Adjusting Fuel Mixture (Problem with checking idle duty cycle)
 

Hey everyone,

I have, for a few days anyway, an Aglient oscilloscope, so i thought I would look at my fuel mixture on my m103. Turns out it was not right.

I'll post up the measurements later, I was pretty close to getting it to spec when I lost any reading @ idle after 30 second from start...

My scope shows nothing but noise..

When you rev the engine to ~1000rpm, I hear a vacuum valve pop open/closed (seems to be connected to a pot of some sort behind the headlamp and the intake manifold) and the waveform comes back to life.

I checked the o2 sensor and its fine. fluctuating voltages.

Anyone have any clue as to why i would get no duty cycle reading from the x11 (pin2 +,pin3 -) port @ idle?

Thanks,

Kris

Your '93 has to be put into 'Diagnostic Mode' to read duty cycle.

really? it was reading duty cycle fine up until that point...how does one enter it into diag mode? i over adjusted lambda took it up to my shop they corrected it for me... havent had a chance to look at it w scope yet..

From the manual:

"@1988 California and
@1991 with diagnostic trouble code
storage:
The CFI control module must be switched over
to output of on/off ratio because the signal for
the diagnostic trouble code (DTC) display is
output as priority.
The output of the on/off ratio is performed once
all the DTCs have been read out and the start
button of the impulse counter scan tool or the
non-locking switch on the diagnostic connector
(California version only) is then once again
pressed (see section m).
The % readout now fluctuates when the engine
is running at normal operating temperature and
there are no DTCs."

I built a homemade LED Tester to put my '93 190E into diagnostic mode to read duty cycle. It also reads DTC's.

If i have to put it in diagnostic mode to read duty cycle..what is it outputting to my scope throu x11/2 x11/3 ?


taken before any adjustment were done. all i did was connect up to x11
http://i98.photobucket.com/albums/l250/MafiaNicky/idleafterafewminutes.jpg


If it is in normal mode..why would it output anything at all?

Was that at idle?
From the looks on that waveform, it looks like a lean mixture. My 2 cents!

Yes, that was @ idle. That reading was done just before I started adjusting things. I did teh EHA first and got it to ~45-55%.

I adjusted the Lambda tower once or twice, then lost signal.

Now when I check duty cycle, it fades out as in this video:

http://s98.photobucket.com/albums/l250/MafiaNicky/?action=view&current=DutyCycleScope.flv

Put an Ammeter at the EHA (in series) and measure the current. You should oscillate around 0 Amps -1mA to +2mA. That should tell you that the EHA is working at regulating the pressure differential inside the fuel distributor based on the feedback.
Check your fuel pressure readings:
Main pressure, that is before the distributor (the cold start outlet is a good place).
Lower pressure, that can be done via the test outlet on the distributor.
You should get a specific difference between the two, check your manual for the value.
If that is off and you tried the EHA to get it then you have a problem with a CIS component. You will have to trace that throughout the fuel lines.
In your case, it's possible that the computer can't keep up and from the looks on that signal you may have set it where it went rich. You could see it drift then it's like the computer decided not to play no more:D

Please note that EHA and Lambda adjustments must be done a hair at a time. NEVER full turn!

I will check the current in the EHA when I get home tonight.

I don't have any tools to check the pressure however...
Any recommendation on a cheap set of gauges?

Yes, Harbor Freight has a nice CIS set. That's the one I use which is meant for occasional use (not a pro that takes a daily beating).
I used it on the Benzes and on a Ferrari it works just fine.

could you point it out to me ?

http://www.harborfreight.com/cpi/ctaf/Category.taf?CategoryID=455

I did find one that like $12 but it says "except Bosch, CIS"
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=92699


Is it this:
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=93547

this one
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=97706

http://www.harborfreight.com/cpi/pho...7799/97706.gif

It's $80 but worth it for occasional use. A professional version will be much higher $$

Checked current flow in the EHA:

(note: my motor is a m103 3L from a 90 300SEL)
(note: my exhaust is straight pipe)

Resistance: 1.116 M Ohm

Key on, Engine Cold: 19.980mA

Key on, Engine On Cold: 4.819mA

After sitting to ~80C : 16.477 mA

As engine was revved it typically went between 4 and 16mA

As engine was held @ higher rpm started @ high number and decresed rapidly to around two or three mA.

Used this: http://www.home.agilent.com/agilent/product.jspx?pn=34410A

just filled up at the gas station

11.02 gallons covered 212 miles....for 19.36 Mpg..