Steve,

I have a question regarding your DIY article "Evaluating Electronic Engine Controls". The KE-Jetronic duty cycle measurements taken at X11 pin 3 are shown together with scope displays of the waveforms. Figure 2 shows a square wave consistent with the 50% duty cycle reading. Figure 1 however, shows a waveform which is at a "high" level for a greater percentage of time than it is at a "low" level. I would normally interpret this as having a duty cycle greater than 50%. The duty cycle displayed is 30% however (rather than 70%). Figure 3 similarly shows 38.9% duty cycle which I would have expected to be more like 61.1%. Is this just a characteristic of your meter or is there a convention with measuring KE duty cycle where the inverted signal is used? Maybe I am missing something here.

I can use my multimeter to measure duty cycle on either my '90 190E-2.3 or my '90 300TE. With the meter negative connected to chassis (or X11 pin 2) and positive connected to X11 pin 3, I get "key on, engine off" readings of 30% for both cars. Obviously if I reverse my meter connections (ie. negative to pin 3, positive to chassis or pin 2) I get a reading of 70%. You suggest 70% is typical for US delivered cars and whilst mine are Australian delivered it appears the same might be the case (using your meter's convention). With the engines running I initially get 50% readings (prior to the O2 sensor reaching working temperature) followed by fluctuating readings slightly below 50% at idle rising to slightly above 50% at say 2000rpm (once system goes closed loop with the O2 sensor at working temperature). Using my meter, I read these figures as indicating the base air/fuel ratio being slightly lean at idle, going slightly rich at higher speeds (perhaps indicating slight vacuum leaks leaning the idle). Both cars incidently, are running fine. My reason for measuring them was purely out of curiosity and to check that they were operating somewhere around the desired 50% duty cycle. I have not yet verified these duty cycle measurements with EHA current measurements, as I have yet to make up a harness to do so.

I have checked my multimeter on the duty cycle range and and the readings are lower (<50%) with waveforms that have short "high" versus "low" states, and the readings are higher (>50%) with waveforms that have long "high" versus "low" states, as I would expect. These readings would be reversed following your meter's conventions.

Sorry to question you on this and I hope I have not "opened a can of worms". I appreciate the wealth of knowledge you bring to this forum and the time you devote to it. Many would not devote their valuable leisure time to doing what they spend much of their day doing at work.

Thanks,
Greg

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