It is pretty sad that their tech support could not answer this question...

It seems to me a simple test would be connect to +12 and it will read 100% and then connect to ground and it will read 0%. I was a bench tech and the MB system of measuring the "percentage off" duty cycle does not strike me as odd since "low side switching" is very common in control systems. You could reverse your leads as stated or just subtract the number from 100.

Mike

Mike,
I ran the test you suggested with one modification. If you connect the red probe to +12v and the black to ground you do not get any reading since the meter is looking for a pulse signal. So I lifted one of the probes for a split sec a few times and the meter read in the high 90% range. When I reversed the leads and did the test again, it read in the low single digits %. I would say that this confirms that this hand held meter reads % ON.

The EXTECH support dude said that all of their meters function in this same manner. He thought you would need to get a higher end scope for it to display the % OFF that Mercedes uses (or as I suggested to him, just reverse the leads on the cheap meter).

The test has to have a pulse to read a duty cycle. .and the pulse has to be within the specs of the meters capacity to read it.
Straight 12v will not have a duty cycle and 100% can not be read by the meter, as 100% is NO duty cycle.
So, your test of interrupting the signal does verify an ON duty cycle meter.

You are correct. Some Extech specs I have here on one of their meters says the PWM range is .4% to 99.6%. So at 0 or 100 you will just get "OL" or whatever as you said. I agree the test you did is a good test and confirms what you thought. Pretty clever reveresing the leads. I don't think I would have thought of that!

Mike

Landis' formula does indicate 100% duty = 0v...

Duty Cycle = [1 - (V{pin 3}/V{max})] x 100%
http://www.landiss.com/mixture.htm

...1 more highly credible rumor...

There are some symbols (in my second sig link ...1st attachment) next to the 'lambda tester type' description that I don't understand the meaning of :

100% IR and 100% (?)---see link...

Anybody?

I think the problem we have here is that we have most members (self included) buying sears $20-30 testers that read percent on while mercedes is calling for % off and then the instructions not making sense...

I know Mark's issue is tricky, because he is not getting a solid LED to switch to the test mode.
For me it was simple... in my post a few weeks ago, I figured it out...
I attached the meter, and looked for the 85% that my sons Cal spec car should have, and read the meter based that way.
The pics I posted on my thread make this very clear... not many words needed!

I think the real problem/question here is why is Marks car NOT giving the correct reading in KOEO mode?

Merkey,
I indicated in another post that my car is the newer style with the 55-pin ECU while yours has the 25-pin ECU. I have no "continuous ON" because I have an extra wire that carries the "Duty" signal to the X11 module at all times. My KOEO signal of 30% is normal as I found out in the MB WIS manual. I confirmed this with a '92 300te that my Indy let me borrow.

Ok so I am in pure hell. I have a very nice 1986 W201 2.3-16 and I decided after all the reading I did about duty cycle and CO setting to check mine out. SHe smelled a bit rich so I thought, hey, I'll do it the right way and buy the meter and test it. WRONG!
I have read everything on the forum about setting the CO by duty cycle and by EHA. I added it to what I already have learned and already knew and put it all together and still have no explaination at to why I cant get the readings correct. I cant even get the 70% or 85% to read indicating a Federal or California car with ingition on and engine off. When I do this simple test at the X11 diag connector on the drivers fender well, I used both the 2-3 pin setup and the 3-2 pin test. Red and black leads in 2-3 and then in 3-2 and neither shows 70 or 85 percent. Red in 3 and black in 2 shows 50.6% plain as day al day. Red in 2 and black in 3 shows 49.8 all day long. Using a different ground instead of the 2 pin gives the same results no matter what. I have a spare ECU computer, correct part and numbers for the car, an exact duplicate and a working pull. I swapped it and is shows the exact same values for duty cycle. If some one wants to tell me I have two bad ECUs that went bad wit the exact same resistance, go ahead, but I think the ECU is good. Can anyone clear this up?

When I start the car , the readings all go way in to the 90's and fluctuate from the initail 50% after the warmup period is over. The o2 sensor is new and the eha is relatively new and does not seem to leak. Other than the car exhaust smelling a bit rich, the car runs great. maybe the idle is a bit low, instead of 900-950 as I think the manual states, it might be in the 750-850 range. But, no stalling, easy starts col and hot, no overheating, ac works fine, etc....just screwy readings that match nothing I can read or hear from anyone else.