O2 sensor test and inverse duty cycle - PeachParts Mercedes-Benz Forum

LeaUK · #1 04-06-2005, 02:43 AM

I've read on Steve B site http://www.peachparts.com/Wikka/EngineControls that when measuring the O2 output with a scope (I hope to do this Saturday) the duty cycle is the ratio of low voltage to high voltage. But this is contradictory to the duty cycle definition in the electronics industry. Where it's always high to low.

ie Steve shows a nice piccy of a scope (Fig 1) showing 30%, but clearly this should be 70% in my book. Is this due to the electronics inverting the output before presenting it to the X11 connector?

I'm pretty sure that O2 sensors themselves o/p a voltage 0.1-0.9V (approx) with the output increasing with detected O2?

Could someone help and clear my confusion..

Cheers
Lea

A264172 · #2 04-06-2005, 10:26 AM

I ran across this recently when trouble shooting my emissions.
Kept getting 30% instead of 70% ignition on engine off
In reading thread after thread I found a reference to Mercedes being the reverse of the industry. To obtain the book value of duty cycle you have to reverse the leads (for me [86 300E] at the X11 diagnostic connector pin #3 the Lambda feed should be connected to the black or ground lead of the meter, and the red "com" lead is connected to pin #2, the ground pin).
This left me wondering for a long time if I had controller problems...
Not sure if the O2 sensor requires reverse leads or not.

Duke2.6 · #3 04-06-2005, 11:48 AM

The duty cycle is a synthesized signal that measures the relative amount of time that the lambda system spends adding a rich or lean bias to the basic mixture provided by the air flow sensor and mechanical section of the fuel distributor and is used to make any necessary mechanical mixture adjustments to keep the lambda system near the mid point of its control authority range.

It's essentially a measure of the mechanical mixture bias. If the duty cycle is 50 percent it means the mechanical mixture is spot on stoichiometric, but the mechanical mixture adjustment is not accurate enough to maintain an exact mixture, so the duty cycle will vary slightly as the lambda system corrects the mixture to stoichiometric with finer granularity and greater accuracy.

The other criterion is that you want the average duty cycle at idle to be within 10 percent of the average at 2000 RPM no load. I prefer a slightly less than 50 percent duty cycle at idle as this indicates a slightly rich bias of the basic mechanically derived mixture, which will aid cold starting and warmup driveability. When I checked my duty cycle it was in the range of 40-45 percent at idle and 50-55 percent at 2000, so I made no adjustments, and the cold start characteristics are excellent.

If you have a scope, you can eavesdrop on the actual O2 sensor output by pulling the O2 sensor connector out just enough to get a scope test lead attached with the other scope test lead to ground. The wave form should jump quickly between about 0.15 and 0.88 volt and back with a frequency of about 1-3 Hz at idle.

Duke

LeaUK · #4 04-06-2005, 03:05 PM

O2 sensor testing

Quote:

The duty cycle is a synthesised signal

Are you describing the signal measured at the diag connector or at the Lambda? If the diag signal represents a synthesised 'input' then can it be used to measure how much compensation the system is making? In a closed loop system, if only the output is measured, we cannot tell how much adjustment has been made.

Quote:

If you have a scope, you can eavesdrop on the actual O2 sensor output by pulling the O2 sensor connector out just enough to get a scope test lead attached with the other scope test lead to ground. The wave form should jump quickly between about 0.15 and 0.88 volt and back with a frequency of about 1-3 Hz at idle.

Looking at the graph for the O2 sensor, I assume this is due to the extremely high transfer ratio about the stoichiometric value. Is this true?

I have a scope and will be doing this check. But I'm not sure what I will achieve. If the closed loop system is running correctly I would expect a reading of 50%. I think my engine is a little lean and so as it's closed loop am I correct in thinking I'll still see 50%. I was hoping to find out how lean.

Which signal reveals more, the diag synth sig or the O2 sensor output direct. Whilst typing this, I'm starting to think the diag o/p....

Failing Cat?

Not only this, but my Cat is potentially failing CO emissions (at tailpipe), these are:

Heated Cat -
Co 0.2-0.3% (UK limit 0.3%)
HC 100ppm
Lambda 0.97-1.02

With such a good Lambda reading I'm guessing that the O2 sensor and control system are performing fine, but my 12 year old Cat is simply starting to give up. Hence I want to check the O2 sensor with a scope just to see what's really going on.

Can anyone recommend a test procedure by any chance?

Thanks
Lea

A264172 · #5 04-06-2005, 03:21 PM

How old are your cap/rotor/plugs?
and How far into your next oil change are you?

I achieved a 1.5% CO reduction and a drop of 100ppm HC with an oil change and a new cap & rotor.

LeaUK · #6 04-06-2005, 03:52 PM

Hi

An interesting thought, but the caps and rotors are only a year old although I've had to clean one recently due to tracking - these cars eat caps! Maybe worth a look and clean again?

More reading about my X11/4 and X11 connector (I have both) seem to suggest that the signal available is simply the O2 sensor output which I think I've convinced myself wont actually tell me much.

How does one measure the compensation value the closed loop system is using to keep the 14.7:1 ratio?

In the electronics industry it's easy, we simply measure the input.

A264172 · #7 04-06-2005, 04:40 PM

On my car, you measure it either at the EHA (with a harness) or at X11 (+ to pin#2 / - to pin#3) fully warmed up.
At X11:
You should get a fluctuating duty cycle (fixed is a fault).
50% fluctuating = stoichiometric
Lower %'s indicate leaning of the base mix
Higher %'s iindicate richening of the base mix

Was that your question?

yal · #8 04-06-2005, 04:47 PM

Quote:

Originally Posted by A264172

Lower %'s indicate leaning of the base mix
Higher %'s iindicate richening of the base mix

Isn't this the other way round?

A264172 · #9 04-06-2005, 04:56 PM

Quote:

Originally Posted by yal

Isn't this the other way round?

Depends on where you stick your multi meter I guess.
I was basing this on Duke 2.6's slightly rich mix of 40%
as in O2 sensor wants to lean a rich mixture... and enrich a lean one.

now I'm confused...

Edit: Ok I just replayed this in my head when I adjusted my mixture last weekend...

KOEO- multi meter + lead in pin #2 (ground) / - lead in pin #3 (system)
duty cycle = 69.3% fixed (49 states controler)

Engine fully warmed and running- multi meter + lead in pin #2 / - lead in pin #3
Adjust mixture adjustment screw clockwise (richen mix) = lowers the duty cycly % reading
Adjust mixture adjustment screw counter-clockwise (lean mix) = raises the duty cycly % reading

Or perhaps I should say leaning or richening of the base mix by the controler.

LeaUK · #10 04-06-2005, 05:26 PM

LOL..

closed loop systems often cause headaches...

Sorry, I'm not making myself clear.

Is the signal at the diag connector simply a buffered version of the O2 sensor's o/p? I think it's not. I've read Duke's comments again and he says

Quote:

The duty cycle is a synthesised signal that measures the relative amount of time that the lambda system spends adding a rich or lean bias to the basic mixture provided by the air flow sensor and mechanical section of the fuel distributor and is used to make any necessary mechanical mixture adjustments to keep the lambda system near the mid point of its control authority range.

You see, the system will try and meet the 14.7:1 ratio by richening or leaning the open loop mixture. If you measure the O2 sensor it should fluctuate around 50% at all times, whether you have air leaks, or a rich system or not -because that's what the system tries to achieve, 50%.

I still don't understand how to interpret the duty cycle from my pin 14 (on-off ratio) X11/4 diag. Duty cycle is the ratio of logic high to logic low but Steve B's scope is clearly measuring the opposite

Please come back Duke...

Lea

Duke2.6 · #11 04-06-2005, 08:08 PM

If the duty cycle reading is as I described, then all should be well with the lambda system. I like to look at the O2 sensor output to make sure it has a nice, crisp jump between the high and low voltage readings within the specifed frequency range, however this test is not prescribed by the Mercedes diagnostic procedure, and it not actually necessary if the duty cycle behavior is within accepatable limits.

Given your CO readings, I don't see anything wrong with your catalyst. These KE-equipped engines are usually higher on HC than other fuel system designs for possible reasons that I've outlined in other posts and is probably one reason why it was eventually abandoned for a pure electronic system with solenoid injectors.

In California and other state emission tests, many KE-equipped engines are just squeaking by due to high HC and many have marginal failures. Further details at:

http://www.peachparts.com/shopforum/tech-help/117048-successful-ca-asm-emission-test-ke-fuel-system.html#post833484

Duke

LeaUK · #12 04-07-2005, 02:21 AM

Duke

An interesting and well written post, thanks for the link. However I still have three questions:

1. Is the purpose of the diag signal to allow visibility of the adjustment (whether richening or leaning) the system is making compared to the 50% base?

2. How to read duty cycle for this diag output?

3. Does residual O2 in the exhaust lead to increased CO too?

I think this thread answers 1+2 (thanks guys) but can someone confirm.

Much appreciated.
Lea

A264172 · #13 04-07-2005, 09:23 AM

1. I think thats a fair description... (- the 50%) 50% (fluctuating) would be a reading of the adjustments being made and the plain old base mix is whats being adjusted.

How about: Is the purpose of the diag signal to allow visibility of the adjustment (whether richening or leaning) the system is making to the base?

2. I am unfamiliar with X11/4 and your specific engine (M119 UK?) but you might find that info. in your factory manual or here: http://mb.braingears.com/default.htm The manuals are incomplete but There should be a section for your engine under a similar model Look under engine, combustion,... in an appropriate manual for the specific diagnostic technique.

3. If there is O2 in the exaust and the cat is heated up that means more combustion could have taken place but did not. Without O2 no further combustion would be possible.

Duke2.6 · #14 04-07-2005, 10:41 AM

Quote:

Originally Posted by LeaUK

Duke

An interesting and well written post, thanks for the link. However I still have three questions:

1. Is the purpose of the diag signal to allow visibility of the adjustment (whether richening or leaning) the system is making compared to the 50% base?

2. How to read duty cycle for this diag output?

3. Does residual O2 in the exhaust lead to increased CO too?

I think this thread answers 1+2 (thanks guys) but can someone confirm.

Much appreciated.
Lea

I don't know what you mean by "diag signal".

A stoichiometric mixture should result in "engine out exhaust" that contains a small concentration of HC, CO, and O2 plus some NOx. A three way catalyst requires this exhaust constituency to provide both oxidation and reduction reactions. The amount of 02 in the after converter exhaust is a good diagnostic tool. If it is zero, the catalyst is operating at maximum efficiency by using all the available O2 to oxidize HC and CO. A tenth of a percent or more indicates that full catalytic oxidation reaction is not being achieved. This is often due to the converter being too cool, and these KE-equipped engines seem to be particularly suspectible to catalyst cool down by improper conditioning prior to testing such as shutting the down before the test or excess idling prior to the test.

Duke

LeaUK · #15 04-08-2005, 02:28 AM

Hi Guys

Thanks for the comments.

Just to be clear, my engine is a LH not KE and therefore has much more control all round.

When previously posted, my referral to the 'diag signal' is the signal that is present in the X11. The R129 (from around '92 upto around '95) was fitted with both x11/4 (38pin diag conn) and the traditional x11 (9 pin) connector for fault diagnostics.

I'm specifically interested by the o2 emissions as these are good clues to whether the Cat is hot enough - thanks for letting me know.

Lea