What is the procedure for checking & adjusting mixture on my 89 260e which is set up to comply with Calif emissions? I have a digital multimeter but it has no duty-cycle function. The car failed emissions here in Az last month and the check engine light gave the fault code for the O2 sensor. I replaced the sensor approx 30k miles ago. I took it to a near-by MB independent shop ( Goody's in Scottsdale-great work & reasonable) and they set it by "feel" based on my failed readings. They were absolutely swamped and said that if it failed again to bring it in for a full diagnostic. The car passed emissions but I'm still getting the fault code for the O2 sensor. I'd like to be able to check and adjust myself . Appreciate any info anyone can offer

To check mixture, follow the procedure outlined on this page:

http://www.landiss.com/mixture.htm

__________________
'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E

I have read in other postings that using a multimeter with pos lead to pin 3 of X11 connector and neg lead to battery neg I should get between 6.8 - 7.2 volts . I get approx 14.0 volts . Since this is a Calif car, do I absolutely need a meter with duty cycle function?

Since my meter has duty cycle (I bought a new one just for this purpose) I never tried reading it in just volts. Perhaps there is a forumla you can apply to the voltage readings you took? Someone will know if this can be done.

You can also set the mixture reading the eha current, reading milliamps. Have heard its a more accurate representation of mixture also.

Pin 2 is the ground of the x11/3 port.

__________________
'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E

What kind of multimeter did you buy, and what was the approximate cost?
Tom

I bought the one sears carries. It was about $30. Might be more now.

Autozone carries some meters too.

Mine:

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__________________
'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E

The voltage method doesn't seem to work on some cars. On my 300E, it did. On my 380SL, it did not - it simply gave battery voltage like yours seems to be doing.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

I bought the much talked about multimeter from Sears for $ 30.00. My 260e is a Calif car so I had to press the button at the X92 connector to give me a duty cycle reading at pin 3 of the X11 connector. I had been getting the code for the O2 sensor (5 flashes) and with the new meter hooked up I was reading approx 6.5 % at idle. When rpm was increased the reading would go to 49.99 and then steadily decrease down to 6.5%. I checked output at the O2 sensor and with engine at operating temp of 80c the output was approx 80mv. I was told it should be 450mv minimum. I changed the O2 sensor and checked with the new meter and got a reading of approx 60% and it was fluctuating like it is supposed to. With the meter hooked up as mentioned earlier (red lead to pin3, black to ground) the duty cycle reading increased as I adjusted the screw CCW. I reversed the leads (black to pin 3, red to ground) and this change in polarity resulted in an increase in duty cycle reading when screw turned CCW (leaner) and a decrease in % when turned CW (richer). I set right at 50% and idle is greatly improved and so is milage. The majority of posts on this subject call for the meter to be hooked up as mentioned in the first part of my post; am I missing something here? Regardless of the polarity, the adjustment for 50% would be the same. Initially I didn't want to believe it was the O2 sensor since I had replaced it 35k miles ago. I love driving this car but but the high maintenance is a bummer. I just replaced the tensioner after getting only 17k out of the last one and six months ago relaced a mb factory rebuilt water pump after 30k.

I just read my own post and was confused myself. Let me clarify. At first with red to pin 3 and black to ground I got an increase in the duty cycle reading when turning cw on the adj screw. CW adjustment will make it richer and as a result the duty cycle reading should decrease. I the switched the polarity of the leads and observed a % increase when turning CCW (lean) and decrease in % when turning adj screw CW. Thanks to all who take the time to post replies and share their knowledge and expertise. This site is an invaluable asset to me and countless others who want to do their own work when they can and a save a few $$ in the process. Thanks again Tom

I think you had it right the first time.

My understanding is that as duty cycle increases, the valve is open more and more fuel is diverted from the FD chamber. So as you richen the mix, the O2 sensor voltage goes up and the computer tries to lean it by keeping the valve open and diverting some more fuel.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe