like it was. That will just give you a rough idea. You will have to adjust like the experts above advised.

Also, if you do a search you will find tons of info.
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  #6  
Old 07-31-2004, 05:19 PM
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Thanks everyone. I will follow your instructions.

Cheers.
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Zorin

'88 Benz 420 SEL
'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery

86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)
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  #7  
Old 07-31-2004, 11:31 PM
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Location: Joliet Illinois
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Sounds like the pot

I took the pot off my '88 260e to try cleaning it and had the same problem when reinstalling it. I eventually got it adjusted correctly again. The adjustment is VERY sensitive.
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  #8  
Old 08-01-2004, 01:06 AM
zorin's Avatar
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Problem Fixed.

Thanks everyone.

I took out the pot, bent the arms forward a bit just to make sure there is contact, and put back the pot. Then I adjusted it by lightly tapping it as Steve mentioned until I got about 0.67 volts. Is it ever sensitive!
I guess I was lucky when I put the pot back on my wife's car. It worked the first time!

Thanks again.
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Zorin

'88 Benz 420 SEL
'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery

86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)
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  #9  
Old 08-01-2004, 05:56 AM
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CONGRATULATIONS

CONGRATULATIONS ZORIN. FEEL HAPPY. That is the Mercedes way, logic, but true.


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  #10  
Old 08-01-2004, 01:51 PM
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hi, how do you check the voltage and which pins do you check, and what do you use to check it? i have a feeling when my friend put it he didnt do it right. thanks.
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