1986 W201 2.3-16 Duty cycle problems
 

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 all 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,checks out fine and heat up and goes to work properly 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 cold and hot, no overheating, ac works fine, etc....just screwy duty cycle readings that match nothing I can read or hear from anyone else.

I would start by watching the voltage at plug on the O2 sensor..should be approx .5 volt & should start moving 1 direction or the other.. if you get a high voltage reading like .9V try leaning the mixture adjustment until the voltage drops to .5V or swings.....IF there isn't any voltage OR voltage changes that O2 sensor IS the problem!

Thanks, I'll try measuring the voltage at the O2 sensor plug. My understanding is to measure voltage at the signal wire ( green) and a good ground location. Is this right for the 0 to 1 volt measurement?

Yep that is correct. You want the connector still connected though so you need to put a little wire in between the plug that hangs out of the plug so you can connect the DVM to it.

Got it, cool , that was my guess. But, then my guess was that my duty cycle would show up as 70% with the ignition on and the engine off and that didnt work out so well. Thanks for the info, i'll try it tonight to see what the voltage at the O2 sensor is.