Air flow meter 1990 300SL - PeachParts Mercedes-Benz Forum

bhatt · #1 06-24-2008, 12:14 PM

Hello all,

I have a 1990 300SL and about a year ago I messed with the air flow meter. The car was stalling just off idle, a typical symptom of the throttle potentiometer (commonly discussed on this forum).

Replaced the pot and couldn't get the car to idle properly. Took it to my MB tech and he said "I replace the entire unit.. I've never had success adjusting those potentiometers". I believe him, because I followed ALL of the rules/voltages/etc, and couldn't get mine adjusted right.

So here's the issue.. he adjusted the pot the best he could to stop the car from stalling. However, the car is down 10-15% on power. He said it the best he could do was balance the two (loss of power vs. stalling). At one point he had good power in the car, but it would stall on occasion and would always stall if the A/C kicked in or power steering pump caused a load on the engine. So he adjusted for the "best we can do" setting

I have now located an identical air flow meter (with pot attached) from the identical car (1990 300SL). Can I swap out my air flow meter/pot with the new one? I've done this before, so I know how to re/re the fuel distributor and fuel lines, etc.

Are there any other adjustments, or (considering the screwed up adjustment of my pot) will this fix everything and put the car back to normal with good power and no stalling??

I should point out that I will not futz with the mixture screw this time to fix stuff when I screw it up.. I promise

Thanks for any help you can provide!

Neal

gavinbr · #2 06-26-2008, 12:05 PM

Hello,

I replaced my AFM on the valve body of my 300e/m103 engine.

The key is to mark the position before removal.

The mercedes work shop details the the voltage range, in my case it was .55 volts to .95 volts. Get the workshop manual and it will tell yout specs. I replaced my AFM because I had high mileage and my symtom was rough idle at low RPM, around 500rpm.

To me it sounds like you have a problems else where in your car. Suggest checking the EHA valve and the idle control valve. Again they have specs listed for checking against.

Good luck.

Gavinbr

http://www.peachparts.com/shopforum/showthread.php?t=145824

bhatt · #3 06-26-2008, 02:24 PM

Thanks Gavinbr,

I did actually check the things you mentioned. I was having a stalling problem and thought (mistakenly) that the pot would fix it. So I replaced the pot and it didn't cure the problem.

MB tech later looked at the car and found the EHA was failing. So he replaced it, put my old pot back on the car, and adjusted it.

So it is definitely an issue with the pot/air flow meter assembly. I'm sure if I swap the air flow meter with a new one, the problem will go away. The tech spent lots of time adjusting the pot (I gave him the documents you refer to below) but we just couldn't get it right. He made the comment that he has tried adjusting them over the years but found that replacing the whole unit is the best thing to do.

I did mark the pot when I removed it, but even a minor adjustment seems to make quite a difference. Even when I put it back according to my markings, I couldn't get it exactly right.

Are there any adjustments I will disturb (mixture?) by just swapping the entire air flow meter/pot assembly with a new one?

When you say you "replaced the AFM" are you just referring to the potentiometer (little black electronic thing) or the entire air meter assembly (big metal thing with round butterfly valve)?