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Duty cycle measurement on Mercedes W124 diagnostic socket (faulty control unit)

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Uploaded on May 16, 2009

This video is about basic diagnosis of a KE jetronic system with lambda control, precisely, faulty control unit recognition. In this example I used W124 230 E KAT from 1988. There is a square wave signal on X11 diagnostic socket. It has 100Hz duty cycle signal between pin 2 (ground) and pin 3. The signal is alternating between battery voltage and zero voltage. The main point is to measure the duty cycle of this signal that is referred as a percentage of the entire pulse period during which the voltage is zero (not the time when it is near battery voltage) First measurement is done with ignition ON and engine OFF as an assurance of KE control unit, whether supports fault recognition or not. If the duty cycle equals to 70%, then it means that faulty recondition is supported. The multimetr shows 50% during first minute. It means that the engine works in open-loop mode (lambda is heating up) When lambda signal is obtained, the engine is controlled in closed-loop mode. As long as the engine is working in closed-loop mode the duty cycle should fluctuate around 50% (considering that the mixture is adjusted correctly)
In this particular example the duty cycle periodically goes down (along with RPM), then returns back on 50 % and goes down again.. The problem is caused by faulty KE jetronic control unit. Control unit does not control an idle valve switch (there is no signal to the idle valve switch at all). Video with properly working control unit can be found here : http://www.youtube.com/watch?v=LFbZ-q...

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