Yet another Duty Cycle question
 

1989 300SEL. Please tell me if this is normal. Checked duty cycle on X-11 with key on engine off. Reading was steady 14.9%. Obviously a California equipped car. Engine running duty cycle hovered around 49%. After reaching normal operating temperature, duty cycle reading started to steadily drop to around 35% with small up and down movement around 35%. The car idles around 550 RPM which is a little on the slow side. I believe specs call for 650 RPM idle speed. Should I adjust the duty cycle so it reads around 50% at operating temperature?

When the engine was revved up, the duty cycle kept going up with the RPM reaching about 85% at about 2000 RPM. Is this normal?

Also, the idle adjustment screw is still covered with the original aluminum tower with the steel ball sealing it. The Mercedes shop manual states something to the effect that on vehicles with electronic idle adjustment there is no need to mess with the screw. However, they describe a procedure to break away the aluminum tower and do some drilling, extraction, etc. that sounds a bit complicated. I also checked EHA current as described in one of Steve Brotherton’s posts on the subject. KOEO current was about 19.9x ma and slowly dropped to around 0.0x ma after starting the engine. I did some search on this subject and found a wealth of information that helped me carry out the two tests. However, I couldn’t find anything that specifically answers the two questions above.

Thanks

Check the voltage of the O2 sensor. Should be between .1 and 1.0 volt when the engine is hot. If it hasn't been replaced lately, it makes sense to invest in a new one and eliminate that as a possible cause.

It's not clear to me that you're measuring the duty cycle correctly, and you may have polarity reversed. Is your engine really a CA model? Look at the emission/tuneup label in the engine compartment. It will say whether it's 49-state or 49-state AND CA. If it's a CA model special test procedures apply.

Make sure you review and understand:

www.landiss.com/mixture.htm

and pay particular attention to the special procedure for CA models if your car really is a CA model.

Duke

I was getting weird readings until I discovered the connector on the ECU wasn't fastened down all the way. I'm guessing it was replaced at one time. In my case it needed quite a bit of pressure to click in place. Its a long shot but worth checking if you're looking around.

Retest
 

I didn’t suspect the car is California equipped until last week when I did the duty cycle test. I checked with the dealer and they confirmed the car is California equipped. The yellow sticker in the engine compartment also states that the car meets California emission requirements.

I did the test again today with multimeter +ve (red) lead in socket no. 2 and -ve (black) lead in socket 3 of X-11 connector. I realize this configuration will reverse the Mercedes spec readings but I did that to conform with the Landis numbers. Something strange happened which made me start the test over again.

Test 1
KOEO
Pushed connector X92 button once -- red LED flashed once.
Pushed button again -- red LED was on continuously
Duty Cycle (DC) reading = 70.1% steady :confused:
Deflected the air sensor -- DC reading = 10.1% steady
Air sensor plate back to original position -- DC reading = 70.1% steady :confused:

The 70% readings didn’t make sense so I turned the key off then on again

Test 2
KOEO
X92 Red light on continuously
DC reading = 85.1% steady
Deflect air sensor -- Duty Cycle reading = 85.1% steady - no change at all :confused:
Start engine
DC goes to 0.00% slowly (noticed X92 red light was off)
pushed X92 button twice again to get continuous red light on
DC reading bouncing around 80%
Engine revving at 2,500 RPM -- DC reading bouncing around 69%
Back to idle -- DC reading bouncing around 78%
KOEO -- DC reading = 85.1% steady

I don’t know what caused the 70.1% reading in Test 1 above. However, it seems that in the second test, DC reading at 2,500 RPM (69%) is within +/- 10% of DC reading at idle (78%) but just barely, hence my original questions: Are these readings normal and should I go through the trouble of breaking away the original aluminum idle adjustment tower to get a better DC reading?

Thank you

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.