Hello everyone,

I have a mercedes 190e 1988 2.0, and i have a idle problem which i suspect is from the O2 sensor. When the car is cold it idles about 1000rpm, but when it warms up the idle goes to 1500-1700rpm.


I've been struggling to read the duty cycle of my car to read error codes so i can fix the idle problem. So when i connect a multimeter positive to pin 3 and negative on pin 2, on the 8 pin diagnostic port on my left fender, i read less than 0.1V. Neither when the engine is on nor engine off/ignition on.

Tonight i figured out that when i connect pin 1 and 3 with a jumper wire and then measure i get 50% duty cycle, also keep in mind im not using a multimeter with duty cycle i measure votage and use simple formula i found on the forum:
1 - (voltage of pin 3/voltage of battery) x100

My questions are why do i need to connect pin 1 and 3 to get duty cycle, and why do i not get any error code when i disconnect the O2 sensor.

Also nothing changes when i dissconnect the o2 signal wire under the passenger seat, the car runs the same way with and without o2 sensor. When i measure the voltage of the signal wire i get 0.03V which is far from normal for a warmed up sensor.

I'm about to buy a new lambda sensor but i wanna make sure that the old one is faulty.

I would appreciate any ideas for how to test the sensor and how to get a propper duty cycle measure.

is this KE Jetronic? You may want to check and see if the control unit is getting power.

I would help if you stated the year. Also, in the USA the 2.0 liter four-cylinder was never available, so I assume you have a model for another market. Details would help.

I had a similar idle issue with my KE system a few year ago. It turned out to the throttle position switch that tells the system the throttle is at idle or WOT. On my 2.6 there's a three pin connecter above the inlet manifold that makes it easy to test the switch.

It's difficult to access for removal, so I just thoroughly doused it with electrical contact cleaner and it gained full function.

Duke

The year is 88’. What i've done so far is check the potentiometer, the TPS switch and today i’ll check the EHA valve today. Also i found that static duty cycle at 50% means bad O2 sensor so i got a new one and now the duty cycle is stuck at 93%. I got mixed information on what it means but in most places i’ve read it means no connection with the EHA valve or bad valve(very unlikely). If you have any other ideas I’m all ears

Thanks

Update, i feel a little stupid but the duty cycle is actually 10% because the formula is 1-(Voltage pin 3/Voltage battery). Thanks again

So i bought a proper digital multi meter, and i get 0% duty cycle, yet i can measure voltage: ignition on engine off 11.73V at pin 2 and 3; 12.50V at pin 2 and 5. If anyone can help me with what that means would be great. Thank you

Update, i went to a electrician and asked him to check and change if needed the two capacitors near the cooling plate of the ECU(i’ll add picture). So he checked them change one and now i get proper duty cycle at pin 3 and ground, don't use (pin 2 as ground mine doesn’t work).
All good now i can start to fix the ke jetronic system, i get 60% which i think means either failed potentiometer or car speed signal. Either way i count that as a victory for me as a person with no experience nor education in this field. All you need to fix this car is a DMM, internet and some common knowledge.

I do not know how to post the picture so this is a link where i uploaded it https://postimg.cc/w35PxKp6

Kudos to your persistence. Good work!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer