Troubleshooting Oxygen Sensor (Lambda) circuit on a 280SL -85…
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Have done a little troubleshooting guide, feel free to edit the text since my grammar could have been better...
Troubleshooting Oxygen Sensor ( Lambda) circuit on a 280SL -85…
Problem:
The spark plugs are totally dry, black, sooty, poor mileage and when the car idles the ground behind the exhaust pipes get fouled and black.
This is probably caused by too rich fuel mixture. Since this is a common WUR-problem I first started troubleshooting the WUR then the Lambda circuit.
This is how I troubleshoot it, text fetched from original thread with some comments.
http://www.benzworld.org/forums/r-c107-sl-slc-class/1545859-finally-made-my-decision.html
Correct system fuel pressure due to manual
System Pressure: 5.2 bar
Control pressure from WUR at outside temperature (temperature in degrees Celsius).
Cold motor
10 degrees -> 0.7 - 1.1bar
20 degrees -> 1.0 – 1.4bar
30 degrees -> 1.4 – 1.7 bar
40 degrees -> 1.7 – 2.1bar
Warm motor
3.5 – 4.0bar
Troubleshooting the WUR (Warm Up Regulator).
I have checked the air filter and it's ok (an open one)...
The timing when idling was 5 degrees but I will check it at the right rpm (due to the label in car) this weekend when I have some help.
Checked the timing... 25 degrees / 3500rpm, was ok due to label in car...
I've cleaned all contacts and checked the vacuum.
I have also disconnected the start injector's termo switch so I can be sure that this isn't open all the time.
Connected a 12V lamp-bulb on the start injector's plug and it lit for 2-3 seconds which is ok.
I will do a quick and dirty fix... I will squeeze the Frequency valves rubber fuel line so that no fuel can go that way, this way should be able to exclude any impact from the lambda system, then start checking the fuel pressure to see if it can be the WUR.
Manage to borrow a fuel pressure gauge and measured the control pressure from the WUR...
Cold motor: 1 bar
"Half warm" motor: Pressure started to rise and stopped at 2 bar
Very Hot motor: still 2 bar???
Comment: Something is obviously wrong with the WUR…
Measured the electrical connection for the WUR’s warm up element and there were no 12V.
Removed the rubber-hose which protect the contacts and there was the problem, the +12V cord was broke, fixed the cable and measured the pressure again and now it went up to 4 bar which is correct due to the book I borrowed from the local Bosch firm.
It's a little strange that the motor didn't manage to warm the WUR enough to raise the pressure since some say "disconnect cable when testing WUR", seems like my WUR need some help from the warm-up element to work properly. It could still be something wrong with it but at least I know what’s wrong now...
Have measured the system pressure and it was 5.5bar, exactly as it should be.
Comment: Took the car for I ride and what a difference, now I see how quick and fast the car really is. Checked the spark plugs and they look as they should.
Troubleshooting the lambda circuit (when WUR was ok).
Comment: I did know that the FV should work as soon as the motor was running and usually buzz while it opened and closes but my FV didn’t make a sound so I started to troubleshooting it…
Glossary:
Oxygen Sensor (also called Lambda Sensor).
Frequency Valve (FV): A valve which is controlled by the Oxygen sensor.
Duty cycle: Usually measured in % and is the amount of time that the Frequency Valve is switched on. On time/(on time + off time)
This is how it works…
The Oxygen Sensor (also known as Lambda Sensor) works like a battery and is located in the exhaust manifold and measures the amount of unburned oxygen in the engine's exhaust. Based on the amount of oxygen, it sends a signal to the ECU which then adjusts the air/fuel mixture for optimum engine performance and emission control.
To rich fuel mixture the output from the sensor: 0.9V.
Perfect fuel mixture the output from the sensor: 0.5V (Lambda value=1… which is the perfect mixture value… 1 part fuel / 14,7part Air… also known as stoichiometric mixture).
To lean fuel mixture the output from the sensor: 0.2V.
The signal controls the FV (via the Oxygen sensors ECU) which works in the same way as the WUR.
To lean mixture -> 0.2V -> open the FV (90% duty cycle) -> lower the pressure in the Fuel Distributor’s lower chamber which increases the amount of fuel to the injectors until the oxygen sensor detects richer mixture.
To rich mixture -> 0.9V -> close the FV ( 0% duty cycle) -> rises the pressure in the Fuel Distributor’s lower chamber which decreases the amount of fuel to the injectors until the oxygen sensor detects leaner mixture.
This is the way the oxygen sensor works in (closed loop), by detecting the amount of oxygen in the exhaust and fine tuning the fuel pressure by open/close the FV until the perfect state occurs (Lambda =1).
I've bought a "meter" to measure the duty cycle on the frequency valve to see if there is anything wrong with the oxygen sensor (there is a diagnostic plug, use no3).
Ignition on...
Measured between the 2 white cables which should be the Oxygen sensors heating and it says 12V so this seems ok.
Measured the Black cable which should be the lambda signal and it shows 0.9V (cold motor). Took it for a drive and measured the signal when the motor was warm, it still says 0.9V. Checked the voltage at different rpm and I can see the voltage changing a little but still around 0.9V.
I then disconnected the Lambda and then the Frequency valve started to tick which it doesn't do when the lambda sensor is connected, should indicate that the valve is ok. Took the car for a drive with the lambda disconnected which didn't go well, black smoke and very poor acceleration, had to stop and connect it again (very very rich mixture but this was before the WUR was fixed).
So what’s the conclusion here!
Frequency valve is OK since it ticks when lambda sensor is disconnected.
Is the mixture so high because of other reasons and therefore the lambda sensor indicates 0.9V and closes the Frequency valve (no ticking), sure looks like it?
Since I didn't get a proper reading for the duty cycle, I built a voltage regulator handling 0 - 1V, which I connected instead of the oxygen sensor, changed the signal between 0.2 - 0.9V and the frequency valve started to tick as it should, following the signal... So the oxygen sensor's ECU was ok...
Measured on number 3 in the diagnostic plug and could follow the duty cycle while I simulated the oxygen sensor.
Noticed that oxygen sensor signal was almost constantly 0.9V when the car was idling ...
Adjusted the fuel mixture screw, almost a full turn counter clockwise to leaner the mixture and then the valve began ticking, the problem was apparently that the mixture was so rich that oxygen sensors circuit was out of range and the lambda system was working overtime to lower it..
Comment: Was difficult to know how much I should dare press down the adjustment screw but I could feel when it gripped the adjustment inside the fuel distributer since it was “sluggish” when I turned it counter clockwise.
Now I can clearly see both the duty cycle changing between 10% to 90 % and hear the FV working, can hear the idle speed following the duty cycle a little but I will adjust the mixture better with a CO meter, hopefully I can fine tune a little bit more then.
