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#1
03-08-2009, 12:25 AM
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1986 560SL Duty Cycle Madness
Chasing down my high idle gremlins, I was exposed to the concept of the diagnostic utility of Duty Cycles (a.k.a. Lambda On/Off Ratio) by the following posts and website:
'Adjusted' lamda tower screw...car won't start. 'Adjusted' lamda tower screw...car won't start. 560SL diagnostic port/tool 560SL diagnostic port/tool 380 SL idle, oscillates between 500-750 rpms 380 SL idle, oscillates between 500-750 rpms ODB 1 code reader ODB 1 code reader Bosch KE3-Jetronic Mixture Adjustment http://www.landiss.com/mixture.htm Unfortunately, my current arsenal of automotive tools does not include an oscilloscope or a Duty Cycle capable multimeter (although I ordered one last night from Sears).  In the meantime, I was determined to try to decipher the method of getting the car to tell me what was wrong with it. 
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#2
03-08-2009, 12:27 AM
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X11
The diagnostic port identified as "X11" is located under the hood on the driver's side fender. It's a round 9-pin socket covered by a screw-on dust cap.
 Its intended purpose is to give MB mechanics quick access to vital engine performance information by means of purpose-built electronics such as the Lambda Control Tester (http://www.boschcarservice.pt/boaa-pt/Product.jsp?prod_id=155&ccat_id=74&language=en-GB&publication=3). The MercedesShopWiki has a great write-up on the usage of such tools at http://www.peachparts.com/Wikka/EngineControls. The pin assignments as explained by http://www.landiss.com/mixture.htm are briefly listed here as:
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#3
03-08-2009, 12:30 AM
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Poor Man's Testers
From what I had read, a Dwell Meter (used by mechanics in the bygone days of ignition distributor contact breakers) was usable for measuring Duty Cycles. An alternative way was to measure the Voltage between X11's pins 2 and 3. I opted to use both devices simultaneously to double my results and enable crosschecking of each method.
Here's a photo of my setup before starting the tests:  The Dwell Meter and tan Multimeter (which is set to measure DC Volts) are both plugged into X11's ports 2 (ground) and 3 (Lambda). The yellow Multimeter is also set to measure DC Volts and is connected X11's port 6 (battery) and a chassis ground. With the ignition switch at Off, only the yellow Multimeter has a result to show, that being the always-on battery voltage.
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#4
03-08-2009, 12:33 AM
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Can She Detect Faults or Not?
An important prerequisite to interacting with CIS-E system's diagnostics is to know whether the car has the fault detection feature. MB Service Manual 07.3-121 Testing electrical components of CIS-E injection system establishes a cutoff date of December 1985 for SL's without fault detection. After 1988, cars came equipped with an additional diagnostic port called "X92" and the thread at 560SL diagnostic port/tool delves into the different (and easier) method of extracting fault codes from it.
Although I found the suggestive date of "07/86" located on the data plate located on the driver's door, I did not know exactly when my car was built. Page 4 of the service manual stated that the system itself would signal what its capabilities are by outputting a specific Duty Cycle value. Having read many posts and articles about the various fault detection systems, I was confused as to when this 'feature' code was presented. I determined that this capability code is displayed as soon as the ignition is switched to Run. As such, my first measurements were 3.59V Lambda, 12.2V Battery and 28 degrees Dwell. For mathematical reasons described at http://www.allpar.com/fix/fixcarb.html and by Blue Bowtie at http://www.automotiveforums.com/vbulletin/t607279.html, it is necessary to use the 6-Cylinder scale on the Dwell Meter for detecting Duty Cycles. As explained by Landiss at http://www.landiss.com/mixture.htm, since MB chose to equate the frequency of low-end voltage occurrence with Duty Cycle (contrary to other makes that spec the high-end) the displayed Dwell reading must be subtracted from 100 to get the appropriate MB Duty Cycle. So my Dwell reading of 28 (on the 6-cylinger scale) equates to a 72% Duty Cycle. Taking a few points off and I matched the initial Duty Cycle code to the service manual table, which presents the following CIS-E feature codes:
1 - ( 3.59 / 12.2 ) * 100 = 70.5% Both the Dwell and Voltage results suggest that my car's CIS-E is capable of outputting fault codes.
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#5
03-08-2009, 12:34 AM
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Please Detect a Fault
To experience how faults are reported by my CIS-E, I tried two tests as outlined by Landiss. At this point, I decided that I would not use the Dwell Meter method to determine the Duty Cycle...it was just too confusing. I felt confident in the results obtained by monitoring the voltage on X11's port 3.
The first test was to press on the Air Flow Sensor plate. This resulted in 10.67V Lambda which converts to 12.5% or pretty close to the service manual table of error values where 10 means that the Air Flow Sensor potentiometer is faulty. To clear the result, I switched the ignition Off and then restored it to the Run position. The second test involved manually opening the throttle to its greatest extent (a.k.a. Wide Open Throttle or WOT). The Lambda voltage output was 9.44V. This converted to a Duty Cycle value of 21.9%. A CIS-E Fault code of 20 indicates that the throttle's Full Load Switch is faulty. The CIS-E fault detection system appears to be working correctly on my car, and she apparently has no reportable errors. To confirm today's results, I will be repeating these tests again when I receive my new Multimeter with its Duty Cycle function. Regardless of these favorable fault detection results, I will continue the quest to reduce my car's idle speed to the manual-specified target range between 600 and 750 rpm. Read my other posts on this forum to see how I progress toward this goal. Thanks in advance for any clarifying comments on these confusing concepts. 
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Linear Mode
