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I followed this procedure to troubleshoot a rough idle and poor fuel economy on my 1986 300E 3.0L. Test 1,2 & 3 were fine, but Test 4 initially starts with the correct reading, but as the engine warms up the voltage drops to below 1 volt.
I disconnected the green wire to the O2 sensor and the idle immediately improved, and the voltage returned to the correct range. While the engine was still running, I connected the green wire to the O2 sensor and the voltage slowly dropped to below 1 volt again.
" Test 1 Average voltage method. Turn on the key, but don't start the engine. Measure the DC voltage between pins 2 and 3. This should be 30% of the injection system voltage as measured usually at pin 6 of the diagnostic socket – it's a little less than battery voltage under the conditions of the test, ie whether the engine is running or not) and correspond to Mercedes' specification of a 70% duty cycle or “on/off ratio”. In fact Mercedes uses the total proportion of its off period as its indicator number, but the multimeter respond to the proportion of time the pulses are on, at injection system voltage. Hence the 30% mention above – 100% minus 70% equals 30%. This initial reading doesn't tell you much, only that the computer is capable of putting out diagnostic codes – but that's something! If you don't receive any readings, maybe your computer is past its prime.
Test 2 Same test conditions Deflect the air flow sensor plate. The duty cycle should decrease to 10%, that is, the multimeter should read about 11 volts. If no change occurs, that is the reading still is about 3 volts, test the throttle valve switch (Job 07.3-121 in Mercedes shop manuals). Please note that the 10% is nominal, 9% to 11% is seemingly OK.
Test 3 Same test conditions Deflect the throttle fully, the duty cycle should read 20% or about 10 volts, indicating that the Full Load Contact switch is OK, unless it decreases to only 40% or about 7 volts. In this case test the air flow sensor potentiometer (Job 07.3-121 in the Mercedes shop manual). Again 20% or close to it is OK.
Test 4 Engine operating at 80 degrees C. Check the Lambda Ratio. The Lambda Ratio is the ideal ratio of air/fuel that enables complete combustion of the fuel. It is 14.7/1. The controlling element is the Oxygen sensor in the exhaust gas stream. This sensor operates only at high temperature, so is provided with a heater supplied by the battery, via the fuel pump relay. In operation, it determines the proportion of oxygen present in the exhaust gas and signals the computer to provide more or less fuel to maintain an average value corresponding to the ideal Lambda ratio. Measure the voltage between pins 2 and 3. It should fluctuate, showing that the oxygen sensor is causing the computer to adjust the mixture on a continuous basis. The duty cycle readings should be between 45% and 55 % . Higher than this indicates a lean mixture and lower indicates rich. The voltage levels corresponding to these values are, depending on the battery voltage while running are 7.4 to 6.1. (These values assume a system voltage of 13.5.)
If there is no fluctuation at all, just a steady reading, something is wrong with the oxygen sensor or its supply. It is located not far along the exhaust pipe and replacing it involves a big spanner and a lot of effort. Its wiring is located under the floor mats on the side of the vehicle above the exhaust system."
190E 2.6L 1989, 190E 2.6L 1988, BMW 535dXdrive 2014, BMW 428i 2015
It could be the O2 sensor or something else. Let us know what the O2 sensor test reveals. You did not say exactly what happens to the duty cycle exactly when you unplugged it.
I suspect it should have just given a steady duty cycle indicating O2 sensor error.
It could be the O2 sensor or something else. Let us know what the O2 sensor test reveals. You did not say exactly what happens to the duty cycle exactly when you unplugged it.
I suspect it should have just given a steady duty cycle indicating O2 sensor error.
When the green O2 sensor wire is unplugged, the idle raises to about 900 rpm, and the duty cycle voltage varies between 5.9 to 8.3v. When the O2 sensor is plugged back in, the idle drops and the duty cycle voltage drops to 0.9 to 1.2v within 30 seconds of being plugged in.
When the O2 sensor is plugged in, I will see the O2 warning light when the ignition is turned on but not started. As soon as the engine is started the light goes out. If I unplug the O2 sensor while the engine is running, the O2 warning light does not come initially, but lights up after 5 minutes of driving.
