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Old 02-10-2006, 06:43 AM
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Location: Los Angeles, CA
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Failed smog, check mixture setting on 83 SL

I've got an 83 380SL. It failed smog on low and high idle for CO and HC. Plugs are less than 10k old but are coated with fluffy black carbon. Will change those. Back probed the O2 sensor with and without an induced air leak (by disconnecting the vacuum booster). It was not varying or hunting much from about 300mV so changed that. Now the sensor bounces arround between 200~750mV and stays down arround 220 with the above mentioned air leak. I assume the engine goes to closed loop after warm up.

Ignition was 3 degrees off of engine specified 0 (TDC) so I set the timing right. No big difference here.

The cat is about the same temp (+/- 10%) from end to end after a long warm up at idle. Doesn't seem plugged (car has plenty of power). I guess I could take a shot at smog again but it may be a good time to check how rich the car is running and also if the cat really needs replacing.

Engine has no vacuum leaks and the engine doesn't missfire but at idle the engine occasionaly revs up a bit. I took off the idle control valve and cleaned it with carb cleaner (not the electrical part of course). Looking under the sensor plate with a flex light I could see the throttle and it needed cleaning. With a 1/2 cans worth of carb cleaner and a clean rag I did my best to remove the carbon from the throttle plate and throat--delicately using a screw driver and very long nose pliers to pull and push the rag). Now the idle is stable.

I would like to at least check that the richness control is working correctly. Could someone tell me how to hook up my meter and what to expect? I don't want to mess with the mixture control screw if it's not needed. My multi meter measures duty cycle, freq and dwell. I don't have an analog meter. I searched this forum for the correct meter hookup after moving on from the MB shopCD. MB uses their tool and targets 40-60% on/off ratio--not to deviate by more that +/-10% at 2500RPM.
CW on the mixture screw is richer and
CCW is leaner.

I probbed the car's diagnostic (terminal 3) and got either nothing, 10 or 95 for the duty cycle with one probe on terminal 3 and the other one put to battery positive, ground or left unconnected. Also did the same three connections with the probes reversed. For whatever it's worth I measured 12V stable at terminal 3 with the meter set to DCV and car warmed up and idling.

Any help would be much appreciated.

1972 450SL
1982 300D Turbo

Last edited by erubin; 02-10-2006 at 07:01 AM.
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Old 02-10-2006, 12:52 PM
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Old 02-10-2006, 03:02 PM
MrCjames's Avatar
California Dreaming
Join Date: Jun 2003
Location: SF Bay Area
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The X11 diagnostic connector

On my 1991 560SEC the "X11 diagnostic connector" is mounted on the inside of the left fender, behind the ABS controller. It has a round screw-cap. On some cars (California?) there is also an "X11/4" between the two firewalls - don't get them confused.

The signal provided at pin 3 of X11 is called the "lambda on/off ratio" signal. It is convenient to use pin 2 of the same connector as a ground reference.

This lambda signal is not a replica of the voltage from the oxygen sensor. Instead, it is a constant 100 Hz pulse whose duty cycle indicates the hunting of the fuel injection for an ideal mixture by switching back and forth between very slightly rich and very slightly lean.

Mercedes has chosen the less common definition of "duty cycle" in this case. They are referring to the percentage of the entire pulse period during which the voltage is zero, not the time when it is near battery voltage (mine was +13.6 V when the battery was +14.0 V). In other words, if the pulse rests at ground for 7 milliseconds and then rises to +13.6 V for 3 ms, the duty cycle is considered to be 70% (see oscilloscope trace at right).

If you have an oscilloscope to measure this timing, fine. If not, it can still be estimated with a voltmeter. Since continuous "0 volts" would be considered 100% and continuous 13.6 V would count as 0%, just measure the voltage between pins 2 and 3 of X11 and divide that by 13.6. Next subtract that ratio from one, and convert the result to percent.

For example, if the meter reads 4.0 V, first divide 4.0/13.6 = 0.294. Subtract 1.0 - 0.294 = 0.706, or 71%. In equation form:

Duty Cycle = [1 - (V{pin 3}/V{max})] x 100%

Remember that if the oxygen sensor is doing its job and the system is operating closed-loop, the reading will jump around, so you might see readings from 5 V to 7 V and have guess at an average.
Follow these instructions to set your CO mixture and with that new O2 sensor your follow up test will have some surprising results
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Old 02-10-2006, 05:08 PM
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Location: Los Angeles, CA
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Thank you for your responses. The systems you refer me to are for newer cars. Mine is an 83 380SL so I it may be different particularly with KOEO.

Digital meter hooked up as follows: positive lead to #3 term and neg lead to ground (available at term 4). edit:<>

Here is what i got (different than last night):
KOEO: 0Vdc on terminal 3 and ground; 0% duty cycle
Cold idle: batt voltage btwn term 3 and ground; 26% duty cycle
2 min warm up (below 80C): 13% duty cycle
Warmed up and idling at 1200RPM: 26% duty cycle

In my car it seems that with KOEO terminal 3 gives useless info (is this a problem?). BTW, i am using a digital meter.

After the work i did yesterday cleaning the ICV and throttle plate the engine idles too high at 1200RPM and doesn't go back down. That's weird because yesterday after cleaning everything I got the idle perfect, something happened overnight.

With the car running at this high idle and in closed loop I back probed the O2 sensor and got values between 300 and 600mV. I measured the cat temp:
inlet 400C
outlet 400C

Smog test results (done before replacing the defective O2 sensor) showed 3.89% O2 at 15mph and 4.23% at 25mph.
1972 450SL
1982 300D Turbo

Last edited by erubin; 02-12-2006 at 04:37 AM.
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Old 02-10-2006, 05:14 PM
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Join Date: Dec 2003
Location: Metro Detroit, Michigan
Posts: 17,386

Emission testing and troubleshooting Emission test failures
Emission testing and troubleshooting Emission test failures
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Old 02-10-2006, 09:38 PM
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Location: Los Angeles, CA
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By disconnecting and connecting the ICV a few times i got the engine to idle at about 750~800. The duty cycle was at 22%. I went ahead and started backing out the mixture screw (3mm allen) and the duty cycle climbed up to 36% (leaner). If i continue CCW on the screw to make it even leaner it doesn't budge from 36% but if i go far enough I stall the engine. When I raise the RPM to 2500 the duty cycle doesn't change more than 0.2%. Any suggestions please?

Update: I took the car out for a hard drive. Immediately after returning I measured the duty cycle as 31% at idle and 37% at 25000 RPM (no load). Based on what I have read this is not acceptable because the difference is greater than 10%. Should I turn the screw richer to get it within 10%. I don't think it will let me go any leaner. What is more important the absolute duty cycle at idle or respecting the +/-10% between idle and 2500?
1972 450SL
1982 300D Turbo

Last edited by erubin; 02-10-2006 at 11:33 PM.
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Old 05-15-2007, 09:09 PM
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Join Date: Jul 2003
Posts: 555
what was your final repair to lower the idle?

Thanks, Juan
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