My 89 300SEL's CEL has been on for a while now, I checked codes, and it gave me a code 4, which has to do with the air sensor pot... So I checked the duty cycle, and it is showing a constant 85%...what does this mean?

Must be 100's of posts discussing "duty cycle". Do a search on the topic and I'm sure your answer will be found.

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Question Authority before it Questions you.

* Need a little more information. Is your '89 300SEL (W126.025) a California or Federal car? Look at the plate/decal under the hood next to the radiator. I think black is Federal and green is California.
* You read the code 4 from the rectangular 8-pin data plug (X11/4) over by the battery. Which pin were you using (i.e., which system is the fault code in)? Fortunately, on an '89 you don't have many systems to choose from.
* Before you try to check duty cycle (sweep) at the round diagnostic plug on the left fender well (X11), check for a malfunction with the on-off ratio. Using pins 2 and 3 turn the ignition on, leave the (warmed up) engine off, and note the reading. Compare to an on-off ratio malfunction table.
* After you've recorded all the codes, it's a good idea to reset all the codes (disconnect the battery briefly), and use the car through several drive cycles. Then retest for fault codes. Some of the old codes may not reappear.
* These tests assume a few conditions. Engine at operating temperature, over-voltage protection relay and fuse in good shape, and climate control off.
* This might help you a little. I would recommend doing a search for basic theory of lambda, duty cycle, on-off ratio, Mercedes on-board diagnostics, both analog and digital. As long a task as it sounds, it might actually be quicker than picking through dozens of posts for information that's useful.

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ASE Master Tech
Quality Enterprises
Mercedes Svc. & Restoration
Sheridan, IN
317.769.3536

Well, I did search, but it seemed like I couldn't find what I was looking for...

I think I may have answered my own question? The duty cycle will show 85 until the car is at operating temp...then the system goes into closed loop and begins using the o2 sensor for correction? I suppose it's possible that the car wasn't FULLY up to operating temp when I performed the test...

And as for the diag codes...I pressed the button on the diag box by the battery, held for 2-4 seconds and watched the blinking LED, it blinked 4 times, held it again for 2-4 secs, and it blinked once. Now the LED light is on at all times...does that mean I'm in diagnostic mode, and the duty cycle can now be read?

Also, how exactly do I test on/off ratio? Can I use my DMM for that? Or do I need to build the special tool with an LED that I have read about?

Ok, I disconnected the battery...the CEL went out. Took it for a drive, enough to get it up to operating temp, and tested the Duty Cycle again, and this time it is showing normal percentages...jumps around between 43%-53% at idle and 2500 rpm. Also, the check engine light hasn't come back...

Not sure what to think at this point...I guess I should just keep driving and see if the light comes back on?

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles

John, Well that's good news.
Sorry for late reply, painting the kids' house today.
Yeah, I think the Calif version will read 85% until it goes closed loop (the 49-state reads 50% unitl it warms up). If you have the LED on the
Anyway, if it is oscillating around 50% +-10% fully warmed up, then you have a lot of things working right. This indicates the O2, EHA, fuel pressure, air valve are working pretty good, cause it's checking and controlling fuel mix just as it should.

I know we discussed head gasket, and we ruled that out, due to the symptoms. And the way the problem came on suddenly, then healed with new plugs, for awhile, tends to point to something non-mechanical that failed.

My thoughts for the day:

Drive it around and see how it does.
Get a piece of speaker wire and splice the meter to it, so you can drive around with the duty cycle meter on the dash. Might see something under load; particularly as it gets nice and hot from driving. Maybe a sensor is going out under temp. If the duty cycle holds good though, that should mean fuel delivery is not causing it to run so bad, correct?

Now, I'm really thinking the code 4 may be an artifact caused as a result of something else going bad - since the duty cycle is right on, seems the air meter must be doing OK.

If it's not fuel, it's likely ignition, if it comes and goes, might be coil or ignition component? Maybe that's why the new plugs helped for awhile.
Might try to see if heat affects it, and if when it runs bad, what happens to the duty cycle.

DG

Thanks^^ I already have been carrying the DMM around in the glovebox with me! LOL I'll be keeping an eye on it...

The CEL hasn't come back on, either...and it is idling and starting better than it has in a long while...really confusing.

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles

Robert was telling you the story. Based on the 85% you probably have a Ca car. The fact you claim to have energized the diagnostic with a button confirms that. Ca cars must be put into the diagnostic mode before the duty cycle is registered. You won't get usable readings till the car is warm but you will get readings as soon as it is running and placed into the diagnostic mode.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Is there a website somewhere that talks about duty cycles, what they should be, etc.? I've got a 560SL that has the diagnostic port to check duty cycle, but I don't know much about it. Is it well covered in the CD?

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Len
'59 220S Cabriolet-SOLD and living happily in Malta
'83 240D 351,500 miles original owner-SOLD
'88 560SL 41,000 miles - totaled and parted out
https://sites.google.com/site/mercedesstuff/home
'99 E300 turbo 227,500 miles
'03 SLK320 40,000 miles - gave to my daughter
'14 Smart electric coupe 28,500 miles
'14 Smart electric cabriolet 28,500 miles
'15 Smart electric coupe 28,000 miles

http://www.landiss.com/mixture.htm

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Excellent - thanks Chuck

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Len
'59 220S Cabriolet-SOLD and living happily in Malta
'83 240D 351,500 miles original owner-SOLD
'88 560SL 41,000 miles - totaled and parted out
https://sites.google.com/site/mercedesstuff/home
'99 E300 turbo 227,500 miles
'03 SLK320 40,000 miles - gave to my daughter
'14 Smart electric coupe 28,500 miles
'14 Smart electric cabriolet 28,500 miles
'15 Smart electric coupe 28,000 miles