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

In theory you should be able to measure duty cycle by reading the voltage at pins 2 and 3 of the X11 connector. In other words, with the key on engine off you should get 4.0 V. Divide 4.0/13.6V = 0.294, subtract 0.294 from 1 = 0.706 or 71%. Simple math right? Well, Ive been getting wierd DC readings on my tester lately so I decided to check this static method out. I get 0.04 V KOEO between pins 2 and 3. I tried 5 different meters and got the same thing. What can affect the static DC reading this way or have I just lost it?

Just checked it with a more reliable meter with auto ranging feature. Got 51 millivolts. Question now is, where does pin three of X11 get it's feed? Apparently I have some sort of interruption.

You can get a very good little digital meter at Sears for $30 with a Duty Cycle range. Then you can leave your calculator out of it. You will also have a great little compact meter that you can throw in your toolbox.

I learned a long time ago that your margin for error is drastically reduced by using the correct test instrument for the job rather than extrapolating the reading. I reserve that for when I'm in a pinch and can't make the measurement any other possible way.

Good luck,

Right now, the $30 Sears meter is $20 or $18 if you are a "Craftmans Club" member. Model number 82139. Duty cycle and most anything else. I am not sure if you can sign up for the club when you buy or not but even if you cannot, it is a great price! I bought two of them.

You have to be careful with less expensive (cheap) meters ,if the internal resistance is low, you can damage an expensive component $$$$$$$$$. The same goes for a test light, always use computer safe test equipment.

I know all about the Craftsman. Remember this thread? http://www.peachparts.com/shopforum/showthread.php?threadid=67067&highlight=favorite+and+multimeter

Anyway, I'm getting 00.00 on the duty cycle, that's why I did the static test.

I only have information for a 300se. For what its worth pin 3 of x11 comes from pin 23 of the CIS-E control, and it is labeled lambda control output.
Mercedes info says for 0 duty cycle check for open wire on x11, or bad CIS-E control.
The OVP does supply 12v to CIS-E control.

Why don't you test directly from the EHA? I don't remember the exact numbers you should see, but they are all over the forum. 20mA KOEO, ~0.0= 50%...something along those lines.

Have you blown a fuse in your testing instrument?

For complete accuracy, use a scope. Meters only average the voltage. There are also expensive handheld LCD scopes available.

Look on e-bay for old Heathkit scopes. Make sure you have the manual. An old vacuum tube scope will work fine as well.

For meters, make sure you get one with a sensitivity of 20,000 ohms per volt or higher. The small inexpensive 1000 ohms per volt meters are far too inacurate due to circuit loading.

Does anyone know where I can get the connectors for the aforementioned X11 connector, and others on the car?

The Sears meter is indeed a high input impedance meter. It is PLENTY accurate for this job. When drawing a code, you would know the code even with a 5% error. This meter is way better than that. When setting the 50% duty cycle during closed loop operation the signal bounces all over the place anyway.

I know the hesitance to believe such a meter. How can you possibly get a decent meter for $30. I've been around electronic test equipment for about 37 years. To get a meter like this in years past would have been several thousand dollars. With microelectronics this kind of functionality and quality is possible.

There are indeed some measurements of duty cycle that would best be done with a scope, but this isn't one of them. Even with a scope you don't have the resolution for such a measurement as you do with a digital meter. One cycle would be several inches wide on the scope, measuring against a scale on the scope face. 1% accuracy would be about the best you could count on.

Don't knock the meter until you've tried it.

Good luck,

At this point I will rephrase the question since we've gotten a little off track. Can lambda be read when the system is in open loop? Would this cause a 99.99 or 00.00 duty cycle? Where is pin 3 on x11 getting 50 mv from?

lambda is read only after engine is warmed up and system goes to closed loop.

The .050 volts might just be the base voltage when duty cycle is zero, I don't know. Do you have a good ol' fashioned dwell meter. Dwell is nothing more than duty cycle measured in rotation degrees. It would be a good way to cross check your other instruments whatever they are. Center of the dwell meter scale is 50% duty cycle.

Hope this helps,

OK, I'll get one. Do you have the Sears model number by chance?

I don't have the number at my fingertips, but there is another thread going on right now with the number and further info. They give info on how to get in online for $20.

Good luck,

Sears 82139

Sears DMM MODEL
 

82139

With the key on, engine off the duty cycle should read a steady 70% on a Federal and 85% on a California M103. Mine reads 00.00 or 99.99. Before warm up it should read about 50%. Mine still reads 00.00.

capn, i would recommend you check your eha readings cause the x11 (which i believe is getting the info from the lambda brain) is in open loop.
m

I would look at the OVP. It sounds like you are dead in the road. You can verify it by looking at the EHA current.

The EHA current is what matters so it is the bottom line on control. I forget what car you are working on but there is no version I know of that goes 0% at KOEO (key on engine off). Late cars require the term 3 to be initiated in diagnostic mode via an impulse request at the right side diagnostic socket. These cars do read key KOEO properly though.

Larry, I bought the Sears 82400. It has a frequency meter and duty cycle as well.

Other features: (partial)

Capacitor tester
Frequency 4 hz to 4mhz
Transistor tester
temperature probe

all for 30 bux.

Pretty tough to beat for $30.

Have a great day,