I've heard this question about if you could check the lambda on '86 and newer Mercedes with a good multimeter and here is the answer. YES.
This is written for the Fluke 87.
1. Connect the black test lead to a good ground and the red one to pin # 3 of the diagnostic socket. (located on the l/f fender under a black round cap)
2. Set the meter to DC volts.
3. Push the "Hz" button 2 times. This will convert the scale to the %.
4. Push the "))))" button once to change from positive polarity to negative.
When you read the meter at this point, you will be getting the real value of the lamba as it is seen by the ECM. (the scale is from 0=richest to 100=leanest) Ideal is from 45 to 55% on most Mercedes.
5. When the engine is hot the value will be fluctuating all of the time if everything is well. This means the system is in closed loop. You will want to check at idle and at 2500 rpm for 15 seconds or so.
6. To get the average you will push the min max button once and wait until the meter stops beeping then press it 3 more times.
7. If you want to repeat the average just hold down the min max button for 2 sec to clear the meter and do 6 over.
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
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