Duty Cycle - trap for young players - Mercedes-Benz Forum
 
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post #1 of 10 (permalink) Old 06-24-2013, 09:54 PM Thread Starter
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Duty Cycle - trap for young players

Just wanted to give a heads up to hobby mechanics (like me) who screw up when setting their duty cycle. I have a 1987 190e. (pre error code)

My story began when my car started running a bit rough and developed a slight miss on idle. I read these forums in detail and addresed all the basics like replacing all old and cracked hoses and traced down a number of air leaks in the vacuum tubes. What happened next was the car actually started to run rougher and made the problem a whole lot worse. I replaced all other consumables like filters, plugs etc and stripped the entire injection system down into pieces and completely cleaned it out like new as it was a bit gummed up thinking this may have been the problem. All to no avail as the problem remained unchanged.

Then I thought I would go down the diagnostic route and investigate the voltages for each component. I purchased a meter with duty cycle and started testing all the voltages for all the components as per the manual. They all checked out fine.

Then I checked the duty cycle. It was way off. Great, I am onto something here. I followed the procedure and adjusted it back to 50%. The miss at idle was still there and after a while it would stumble and die completely, especially as it moved into 80deg range it would barely run at all.

After pulling the plugs I noticed it was running very rich and they were very gummed up. After cleaning the plugs, the car would run fine initially but shortly after they would foul up again and the car would once more stumble and die. I checked and rechecked the duty cycle and it was set correct, even in relation to the duty cycle at 2500rpm it was +10% at idle. It all seemed ok according to the book but I cold not figure out why it ran so rich.

Eventually I went back to basics and started from scratch, then I noticed that when the engine was off and ignition on, the static reading was 30% and not 70%as per the manual.

I reversed the cables from the meter into Pin 2 and Pin 3 which then gave me the correct 70% static reading. With cables reversed the duty cycle was of course way off. I then reset it back to 50%, recleaned the plugs and the car now runs better then it ever has.

So, the moral of this story is, don't always assume that the black cable from your meter goes to Pin 2 (earth) and the red cable from your meter goes to Pin 3 (duty cycle). Always check the static reading is at 70% before starting the motor and adjusting the duty cycle and avaoid the drama I had.

I can only assume that I was setting it way too rich causing the fouling and poor running condition. Perhaps in an earlier life, another mechanic had richened the duty cycle to compensate for air leaks in the hoses and vacuum tubing. Once I had fixed the air leaks it was then running too rich, with me adjusting it richer I was making it worse and heading down a road of pain.

I hope this information is of help to others in the future.
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post #2 of 10 (permalink) Old 06-25-2013, 04:04 AM
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which manual were you using?


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i need my star cleaned too
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post #3 of 10 (permalink) Old 06-25-2013, 04:17 AM
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There seems to be some understanding that adjusting the air/fuel mixture on the "tree" will correct duty cycle calibration...and it does to some degree.

The actual correct procedure to adjust the air/fuel mixture is actually done via the EHA. You connect an ammeter inline and adjust to reach an average of 0 ma at idle. This is why the setting is done and sealed at the factory as it should not change for the life of the EHA/Fuel Distributor.
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post #4 of 10 (permalink) Old 07-01-2013, 02:26 PM Thread Starter
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I used the Star Manual and stepped through the tests listed in the section 07.3-121 "Testing Electrical Components of the CIS-E Injection System".

As I stepped through each test, they all checked out correct, I could find no faults. Which is why I went down the path of chasing some type of mech or elect fault (ie plugs, wire, distrib, accumulator etc). But these all checked out fine.

With regard to the EHA, I never touched it and to my knowledge it is still the factory setting. It beats me how people actually adjust it since you need to remove it to get ad the plastic plug and adjust the small hex bolt.

In relation to my meter, I read somewhere (that I can't find now) that the Red cable goes to Pin 3 and the Black goes to Pin 2. Seemed logical as black is always earth/negative in my book and red is live. But I got 30% with ign on and engine off and not 70% as per the manual. I got 70% when they were reversed and started from here to adjust the tower and bringing the average DC back 50%. Maybe it is my meter that has a problem?

The car continues to run fine. Before, the plugs would foul up after only a few hours of running and then the car would run so rough and stumble & stall all the time.
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post #5 of 10 (permalink) Old 07-01-2013, 06:26 PM
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just to humor me, how did you go about cleaning your plugs? did you just use some brake clean? scrub it off with a brass brush?


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i need my star cleaned too
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post #6 of 10 (permalink) Old 07-01-2013, 09:46 PM Thread Starter
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I have a brass wire wheel on my bench grinder. I lightly brush them to remove the carbon then blast them with a bit of carby cleaner and then recheck the gap, they never move. They are brand new plugs. Having said that, the last check they came out a nice light brown color and not black & sooty. I will recheck them again which will record about 4 weeks of city driving to see how it is running.
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post #7 of 10 (permalink) Old 07-02-2013, 03:40 AM
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Sorry but I reread my post and I did not explain that properly. You don't adjust the eha. You connect an ammetter inline with the eha and adjust the tower based on that reading.
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post #8 of 10 (permalink) Old 07-02-2013, 02:22 PM Thread Starter
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I did check the current at the pins on the EHA and from memory it matched the value in the manual but I will recheck.
In order to determine if the DC was correct in relation to the EHA, the test I applied was to adjust the tower to get an average of 50% and then check the average at 2500rpm which returned about 40% to 41% which I figured was close enough to the -10% value according to the manual.
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post #9 of 10 (permalink) Old 07-02-2013, 05:12 PM
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If I remember correctly you connect an ammetter inline with the eha and you read the average oscillation and adjust the tower to get as close to midpoint as possible. I believe the average is zero or as close as possible.

The thing to remember is duty cycle is the reading of the output of the entire system. The tower adjustment is air fuel ratio (input). If you adjust the tower based on duty cycle then it is entirely possible to compensate for a failing component and a perfect duty cycle will still produce a poor idle or other ignition system issues.
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post #10 of 10 (permalink) Old 07-03-2013, 01:45 AM Thread Starter
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That is interesting. Let me try the adjustment this weekend and report the difference in DC %age when tuning the EHA back to zero. It is running quite nicely at the moment so I suspect it should be pretty close to zero as it is. I will report back next week.
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