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#1
10-21-2006, 09:47 PM
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Duty cycle anyone?
Just replaced my oxigen sensor to correct a "rythmic idle". Car is an 88 560SL, I used an universal Bosch sensor which I spliced into the old connectors. Idle is not rythmic anymore with fluctuating duty cycle readings as expected. There are hundreds of postings on this subject, 500 threads - about 17 pages worth. The consensus is that reading the duty cycle with the negative lead on terminal 2 and the + on three, the mixture should be set for a duty cycle oscillating around 40%. When I did this this I noticed that the readings would never go over 49.9%. I could get a setting with readings ocasionally going over 50%, but then the economy gage woud read about 25% at idle when normally it would be pegged to the left. This would also happen whe driving and coming off the throttle. I am back to my original setting, idle is smooth and slightly high, around 700 rpm. Duty cycle still does not go over 50%. Additionally I checked the duty cycle at idle after driving on the highway and I found a steady value of 0.8%. This reading does not make sense so I checked the signal with a scope verified that indeed there was no cycling. I can get back a fluctuating reading around 40% by inserting the 3mm allen in the adjusting hole and pushing the adjusting screw down a couple of times. Car drives fine as far as I can tell, but I imagine there is something not right though I cannot imagine what. Any ideas?
Jl 
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#2
10-21-2006, 11:02 PM
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If you have a nice steady idle at 50%, leave it there, don't force it to run crappy at a lower value. 50% gives the computer plenty of room to adjust the mixture. Remember that you are dealing with 20 year-old electronics.
According to the CD, an 80% constant value is a bad altitude sensor or associated wiring.
__________________
Chuck Taylor Falls Church VA '66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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#3
10-22-2006, 12:16 AM
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You are right, it is really amazing how sturdy these cars are. I plan to leave things as they are. I have a nice steady idle around 40%. I do not know if not going over 49.9% (lean) will matter much. As I understand it, there is no control under acceleration. After re-reading the CD, it seems that you should not check the duty cycle after hard driving, this might explain the crazy 0.8% reading (which is the same as 0.0 because it is probably at the lower limit of the meter) . At this point I am doing all this for my own education, and so far I am enjoying it.
Thanks. Jl
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