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Friday, July 27, 2012

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

This video is about basic diagnosis of a KE jetronic with lambda control, precisely, faulty control unit recongizition. In this example we 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 is an assurance of KE control unit, whether support fault recognition or not. If duty cycle equals to 70% it means that faulty recondition is supported. The multimetr shows 50% during first minute. It shows that the engine works in open loop mode. (without lambda control) When lamba signal is obtained, engine is controlled in closed loop mode (with lambda control) and duty cycle should then fluctuate around 50%. In this partucular example the duty cycle periodically goes down (together with RPM), 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). A video with properly working control unit can be found here : www.youtube.com