The higher the duty cycle reading, the richer the mixture. You need to establish a base line with the car, before making any sort of dramatic adjustments. Correct plugs, good wires, cap, rotor etc. What's the condition of the O2 sensor? If you don't know, it's probably time for a change. They don't have an indefinite life span and play a major role in proper A/F ratio. Do all of your checking with the motor at operating temperature.

Good Luck

Jayare

 

I don't think so ...

The higher the duty cycle reading, the richer the mixture.

Jayare

No, the higher the duty cycle, the WEAKER the mixture.

Otherwise, Jayare, i agree.

RayH

 

Thanks for the response!

Plugs, wires, cap and rotor all within the last 20,000 miles - although it's been 5 years (PO didn't drive car much - she changed all of this about 52,000 miles in '05 and I bought the car from her last May at 57,000 miles - currently it has 72,500).

I have a new O2 sensor to install, but I was trying to establish some understanding of the readings before I just replaced the O2 sensor.

Why do my Volts and Duty Cycle % readings not make sense? Should I reverse the black and read on pins 2 & 3? If I do, I get a static 30% DC...

ryan

 

Lower voltage at the X11 connector means lean. Higher = rich.

0v = lean
13.6v = rich

If you don't have a frequency feature on your DMM, you can figure it out by diving what you get by 13.6 then taking that number (1 - x) that will give you the percentage you are looking for.

 

Thanks guys. Seems I've been reading something incorrectly with my duty cycle meter. When I lean out my mixture (CCW rotation) my meter % comes down.

Regardless, the car runs best when I'm showing a reading of 46-49%. Never hesitates, never bogs down.

Jayare

 

Which is correct. Voltage...higher = richer....Frequency...lower = richer.

It is a bit confusing though. I constantly keep going back to remember after a few months to double check.

 

Well, using the formula you posted (volts / 13.6 = XXX) indicates to me that higher voltage is a richer mixture.

With my meter, which has a duty cycle setting, if I lean it out (CCW) while the meter is attached, I can watch the % fall until the car will hardly run because it's so lean. That was the basis for my comment in the first place. I've adjusted my A/F ratio, using this logic, on all my previous M103 powered cars, my M104 24-V 300CE and my 190 2.3 16V. All ran/run perfectly.

Jayare

 

You must do 1 minus the number you got though as I said...

Example:

Read duty cycle....You got 3v.

3 / 13.6 = 0.22.

1 - 0.22 = 0.78 or 78%....You are running very lean. As I said, the frequency you will read on the DMM reverses the rich/lean. Confusing yes...

0v = lean
13.6 = rich
0% = rich
100% = lean

:surrender:

From landiss...

The signal provided at pin 3 of X11 is called the "lambda on/off ratio" signal. It is convenient to use pin 2 of the same connector as a ground reference.

This lambda signal is not a replica of the voltage from the oxygen sensor. Instead, it is a constant 100 Hz pulse whose duty cycle indicates the hunting of the fuel injection for an ideal mixture by switching back and forth between very slightly rich and very slightly lean.

Mercedes has chosen the less common definition of "duty cycle" in this case. They are referring to the percentage of the entire pulse period during which the voltage is zero, not the time when it is near battery voltage (mine was +13.6 V when the battery was +14.0 V). In other words, if the pulse rests at ground for 7 milliseconds and then rises to +13.6 V for 3 ms, the duty cycle is considered to be 70% (see oscilloscope trace at right).

If you have an oscilloscope to measure this timing, fine. If not, it can still be estimated with a voltmeter. Since continuous "0 volts" would be considered 100% and continuous 13.6 V would count as 0%, just measure the voltage between pins 2 and 3 of X11 and divide that by 13.6. Next subtract that ratio from one, and convert the result to percent.

For example, if the meter reads 4.0 V, first divide 4.0/13.6 = 0.294. Subtract 1.0 - 0.294 = 0.706, or 71%. In equation form:

Duty Cycle = [1 - (V{pin 3}/V{max})] x 100%

 

Makes sense. Now I'm itching to go out and fire up the 300CE, get it warm and start checking V/duty cycle. Then again, the feel like temp outside is well over 90, so I may wait.

Jayare

 

Using my meter, set on duty cycle %, I get a reading of 49-51%, Voltage is approx. 7.1. Using the formula, it pretty much works out the same. Now, turning the adjuster ccw, the cycle % drops, voltage drops.

I need to run the CE for awhile. It's been sitting, quietly, for about a month now.

Jayare

 

Can I refer you gentlemen back to my original question in post 1?

"Just bought a Sears multimeter 82139 with duty cycle and I have been taking some readings but I can't figure things out for sure. I put the red lead on pin #3 and the black lead on pin #2 and I get a a static reading of 69.9% - so far so good. Start the car and readings fluctuate around 50% until the car reaches operating temp - then readings go from 33%-41% at idle and 35%-43% at 2500 rpm. So I'm running rich, right?

So I check the volts and they read 4.7v-5.7v at idle and 5.1v-5.9v at 2500 rpm - BUT, according to the Landiss formula that doesn't make sense! A 4.7v reading should correspond to a 66% duty cycle and 5.7v should correspond to a 58% duty cycle - backwards of what I am getting! What's going on here?"

These numbers above don't make sense to me! Does this have something to with the way the Sears 82139 measures duty cycle frequency? Should I hook the leads up "backwards" - red to 2 and black to 3?

Does anyone understand my confusion - I don't know if I am communicating my problem clearly???
ryan

 

Let's start all again and not over think the concerns you have. Set the meter to duty cycle and measure what you have with the car a full operation temperature. If you get a 58% reading, depress and turn the A/F adjuster nut about 1/8 ccw. Rev the motor, let it settle down and check the reading again. Repeat this until you get a reading that varies between 47/48 - 50/51. As you know, the reading jumps around, but should swing between these values.

Jayare

 

OK. That was going to be my next step. Up to this point I haven't attempted to actually adjust the mixture - I have just been trying to figure out how all of this works!

You might read through the following link - I just found it.

Sears Multimeter #82139 for Duty Cycle Check - PeachParts Mercedes ShopForum

Pay particular attention to post #9.

ryan

 

Anyone read through these posts and can offer some advice?

Interestingly enough, I checked the duty cycle on my '86 560SL - red lead in 3 and black in 2 gives me a 30% static reading (KOEO). Reverse the leads and - of course - I get a 70% reading!!

Anyway, I installed my new O2 sensor in my 300CE and it is running even richer (I think) - 22% to 29% duty cycle. Went up a little at 2500 rpm - 26%-35% - give or take...

Does this make sense? I still haven't tried adjusting the mixture - will work on that tomorrow.

ryan

 

ryan
I just did my mixture adj last Fri after replacing the injectors on my 92' CE to resolve a slight off idle stumble. I have also been hesitant previously because of some conflicting information that I've read. I believe your research has come to the correct conclusion that the sears multimeter is measuring pulse when voltage is at max and not at 0 Volts as needed to correctly measure the duty cycle as Mercedes defines it. So as you concluded, reversing the leads will indicate the correct duty cycle. I used a higher end fluke meter that you have to select "duty cycle positive" or "duty cycle negative". When selecting "duty cycle negitive" in order to measure "time duration at 0 volts" I could clearly watch the duty cycle increase (from the low teen's) and the voltage decrease (fm above 12V) as I adjusted the mixture screw CCW to lean out an extremely rich mixture. I only brought it up to 36-38% as it was running fairly well before and i did not want to make a drastic change all at once. So far everything is good the last couple of days.
Good luck with your CE!
Mike

 

I don't see how it can be my the Sears tester when I am getting opposite readings from the 2 cars - again, '86 560SL with red (+) in 3 and black (-) in 2 I get a KOEO static reading of 30% and with the 91 300CE with red in 3 and black in 2 I get a 70% static reading. There must be a difference between the 2 cars - right?

So, I don't know what is going on - and as I have read thru all the posts on this topic, I'm not sure anyone knows for sure!!

Anyway, I did adjust my 300CE mixture this weekend - slowly CCW resulted in a duty cycle reading moving from approx 30% closer to 50% (Still not sure if I'm leaning or enriching the mixture). I set it to right around 50% and have been driving it around for a day or two now. Response is better - no warm start problem yet and still some hesitation, but noticeably better. I am awaiting an EHA harness and I am going to test it today.

ryan